Description:
NONLINEAR SOLID STATE DEVICES FOR OPTICAL RADIATION I
N FAR‐UVC SPECTRUM CLAIM OF PRIORITY [0001] The present application claims priority from U.S. Pro
visional Patent Application No. 63/311,660 filed February 18, 2022, and U.S. Provisio
nal Patent Application No. 63/359,251 filed July 8, 2022, with the United States Patent a
nd Trademark Office, the disclosures of which are incorporated by reference herein in their entiret
ies. FIELD [0002] The present application is directed to UV light sour
ces, and in particular, to far‐UVC light sources and related devices and methods. BACKGROUND [0003] Compact and efficient ultraviolet (UV) light sources
in the wavelength range of about 200 nanometers (nm) to about 400 nm may be desirabl
e for many applications. For example, UV lasers may be used for lithography in semiconduct
or manufacturing. Since short‐ wavelength radiation is easily absorbed by most mater
ials, another application is the detection and classification of materials and substances, such
as in mass spectroscopy. Photons in the UV‐C (or UVC) wavelength range (e.g., about 200 nm
to about 280 nm) can be used to disinfect airborne and surface disease‐causing pathogens while
remaining safe for human exposure. For example, far‐UVC light (from about 200 nm to about
240 nm) may not penetrate through the dead‐cell layer of the skin surface or the tear l
ayer of the human eye, but may be effective against bacteria and viruses. In particular, far‐U
VC light can efficiently cause permanent physical damage to DNA, which can prevent bacteria,
viruses and fungi from replicating. Human‐safe far‐UVC light can thus effectively kill
disease causing pathogens with little to no risk to humans because these wavelengths may be largely a
bsorbed by the stratum corneum (the top layer of dead skin cells in the epidermis). [0004] However, operation in the far‐UVC wavelength range
may present challenges. For example, few available light sources may be configure
d for operation in the far‐UV. Some conventional UV light sources have been implemented b
y gas‐based lamps. An important class of such lamps is called “excimer” (excited dimer)
lamps that employ a mixture of a reactive gas (such as F 2 or Cl 2 ) and an inert gas (such as Kr, Ar or Xe) a
s an active medium. The gas mixtures, when electrically excited, produce a pseudo‐molecule
excited state dimer, or ‘excimer’ with an energy level configuration that allows the generation
of specific ultraviolet laser wavelengths. For example, some KrCl lamps may be used to generat
e Far UVC light for medical applications. However, the inefficiency, large size, and significant
cost of such lasers may be prohibitive for use in many applications. [0005] Also, high power, ultrafast laser systems designed fo
r laboratory use can generate non‐ linear harmonics (e.g., second, third, fourth, and fi
fth harmonic generation) or parametric sum frequency generation to create light in the far‐U
V. Such systems may likewise be large (e.g., table‐top size or macroscopic optical bench size),
expensive, and inefficient (e.g., generating less than a watt optical in the far UV). [0006] Free‐electron pumped field emission lamps with hexag
onal Boron Nitride (h‐BN) target may rely on bulbs that are vacuum sealed to allow
the electron beam to operate, but the power efficiency and reliability of such lamps may be unpr
oven. [0007] Semiconductor‐based LED light sources (e.g., based o
n GaN material system) have also been used to provide UV‐C light, for example, usin
g phosphor‐based wavelength conversion. Such light sources typically have short operating lif
etimes and poor performance at emission wavelengths shorter than about 265 nm. Also, due t
o residual uncertainty about human safety, regulatory limits remain strict. SUMMARY [0008] According to some embodiments, an ultraviolet (UV) li
ght source includes a light emitting element that is configured to generate light
of a first frequency, a nonlinear optical element that is configured to receive the light of
the first frequency from the light emitting element and generate far‐UVC light of a second fre
quency from the light of the first frequency, and an output coupling element that is configured to
selectively outcouple the far‐UVC light from the nonlinear optical element as output light.
[0009] In some embodiments, the output coupling element is
configured to selectively outcouple the far‐UVC light into at least one dire
ction that is different than a direction of propagation of the light of the first frequency to
provide the output light. [0010] In some embodiments, the output light is substantiall
y free of the light of the first frequency. [0011] In some embodiments, the nonlinear optical element, t
he light emitting element, and/or the output coupling element comprise elements of a s
ame material system. In some embodiments, the nonlinear optical element comprises a
luminum nitride (AlN). In some embodiments, the light emitting element and/or the ou
tput coupling element comprise a Group III nitride‐based material. [0012] In some embodiments, the nonlinear optical element is
or comprises an optical cavity that is at least partially resonant at the first fr
equency. [0013] In some embodiments, the nonlinear optical element ha
s a ring configuration that defines the optical cavity. [0014] In some embodiments, the nonlinear optical element co
mprises a plurality of nonlinear optical elements that are arranged to receive the li
ght of the first frequency from the light emitting element. [0015] In some embodiments, an input coupling element is co
nfigured to receive the light of the first frequency from the light emitting element,
and the plurality of nonlinear optical elements are arranged along the input coupling elemen
t. [0016] In some embodiments, respective ones of the nonlinear
optical elements comprise different dimensions and/or materials. The output co
upling element comprises a plurality of output coupling elements that are respectively configu
red to selectively outcouple the far‐UVC light from the respective ones of the nonlinear opti
cal elements. [0017] In some embodiments, the optical cavity includes the
light emitting element and the nonlinear optical element therein. [0018] In some embodiments, the optical cavity has a linear
shape or a closed curve shape. [0019] In some embodiments, output coupling element comprises
at least one of a facet having a refractive index that is configured to selectively
outcouple the far‐UVC light in a first direction,
or a grating having a diffraction order that is c
onfigured to selectively outcouple the far‐UVC light in a second direction, different than the firs
t direction. [0020] In some embodiments, the nonlinear optical element an
d the output coupling element are integrated in an output element that is configur
ed to outcouple the far‐UVC light at a plurality of positions or continuously along a length
thereof. [0021] In some embodiments, the UV light source is configur
ed to provide the output light substantially free of phase matching between the ligh
t of the first frequency and the far‐UVC light of the second frequency. [0022] In some embodiments, at least one of the nonlinear
optical element and the output coupling element is configured to provide phase match
ing between the far‐UVC light of the second frequency and the light of the first frequenc
y. [0023] In some embodiments, the light emitting element is a
laser comprising a lasing cavity. The laser is configured to generate the light of th
e first frequency. In some embodiments, the laser comprises a Group III nitride‐based material.
[0024] In some embodiments, the light emitting element furth
er comprises one or more optical resonators that are configured to reflect the light
of the first frequency and are arranged at first and second ends of the lasing cavity. [0025] In some embodiments, the nonlinear optical element is
configured to receive the light of the first frequency from an intra‐cavity portion be
tween first and second ends of the lasing cavity. [0026] In some embodiments, the nonlinear optical element co
mprises first and second nonlinear optical elements positioned at first and se
cond ends of the lasing cavity, respectively. [0027] In some embodiments, a saturable absorber in the las
ing cavity is configured to generate the light of the first frequency as a plur
ality of light pulses at a predetermined pulse repetition frequency and duty factor. [0028] In some embodiments, at least one tuning mechanism i
s configured to adjust one or more operating characteristics of the nonlinear elemen
t based on the light of the first frequency. [0029] In some embodiments, a monitor element is configured
to measure a property of the output light and generate a feedback signal to a co
ntroller that is configured to operate the light emitting element and/or the tuning mechanism. [0030] In some embodiments, a substrate includes the light
emitting element, the nonlinear optical element, and the output coupling element on
a surface thereof, where two or more of the light emitting element, the nonlinear optical ele
ment, the output coupling element, or connecting waveguides therebetween overlap in a direct
ion perpendicular to the surface of the substrate. [0031] In some embodiments, the output coupling element comp
rises a plurality of output coupling elements that are configured to outcouple th
e far‐UVC light in respective directions, to provide the output light with a desired far field p
attern. [0032] In some embodiments, one or more sensors are configu
red to detect real‐time conditions in an operating environment of the UV lig
ht source, and to transmit detection signals indicating the real‐time conditions to a controller
that is configured to control operation of the light emitting element based on the detection signals
. [0033] In some embodiments, the second frequency comprises a
sum of or a harmonic of the first frequency. [0034] In some embodiments, the first frequency corresponds
to a first wavelength in a range of about 400 nanometers (nm) to 480 nm, and the se
cond frequency corresponds to a second wavelength in a range of about 200 nm to 240 nm.
[0035] In some embodiments, the light emitting element and
the nonlinear optical element comprise respective elements that are arranged on a
non‐native substrate. [0036] In some embodiments, the light emitting element and
the nonlinear optical element are integrated in a monolithic structure. [0037] In some embodiments, the UV light source comprises a
n array including a plurality of the light emitting element and the nonlinear optical
element. [0038] According to some embodiments, a light source include
s a monolithic structure comprising a light emitting element that is configure
d to generate light of a first frequency, and a nonlinear optical element that is configured to re
ceive the light of the first frequency from the light emitting element and generate light of a secon
d frequency from the light of the first frequency. [0039] In some embodiments, the monolithic structure further
comprises an output coupling element that is configured to selectively outcouple t
he light of the second frequency from the nonlinear optical element as output light. The output
coupling element is configured to selectively outcouple the light of the second frequen
cy into at least one direction that is different than a direction of propagation of the lig
ht of the first frequency to provide the output light. [0040] In some embodiments, the nonlinear optical element of
the monolithic structure comprises aluminum nitride (AlN). In some embodiment
s, the light emitting element and/or the output coupling element of the monolithic structu
re comprise a Group III nitride‐based material. [0041] In some embodiments, the light emitting element is a
laser comprising a lasing cavity, and the nonlinear optical element is configured to r
eceive the light of the first frequency from an intra‐cavity portion between first and second en
ds of the lasing cavity. [0042] In some embodiments, the light of the second frequen
cy is UVC light, such as far‐UVC light. In some embodiments, the light of the first
frequency is visible light. [0043] According to some embodiments, an ultraviolet (UV) li
ght source includes a light emitting element that is configured to generate light
of a first frequency, and a nonlinear optical element comprising aluminum nitride (AlN) that
is configured to receive the light of the first frequency from the light emitting element and
generate UVC light of a second frequency from the light of the first frequency. [0044] In some embodiments, an output coupling element is c
onfigured to selectively outcouple the UVC light from the nonlinear optical
element as output light, in some embodiments into at least one direction that is diff
erent than a direction of propagation of the light of the first frequency. The light emitting e
lement and/or the output coupling element may include a Group III nitride‐based material, in some
embodiments in a monolithic structure. [0045] According to some embodiments, an ultraviolet (UV) li
ght source includes a light emitting element that is configured to generate light
of a first frequency, and an optical cavity comprising a nonlinear optical element that is config
ured to receive the light of the first frequency from the light emitting element and generat
e UVC light of a second frequency from the light of the first frequency, where the optical
cavity is at least partially resonant at the first
frequency. [0046] In some embodiments, the optical cavity is at least
partially resonant at the first frequency and at the second frequency. [0047] In some embodiments, the optical cavity comprises a
plurality of optical cavities, each comprising a respective nonlinear optical element and
arranged to receive the light of the first frequency from the light emitting element. In some e
mbodiments, the optical cavities are ring‐ shaped. [0048] In some embodiments, respective ones of the optical
cavities include different dimensions and/or materials, and the output coupling
element comprises a plurality of output coupling elements that are respectively configured to
selectively outcouple the UVC light from the respective ones of the optical cavities. [0049] In some embodiments, the nonlinear optical element an
d the output coupling element are integrated in an output element that is configur
ed to outcouple the UVC light at a plurality of positions or continuously along a length thereof.
[0050] According to some embodiments, a light source include
s a light emitting element that is configured to generate light of a first frequency, a
nd a nonlinear optical output coupling element that is configured to receive the light of
the first frequency from the light emitting element, generate light of a second frequency from t
he light of the first frequency, and outcouple the light of the second frequency as outpu
t light at a plurality of positions or continuously along a length thereof. [0051] In some embodiments, the light source is configured
to provide the output light substantially free of phase matching between the ligh
t of the first frequency and the light of the second frequency. [0052] In some embodiments, the nonlinear optical output cou
pling element is configured to selectively outcouple the light of the second frequen
cy into at least one direction that is different than a direction of propagation of the lig
ht of the first frequency to provide the output light. [0053] In some embodiments, the nonlinear optical output cou
pling element includes or is coupled to an optical cavity that is at least parti
ally resonant at the first frequency. [0054] In some embodiments, the nonlinear optical output cou
pling element comprises a plurality of alternating nonlinear optical element sec
tions and output coupling element sections along the length thereof. [0055] In some embodiments, the nonlinear optical output cou
pling element comprises first and second materials that are configured to alter li
ght propagation at one of a first wavelength corresponding to the first frequency and a second wa
velength corresponding to the second frequency, and do not substantially alter light propa
gation at another of the first wavelength and the second wavelength. [0056] In some embodiments, the nonlinear optical output cou
pling element is a waveguide comprising nanopores or defects therein having respect
ive dimensions that are configured to scatter the light of the second frequency, without s
ubstantially affecting propagation of the visible light of the first frequency. [0057] In some embodiments, the first frequency corresponds
to a first wavelength in a range of about 400 nanometers (nm) to 480 nm, and the se
cond frequency corresponds to a second wavelength in a range of about 200 nm to 240 nm.
[0058] Other devices, apparatus, and/or methods according to
some embodiments will become apparent to one with skill in the art upon review
of the following drawings and detailed description. It is intended that all such additiona
l embodiments, in addition to any and all combinations of the above embodiments, be included wi
thin this description, be within the scope of the invention, and be protected by the acc
ompanying claims. BRIEF DESCRIPTION OF THE DRAWINGS [0059] FIG. 1A is a schematic block diagram illustrating a
UV light source according to some embodiments of the present disclosure. [0060] FIG. 1B is a schematic block diagram illustrating el
ements of a UV light source according to some embodiments of the present disclosure in gre
ater detail. [0061] FIG. 1C is a graph illustrating an emission range
for light output from a UV light source according to some embodiments of the present disclosu
re. [0062] FIGS. 2A1 and 2A2 are schematic perspective and side
views, respectively, illustrating elements of a UV light source in a vertical linear
arrangement according to some embodiments of the present disclosure. [0063] FIGS. 2B1 and 2B2 are schematic perspective and top
views, respectively, illustrating elements of a UV light source in a horizontal linea
r arrangement according to some embodiments of the present disclosure. [0064] FIGS. 2C1 and 2C2 are schematic top views illustrati
ng elements of a UV light source in a spiral arrangement according to some embodiments of t
he present disclosure. [0065] FIGS. 3A1 and 3A2 are schematic block diagrams illus
trating elements of a UV light source including optical cavity enhancement according
to some embodiments of the present disclosure. [0066] FIGS. 3B1 and 3B2 are schematic perspective and top
views, respectively, illustrating elements of a UV light source including optical cavi
ty enhancement in a horizontal linear arrangement according to some embodiments of the pres
ent disclosure. [0067] FIG. 4A is a schematic block diagram illustrating el
ements of a UV light source including a nonlinear optical element in a ring cavity configu
ration according to some embodiments of the present disclosure. [0068] FIG. 4B is a schematic top view illustrating element
s of a UV light source including a plurality of nonlinear optical elements in ring cavit
y configurations that are sequentially arranged according to some embodiments of the present
disclosure. [0069] FIG 4C1 is a schematic top view illustrating element
s of a UV light source including a nonlinear optical element coupled to an intra‐cavity
portion of the light emitting element according to some embodiments of the present disclosu
re. [0070] FIG 4C2 is a schematic top view illustrating an arr
ay of UV light sources that respectively include a nonlinear optical element coupled to an in
tra‐cavity portion of the light emitting element according to some embodiments of the present
disclosure. [0071] FIG. 4C3 is a graph illustrating vernier frequency s
election for determining optical cavity size (including height, width, and circumference/length
) of a ring‐shaped nonlinear optical element according to some embodiments of the present
disclosure. [0072] FIG. 5A is a schematic block diagram illustrating el
ements of a UV light source in which the light emitting element and the nonlinear optical
element are provided in a same optical cavity according to some embodiments of the present
disclosure. [0073] FIGS. 5B1 and 5B2 are schematic perspective and top
views, respectively, illustrating elements of a UV light source in a same optical ca
vity with an output coupling element implemented as a reflective facet configured for sele
ctive light extraction in a horizontal linear arrangement according to some embodiments of the pres
ent disclosure. [0074] FIGS. 5C1 and 5C2 are schematic perspective and top
views, respectively, illustrating elements of a UV light source in a same optical ca
vity with an output coupling element implemented as an optical grating configured for sele
ctive light extraction in a horizontal linear arrangement according to some embodiments of the pres
ent disclosure. [0075] FIG. 6A is a schematic block diagram illustrating el
ements of a UV light source in which the light emitting element and the nonlinear optical
element are provided in a same optical cavity in a closed curve or racetrack configuration
according to some embodiments of the present disclosure. [0076] FIGS. 6B1 and 6B2 are schematic perspective and top
views, respectively, illustrating elements of a UV light source in a same optical ca
vity in a closed curve or racetrack configuration with an output coupling element implemen
ted as an optical grating configured for selective light extraction according to some embo
diments of the present disclosure. [0077] FIGS. 7A, 7B, and 7C illustrate example drive signal
s to provide pulsed light output from light emitting elements in a UV light source accordi
ng to some embodiments of the present disclosure. [0078] FIG. 8 is a schematic perspective view illustrating
respective elements of a UV light source arranged in a hybrid configuration on a commo
n non‐native substrate according to some embodiments of the present disclosure. [0079] FIG. 9 is a schematic block diagram illustrating an
array of UV light sources that respectively include a light emitting element and a
nonlinear optical element according to some embodiments of the present disclosure. [0080] FIGS. 10A and 10B are schematic perspective and top
views, respectively, illustrating elements of a UV light source including optical cavi
ty enhancement with an output coupling element configured to provide distributed emission and
selective light extraction in a horizontal linear arrangement according to some embodiments of t
he present disclosure. [0081] FIG. 11A is a schematic top view illustrating an ex
ample combination of various elements of a UV light source including a nonlinear
optical element coupled to an intra‐cavity portion of a light emitting element, in combination
with ring resonators at respective ends of the light emitting element and an output coupling el
ement configured for selective light extraction according to some embodiments of the prese
nt disclosure. [0082] FIG. 11B are graphs illustrating a vernier frequency
effect according to some embodiments of the present disclosure that provides f
or selection of a subset of possible longitudinal modes supported by the light emitting el
ement. [0083] FIG. 12 is a schematic top view illustrating an exa
mple combination of various elements of a UV light source including nonlinear optical ele
ments coupled to an extra‐cavity portions of a light emitting element, in combination with ring r
esonators at respective ends of the light emitting element, tuning mechanisms, output coupling e
lements configured for selective light extraction, and an output monitor according to some
embodiments of the present disclosure. [0084] FIG. 13 is a schematic top view illustrating an exa
mple combination of various elements of a UV light source including nonlinear optical ele
ments coupled to an extra‐cavity portions of a light emitting element including a saturable absorb
er element, in combination with tuning mechanisms, output coupling elements configured for se
lective light extraction, and an output monitor according to some embodiments of the present
disclosure. [0085] FIG. 14 is a schematic top view illustrating an exa
mple combination of various elements of a UV light source including a plurality of diffe
rent or non‐identical nonlinear optical elements 120 in ring cavity configurations with respective out
put coupling elements according to some embodiments of the present disclosure. [0086] FIG. 15 is a schematic top view illustrating an exa
mple combination of various elements of a UV light source in physically overlapping confi
gurations according to some embodiments of the present disclosure. [0087] FIG. 16 is a schematic top view illustrating includi
ng output coupling element configurations of a UV light source to provide a de
sired far field emission pattern according to some embodiments of the present disclosure. [0088] FIG. 17 is a schematic block diagram illustrating co
mponents of a sensor feedback‐based “smart” illumination device that includes a germic
idal UV (GUV) light source communicatively coupled to sensors that are configured to feedback i
nformation to a controller of the GUV light source according to some embodiments of the present
disclosure. DETAILED DESCRIPTION [0089] Embodiments of the present disclosure provide solution
s for generating electromagnetic radiation in the far‐UVC wavelength
band (about 200 nm to 240 nm, for example, about 207 nm to 222 nm) and related contro
l of illumination patterns, which can be useful for numerous applications, including (but not
limited to) germicidal applications for disinfecting airborne and surface disease‐causing pat
hogens, and detection of trace chemical or biological species in various field environments (air,
water, etc.), while simultaneously remaining safe for human exposure and complying with
human safety regulations and requirements. In particular, embodiments of the pres
ent disclosure provide a solid state system and method for generating coherent or non‐co
herent, collimated or non‐collimated, electromagnetic, non‐ionizing radiation in the far‐
UVC wavelength band, based on nonlinear optical processes and using photonic integrated circui
ts (PIC). As used herein, “far‐UV” or “Far
UV” wavelength band or range refers to wavelengths
greater than about 200nm (such that the radiation is non‐ionizing in the atmosphere), and l
ess than about 240 nm, for example, about 200 nm to 230 nm. [0090] Embodiments of the present disclosure allow generation
of light in the far‐UVC band using compact sources based on materials and processe
s from the semiconductor industry which will allow rapid volume scaling reduction of c
ost that may not be available by other methods. Embodiments of the present disclosure may
provide a far‐UVC light source including a semiconductor light emitting element, such as a pu
mp laser (e.g. a Group‐III nitride‐based laser diode) configured to generate light of a first
wavelength (e.g., in the visible spectrum, also referred to herein as visible light), which is coupl
ed into a nonlinear optical element (e.g., a monolithically integrated waveguide with nonlinear opti
cal properties) for generation of light that is a sum of the frequency of the visible ligh
t (also referred to herein as sum frequency generation (SFG), e.g., Second Harmonic Generation (SH
G) of frequency doubled light). Sum frequency generation may include both frequency doubli
ng (combination of photons of the same wavelength) and optical parametric conversion (i.
e., from combination or difference of two photons of unequal wavelength). The nonlinear o
ptical element may be referred to herein as an SHG element, or more generally, an SFG elemen
t. The sum frequency generation or frequency‐doubling converts a portion of the visible
light emitted by the light emitting element into far‐UVC light. In particular, some embodiment
s of the present disclosure provide a monolithic, solid‐state Far UV Photonic Integrated C
ircuit (PIC) (for example, based upon the AlN/GaAlN material system), which may be scalable to
high volumes, low cost, high WPE, and small form factors without the need for an optical
filter that discriminates or transmits light only within a range of far‐UVC wavelengths. [0091] As used herein, the term monolithic structure or mon
olithic integration may refer to any arrangement of active elements (e.g., light emitting
elements) and/or passive elements (e.g., waveguides or other optical coupling elements) in a
unitary structure with no air interfaces or free propagation of light between elements, including
structures formed by epitaxial growth, wafer bonding, and/or microtransfer printing or other
forms of mass transfer for solid state integration. Monolithic integration may thus include
elements of the same material system or multiple materials, and may be provided on a native
(e.g., growth) substrate or on a non‐native substrate (which is different from the native or sou
rce substrate on which the elements are grown or otherwise formed). In contrast, a hybrid
structure or hybrid integration may refer to arrangement of separate or discrete elements (e.g., r
espective semiconductor chips) with air interfaces between elements and/or assembly of such d
iscrete elements on a non‐native substrate. Elements that are “coupled” may refer
to physical and/or optical coupling. [0092] U.S. Patent 9,159,178 describes the use of a semicon
ductor diode laser as the pump frequency which has a single pass through a non‐li
near crystal (BBO) and is critically phase matched by means of angle tuning. The publication
“Periodically‐Poled AlN for frequency doubling” to Sitar et al. describes AlN as a nonl
inear material, but seeks to achieve phase matching by periodically poling the AlN, with a macr
oscopic pump laser that is externally coupled into the AlN ridge waveguide with major opti
cal losses. [0093] Second (or third, fourth, etc.) harmonic frequency ge
neration using nonlinear optical materials in accordance with some embodiments may req
uire several components or characteristics for efficient conversion. For example
, a nonlinear crystal that is non‐ centrosymmetric and highly polarizable may lead to no
n‐zero elements of its second order non‐ linearity tensor, where the higher this coefficient,
the higher the conversion rate. The nonlinear crystal should be optically transparent at
the wavelength of the frequency doubled light; otherwise the crystal would absorb the newly
generated light. Also a pump light source that is coherent (second harmonic generation is a co
herent effect relevant to a single wavelength, so the pump laser may have a narrow lin
ewidth with sufficiently long coherence length) and high power (the output power of second
harmonic generation scales with the square of the pump power; therefore the higher the
power of the pump laser the higher the efficiency of conversion) may be used. In some emb
odiments, pulsed lasers, which generally have higher peak pulse power than continuous wave (C
W) lasers, may be preferred. In some embodiments, phase matching methods may be used to m
atch the phase speed of the pump wavelength to that of the second harmonic wavelength
such that coherent addition of the electric field from both waves is maintained over th
e entire propagation length of the nonlinear crystal. [0094] According to some embodiments of the present disclosu
re, a UV light source includes a light emitting element (e.g., a Group‐III nitride‐
based laser diode, such as a blue pump laser diode) configured to generate light of a first (fund
amental) frequency or wavelength (e.g., visible light), a nonlinear optical element (e.g., a
nonlinear optical crystal, such as a SHG element) that is optically transparent to wavelengths
at or below the desired output wavelength (e.g., the UVC wavelength range of about
200 nm to about 280 nm, or the far‐UVC wavelength range of about 200 nm to about 240 nm)
and is configured to generate UVC or far‐ UVC light of a second frequency or wavelength based
on sum frequency generation of the light of the first frequency; an input coupling element co
nfigured to couple the light from the light emitting element into the nonlinear optical element (
e.g., a continuous waveguide or optical fiber or a photonic wirebond that connects radiatio
n from the pump laser to the nonlinear optical crystal; also referred to herein as an input
waveguide); and an output coupling element configured to selectively outcouple the UVC or far‐
UVC light from the nonlinear optical element. In some embodiments, one or more elements m
ay provide phase matching between the UVC or far‐UVC light of the second frequency
or wavelength and the fundamental (pump) frequency or wavelength of the visible light. [0095] FIG. 1A is a schematic block diagram illustrating a
UV light source 100 according to some embodiments of the present disclosure. As shown in
FIG. 1A, an ultraviolet (UV) light source 100 includes a light emitting element 110, 110’ th
at is configured to generate light 111 (e.g., visible light 111’) of a first frequency, a nonlin
ear optical element 120, 120’ that is configured t
o receive the light 111, 111’ from the light emittin
g element 110 and generate light 121 (e.g., far‐ UVC light 121’) of a second frequency based on su
m frequency generation of the light 111, 111’ of the first frequency, and an output coupling eleme
nt 130 that is configured to selectively outcouple the light 121, 121’ from the nonlinear
optical element 120, 120’ as output light 131, 131’. While described herein primarily with refe
rence to generation of far‐UVC light (including wavelengths within about 200 nm to about 240 nm), i
t will be understood that embodiments of the present disclosure are not limited to generations
of far‐UVC light. For example, some embodiments may include nonlinear optical elements 120
configured to generate light over a wider wavelength range, such as UVC light (including
wavelengths within about 200 nm to about 280 nm). [0096] In some embodiments, the light emitting element 110
may be a blue pump laser 110’ that produces (high power) coherent radiation at wave
lengths between about 400 nm to about 460 to 480 nm with good wall plug efficiency (optic
al power output per unit electrical power consumption). In some embodiments, the laser 110’
may be a laser diode (for example, an edge emitting laser or a vertical‐cavity surface‐e
mitting laser (VCSEL)). However, other lasers (for example, a frequency doubled fiber laser) may a
lso be used. The light emitting element 110 may be formed of or otherwise include a Group‐III
nitride‐based material (such as gallium nitride (GaN)). [0097] The nonlinear optical element 120 may be configured
to generate far‐UVC light 121’ of a second frequency based on sum frequency generation of
the light of a first frequency that is output from the light emitting element 110. The se
cond frequency may be a harmonic (e.g., integer multiple) of the first frequency. The nonli
near optical element 120 may be a nonlinear optical crystal that is optically transparent to wave
lengths at or below the far‐UVC output wavelength of about 200 to 240 nm. Examples of su
ch nonlinear optical crystals may include, but are not limited to, BBO, aluminum nitride (AlN),
lithium niobate (LiNbO 3 ), etc. [0098] AlN is not (to the inventors’ knowledge) generally
used to provide nonlinear optical elements. Rather, those of skill in the art in
the field of nonlinear optics (as distinct from thos
e of skill in the art in the field of semiconductor
processing) have typically relied on bulk crystals, for example, using angle tuning of birefringent mater
ials to achieve phase matching. Common suppliers of such bulk nonlinear crystals likewise ha
ve not recognized and do not sell AlN nonlinear optics. Also, bulk crystalline AlN may no
t be well suited for operation in the UV wavelength ranges due to a large quantity of point
defects therein, which can absorb light having shorter wavelengths. [0099] While thin‐film AlN may be optically transparent to
wavelengths of light as short as 200 nm, such thin films of AlN have been limited to us
e of waveguides. Research using thin‐film AlN for nonlinear optical conversion has typically
been limited to longer wavelengths of light, for example, due to some inherent challenges with ac
hieving similar results at shorter wavelengths of light. For instance, polycrystalline
thin films fabricated by sputtering may include grain boundaries that create absorption and s
cattering at short wavelengths of light(similar to point defects in bulk AlN). Fabric
ation of PICs in AlN with the fidelity required for acceptable nonlinear conversion may likewise be d
ifficult at shorter wavelengths of light. Such challenges may manifest as losses in the nonlin
ear optical element, and thus may present barriers to realizing desired performance at short wa
velengths. In particular, fabrication fidelity may present difficulties in achieving sufficient phase
matching. [0100] Some embodiments of the present disclosure may arise
from realization that delivery of higher intensity pump laser light into the nonlinear
optical element may overcome the aforementioned optical loss challenges. Embodiments o
f the present disclosure provide various configurations for obtaining higher optical in
tensity inside the nonlinear optical element, including (but not limited to) monolithic in
tegration, cavity enhancement, and intra‐ cavity‐tapping as described herein. Embodiments of
the present disclosure also address challenges with respect to phase matching, which may
be more difficult at shorter wavelengths. [0101] AlN may be advantageous for nonlinear optical eleme
nt 120 formation (e.g., by epitaxial growth) in combination with a Group III ni
tride‐based light emitting element 110 material, such as GaN, due to lattice compatibility
or similarity of material processing to that of GaN. More generally, the nonlinear optical element
120 and the light emitting element 110 may include common elements or materials that belong
to the same material system (e.g., AlN may be used as a nonlinear optical element 120’ b
ecause it belongs to the same AlGaInN material system from which a GaN light emitting elem
ent 110’ is formed). Particular embodiments are described herein with reference to Al
N‐based nonlinear optical elements 120 (and in some embodiments, with reference to light so
urces where the light emitting element 110, the nonlinear optical element(s) 120, and the c
oupling elements 115, 130 are all nitride‐ based materials), but are not limited thereto. [0102] The output coupling element 130 may refer to an opt
ical element that is configured to provide the output light 131 (e.g., the far‐UVC li
ght 121’) for propagation through free space. In some embodiments, the output coupling element 130 may
be configured to provide selective light extraction, such that the output light 131 may
include primarily the far‐UVC light 121’ of the second wavelength or frequency (and in some inst
ances, may be substantially free of the visible light 111’ of the first wavelength or freq
uency), in one or more directions that differ from the direction(s) of propagation of the light 11
1 of the first wavelength or frequency. The output coupling element 130 may be implemented as pa
rt of a waveguide (also referred to herein as an output waveguide) and/or an edge facet
in some embodiments. In some embodiments, the output coupling element 130 may be
a grating for generating the different direction(s) of propagation of the output light 131
as surface emission, such as surface normal (or near surface normal) emission. In some embodime
nts, the output coupling element 130 may be integrated with the nonlinear optical element
120 (also referred to herein as a nonlinear optical output coupling element 120/130). [0103] FIG. 1B is a schematic block diagram illustrating el
ements of a UV light source according to some embodiments of the present disclosure in gre
ater detail. As shown in FIG. 1B, the UV light source includes the light emitting element 110
(shown as pump laser 110’ configured to output visible light 111’ having a wavelength λ 0 between 400 nm and 480 nm) and the nonlinear optical element 120 (shown as a SFG elemen
t configured to output light 121’ having a wavelength λ 2 between 200 nm and 240 nm, e.g., ½λ 0 ). Some subcomponents of the laser 110’ include the gain medium and the optical structures w
hich form an optical cavity 125. The gain medium is pumped (electrically, optically, and/or by
other means) to achieve population inversion, and the “pump light” 111 (used herein
to refer to the output of the laser 110’ or other light emitting element 110) experiences cavity
effects and propagates through elements. In the example of FIG. 1B, a simple Fabry Perot ca
vity is shown with an asymmetry in the reflectivity, such that emission is favored on one s
ide. This embodiment illustrates a single pass of the pump light 111 through the SFG element,
generating SFG light (λ 2 ) along the way, but other embodiments may employ optical cavity 125
at either ( λ 0 or λ 2 or both ) to generate resonant enhancement of the SFG element. That is,
it is understood that this example is by way of illustration only, and may be used with other se
miconductor diode lasers including (but not limited to) edge emitters with cleaved facet end mir
rors, distributed feedback grating structures or photonic crystal structures for generati
ng the optical cavity 125. The example illustration may also represent a vertical cavity sur
face emitting laser (VCSEL) in which the gain is a multi‐quantum well (MQW) structure and the en
d mirrors are dielectric Bragg reflectors (DBRs). Optional active or passive elements, such
as optical amplifiers, mode converters, etc., may be provided at the output of the light emitting
element 110 to enhance performance in some embodiments. [0104] The input coupling element 115 is configured to coup
le the visible light 111’ from the light emitting element 110 into the nonlinear optical
element 120. The input coupling element 115 may be a continuous waveguide that connects radi
ation from the pump laser to the nonlinear optical crystal. Some embodiments may incl
ude optical coupling by non‐waveguide means, for example, free space propagation and focusi
ng with lenses; optical fibers; etc. That is, the input optical coupling element may be implem
ented by any optical element that is configured to relay the light output from the light
emitting element 110 to the nonlinear optical element 120. [0105] In some embodiments, the UV light source may be con
figured to provide phase matching between the second frequency ω 2 or wavelength λ 2 of the far‐UVC light 121’ generated by the nonlinear optical element 120 (also
referred to herein as the SHG or SFG wavelength or wavelength range) and first frequency
1 or wavelength λ 0 of the visible light 111’ generated by the light emitting element 110 (
also referred to herein as the fundamental or pump wavelength or wavelength range). For example, th
e phase matching may be provided by implementing the nonlinear optical element 120 as a
waveguide (which may or may not have optical resonance) that is configured such that the
speed of propagation modes supported at fundamental and SHG/SFG wavelengths are identical and
thus phase matched. However, in some embodiments other means of phase matching may b
e used, such as (but not limited to) periodically poled crystals (i.e., quasi phase matchin
g, type 0) or birefringence in the nonlinear crystal for type 1 or type 2 phase matching. In
other embodiments, the UV light source may be substantially free of phase matching (i.e., may be c
onfigured to provide the output light without phase matching methods or with relaxed phase matching
requirements to match propagation of the visible light 111’ of the first frequency
with the far‐UVC light 121’ of the second frequency). For example, some embodiments may provid
e distributed, selective outcoupling of the SFG/SHG light 121’ such that light of the sec
ond frequency does not propagate long distances in the waveguide and thus allows the devic
e to achieve satisfactory performance instead of or in the absence of phase matching. [0106] FIG. 1C is a graph illustrating an emission range
for light output from a UV light source according to some embodiments of the present disclosu
re. As shown in FIG. 1C, the output light 131 includes far‐UVC light 121’ having a w
avelength in a range of about 200 nm to 240 nm, for example, about 207 nm or 222 nm, and may or m
ay not include the visible light 111’. UV light sources in accordance with embodiments of the
present disclosure may lack requirements as to the nature or quality of the emitted SFG (e.
g., far‐UV) light. Any generation of photons at
the required wavelength in sufficient quantity to mee
t the purposes of the intended application may be used in embodiments as described herein. In
other words, some embodiments may operate without specific requirements on the beam qua
lity of the SFG light, or may not require a beam at all. The emitted SFG light can diverge,
can be unpolarized, and/or can scatter, Embodiments described herein thus span producing light
of all levels of quality (e.g., coherent or non‐coherent, collimated or non‐collimated) so
long as the wavelength is contained in a well‐ defined band (e.g., the far‐UV) based on second
(or higher) harmonic generation from a coherent pump laser. [0107] Referring again to FIG. 1A, embodiments of the prese
nt disclosure may monolithically integrate active optical components (such as the ligh
t emitting element 110) and passive optical components (such as nonlinear optical elements 120, l
ow loss optical waveguides, resonant cavities, optical couplers, etc.) onto a single chip
(e.g., a single PIC) that is configured to generate human‐safe far‐UVC light 121’, without
free‐space propagation of the light between elements or components. Monolithic integration may b
e advantageous by reducing losses in coupling of light from the active element 110 (e.g.,
laser 110’) to the passive element 120 (e.g. SHG/SFG element 120’). As used herein, a “die
or chip may refer to a small block or body of
semiconducting material or other substrate on which e
lements are fabricated, for example, to provide monolithic integration of active and passive
optical elements. In other embodiments, the active and passive optical elements may be respe
ctive elements that are arranged or assembled on a non‐native substrate. Such hybrid
integration may include embodiments where the light emitting element 110, the nonlinear
optical element 120, and/or other passive components are separately packaged or are on separate
dies, and optical coupling is implemented by fiber or free space propagation. [0108] In some embodiments, the output coupler(s) may be co
nfigured such that the desired wavelength of light (e.g., 220nm) is preferentially s
upported and outcoupled (rather than the fundamental frequency or wavelength of the laser 110
, e.g., 440nm), referred to herein as selective light extraction or selective outcoupling.
However, it will be understood that selective outcoupling of the far‐UVC light 121 does not requ
ire an absence of the undesired wavelengths of light (e.g., the pump light 111) in the output
light 131. For example, because the optical power of the visible light 111’ may be an order
of magnitude stronger than the SFG/SHG light 121’, even a 2x selective output of the SHG/SFG l
ight 121’ may provide output light 131’ that includes less SHG/SFG light 121’ than the visible
light 111’. The selective outcoupling of the far
UVC light 121 as the output light 131 may be provi
ded in one or more directions that differ from a direction of propagation of the pump light 1
11 in some embodiments. [0109] While described in specific embodiments herein with r
eference to lasers that emit light 111’ at 440 nm and output couplers that output li
ght 121’ at 220 nm, it will be understood that such specific emission wavelengths are mentioned by w
ay of example only, and that any of the embodiments described herein (and/or components thereof
) may be configured to emit or operate using other emission wavelengths such that th
e overall light output includes light in the far‐UVC wavelength range. Likewise, while described
primarily below with reference to example implementations of the nonlinear optical eleme
nt 120 for second harmonic generation (referred to as an SHG element), it will be underst
ood that, in any of the embodiments described and illustrated herein, the SHG element may
be replaced by any nonlinear optical element 120 that is configured to generate light by
sum frequency generation (referred to as an SFG element). [0110] FIGS. 2A1 and 2A2 are schematic perspective and side
views, respectively, illustrating elements of a UV light source 200a in a vertical l
inear arrangement (e.g., a Vertical External Cavity Surface Emitting Laser (VECSEL)) according to
some embodiments of the present disclosure. FIGS. 2B1 and 2B2 are schematic perspec
tive and top views, respectively, illustrating elements of a UV light source 200b in
a horizontal linear arrangement according to some embodiments of the present disclosure. [0111] The linear UV light sources 200a, 200b (in both the
vertical arrangement shown in FIGS. 2A1 and 2A2 and the horizontal arrangement shown in
FIGS. 2B1 and 2B2) include the light emitting element 110 and the nonlinear optical elemen
t 120 integrated in a monolithic structure 190. In particular, the linear UV light
sources includes a light emitting element 110 (e.g., a laser diode 110’ including a gain materia
l configured to generate light output in the blue part of the visible spectrum with a wavelength of a
bout 440 nm), an input coupling element 115, an optional semiconductor optical amplifier (SOA)
with gain at about 440 nm, a monolithically integrated nonlinear optical element 120
(e.g., an AlN waveguide with nonlinear optical properties configured for Second Harmonic Gene
ration of frequency doubled light in the far UV part of the visible spectrum near 220 nm),
and two facets 129‐1, 129‐2. The first facet 129‐1 has high reflectivity for both approximately
440 and approximately 220 nm light, and the second facet 129‐2 has higher reflectivity for 440
nm light and lower reflectivity for 220 nm light to provide selective outcoupling of the far‐U
VC light 121’. [0112] FIGS. 2C1 and 2C2 are schematic top views illustrati
ng elements of UV light sources in a spiral arrangement according to some embodiments of t
he present disclosure. The spiral UV light sources 200c‐1, 200c‐2may integrate the nonl
inear optical element 120 (and in some embodiments, the output coupling element 130) in a s
piral‐shaped waveguide 220, which may be adjacent (in FIG. 2C1) or extend around (in FIG.
2C2) the light emitting element 110 (e.g., a laser diode 110’ including optical resonators 1105
configured to generate visible light 111’ with a wavelength of about 440 nm). The embodiments of
FIGS. 2C1 and 2C2 may allow for increasing optical length substantially while maintaini
ng a relatively small footprint. While bending of the waveguide may increase radiative losse
s, this may be advantageous in some embodiments, particularly for embodiments in which the
far‐UVC light 121’ can be continuously or quasi‐continuously outcoupled (also referred to h
erein as distributed emission) along a length of the output element, which may avoid or mi
tigate phase matching requirements. [0113] FIGS. 3A1 and 3A2 are schematic block diagrams illus
trating elements of UV light sources 300a‐1, 300a‐2 including optical cavity en
hancement according to some embodiments of the present disclosure. In the examples of FIGS
. 3A1 and 3A2, the output coupling element 130 is configured to receive the far‐UVC light 121
’ from an optical cavity 125 that is at least partially resonant at the first frequency. For exam
ple, the optical cavity 125 may be optically resonant at the first (fundamental) wavelength/frequenc
y of the visible light 111’, the second (e.g., harmonic) wavelength/frequency of the far‐UVC
light 121’, or both. The optical cavity 125 may include or may surround the nonlinear optical el
ement 120 (e.g., separate from the lasing cavity 105 of the laser 110’) in some embodiments.
By providing the non‐linear crystal inside an optical cavity 125, the efficiency of conversion
can be increased by providing for many passes of the pump light 111 through the nonlinear
crystal. In this way the optical cavity 125 acts to optically “lengthen” the nonlinear optical
element 120 (with respect to distance of light propagation) beyond the physical dimensions of the no
nlinear optical element 120. Alternatively, cavity enhancement can be considered as
recycling the pump light 111 over multiple passes. Furthermore, the optical cavity 125
may greatly increase the pump light field intensity in accordance with the quality (Q) factor
of the cavity 125. Because the efficiency of nonlinear wavelength conversion (e.g., SFG/SHG) may de
pend monotonically on pump field intensity, the use of an optical cavity 125 can imp
rove SFG/SHG efficiency for each of the passes. [0114] In the example of FIG. 3A1, the nonlinear optical
element 120 (or the optical cavity 125 in which the nonlinear optical element 120 is provid
ed) is shown as resonant at the pump wavelength, and is non‐resonant (or has high loss,
i.e., emission) at the SHG/SFG (e.g., far‐UV) wavelengths, also referred to herein as a singly res
onant configuration. In a singly resonant configuration, the pump wavelength is resonant such t
hat its intensity will “build” (i.e., increase) in the optical cavity 125, while the SHG/S
FG wavelength will not build up substantially because the optical cavity 125 is not resonant at t
he SHG/SFG wavelength. While not wishing to be bound by theory, in such a singly resonant c
onfiguration, requirements for phase matching may be diminished or relaxed. Phase matchi
ng may be sufficient to build the intensity of the SHG/SFG wavelength over a single pa
ss over the length of the nonlinear optical element 120. [0115] In the example of FIG. 3A2, the nonlinear optical e
lement 120 (or the optical cavity 125 in which the nonlinear optical element 120 is provid
ed) is shown as resonant at both the pump wavelength, and at the SHG/SFG (e.g., far‐UV) wavel
engths, also referred to herein as a doubly resonant configuration. If both the pump and the S
HG/SFG wavelengths are resonant with the nonlinear optical element 120, then both pump and SH
G/SFG intensities may build up in the optical cavity 125, and phase matching requirements m
ay be stricter. Indeed, doubly resonant operation may not be possible without realizing phase
matching between the two wavelengths. Between each pass, the SHG/SFG light 121’ that is
generated may be (partially or totally) coupled out of the optical cavity 125. The optical
cavity 125 may be configured to couple in the maximum amount of pump laser light 111, while preven
ting the pump light 111 from leaking out on each pass. [0116] More generally, embodiments of the present disclosure
include implementations that are singly or doubly resonant. Although the figures
may show gaps between elements (e.g., in FIGS. 1B, 3A1, 3A2), it will be understood that the
illustration of such gaps may merely be provided to distinguish functional components from one
another, and does not imply that the light necessarily propagates through free space betwee
n components or elements. For example, as noted above, embodiments with monolithic
integration of components may have no free‐space light propagation between components,
while embodiments with hybrid integration may involve some free‐space light propag
ation. [0117] FIGS. 3B1 and 3B2 are schematic perspective and top
views, respectively, illustrating elements of a UV light source 300b including optical
cavity enhancement according to some embodiments of the present disclosure. FIGS. 3B1 an
d 3B2 illustrate an implementation of the elements shown in FIG. 3A1 on a single chip, in a
linear, horizontal geometry. [0118] In FIGS. 3B1 and 3B2, the nonlinear optical element
120 is implemented as an AlN waveguide, and is provided in an optical cavity 125
indicated by Facet#1 129‐1 and Facet#2 129‐2. This optical cavity 125 recycles the pump
light 111 in order to enhance the nonlinear effect of SHG/SFG, and is configured to avoid feedin
g pump light 111 back to the original pump laser diode 110’ in too high of a quantity. The
output coupling element 130 is implemented as a grating on a portion (e.g., some or all) of the
nonlinear optical element 120. The grating is of
a diffraction order that is configured to selectively
outcouple the SHG/SFG (e.g., far‐UV) light out with an efficiency that may be optimized for ov
erall performance. High levels of outcoupling of the far‐UVC light 121’ would provi
de a singly resonant cavity (resonant only at the pump wavelength), while low levels of output cou
pling of the far‐UVC light 121’ would provide in a doubly resonant cavity (i.e., resonant
at both pump and SHG/SFG wavelength). As noted above, doubly resonant designs may have stricte
r requirements on phase matching and component design. [0119] The grating shown in FIGS. 3B1 and 3B2 is one of
many possible implementations of an output coupling element 130 that is configured to se
lectively couple the SHG/SFG light 121’ out of the nonlinear optical element 120. For example,
the output coupling element 130 may be or may include at least one of a facet having a refra
ctive index that is configured to selectively outcouple the far‐UVC light 121’ in a first dire
ction corresponding to a direction of propagation thereof, or a grating having a diffraction order t
hat is configured to selectively outcouple the far‐UVC light 121’ in a second direction, differe
nt than the first direction. The second direction may be orthogonal to the first direction of propagat
ion of the visible light 111’ from the light emitting element 110. For example, the grating or
facet may be configured to outcouple the far‐UVC light 121’ in a direction that is normal
to a surface of a substrate 101 (native or non‐
native) having the UV light source thereon. [0120] FIG. 4A is a schematic block diagram illustrating el
ements of a UV light source 400a including a nonlinear optical element 120 in a ring
cavity configuration according to some embodiments of the present disclosure. This is anot
her variation of a design in which the nonlinear optical element 120 employs resonant effects
, using ring shaped optical cavities instead of linear cavities, to enhance SHG/SFG, shown
in plan view. [0121] As shown in FIG. 4A, the nonlinear optical element
(s) 120 may be implemented in a ring configuration that defines the optical cavity 125, ra
ther than being provided in an optical cavity 125. The ring cavity may be a circle, as shown f
or simplicity, or may be an oval, an ellipse, or other ring or closed shape. The nonlinear optical
element 120 may be an AlN‐based ring cavity in some embodiments. Pump light 111 of a first fr
equency ω 1 is coupled from the input coupling element 115 (shown as in input waveguide) p
artially or fully into the optical cavity 125 defined by the nonlinear optical element 120. The
SHG/SFG light 121’ is coupled out of the nonlinear optical element 120 (selectively) into the
output coupling element 130 (shown as including an output waveguide). The higher the coup
ling of SHG/SFG light 121’ out of the optical cavity 125, the lower the quality factor (Q)
of the cavity and thus the closer the cavity is
to being singly resonant. More generally, the opt
ical cavity 125 may be at least partially resonant at both the pump and the SHG/SFG wavelength
s. [0122] In some embodiments, multiple nonlinear optical elemen
ts 120 (e.g., crystals or cavities) may be provided per pump laser 110’, each with a
respective output coupling element 130. For example, as the input coupling element 115 may not
transfer 100% of the pump light 111’ to the first SHG element, multiple different SHG/SFG ele
ments 120’ or cavities may be arranged to receive light from a common input coupling element 1
15, and thus, to be pumped by a single pump laser 110’. That is, because the input coup
ling element 115 may be imperfect, any light that does not couple into the first SHG/SFG element
120’ or cavity may be provided to the next or subsequent SHG/SFG element 120’ or cavity. As
such, each subsequent SHG/SFG element 120’ or cavity may receive “leftover” light tha
t was not coupled into the previous SHG/SFG element 120’ or cavity. Further embodiments may u
se a photonic integrated circuit (PIC) to divide the pump light 111 prior to distribution acro
ss the various SHG/SFG elements 120’ or optical cavities 125. [0123] FIG. 4B is a schematic top view illustrating element
s of a UV light source 400b including a plurality of nonlinear optical elements 120 in rin
g cavity configurations that are sequentially arranged according to some embodiments of the present
disclosure. As shown in FIG. 4B, the UV light source 400b includes an input coupling elem
ent 115 (shown as a waveguide) that is configured to receive the visible light 111’ from
the light emitting element 110, with a plurality of nonlinear optical elements 120 sequentially arrange
d along the waveguide to receive the visible light 111’ from the light emitting element
110. [0124] As noted above, coupling from the waveguide to a no
nlinear optical element 120 may be less than 100% efficient. In fact, increasing Q
of the ring cavity may demands that the coupling ratio be restrained from being too large.
Understood differently, a larger coupling ratio may mean that a large fraction of the pump l
ight 111 could leak from the ring back into the waveguide with every cycle around the ring cavit
y. As such, it may be advantageous to provide a plurality of nonlinear optical elements 120
along the length of a waveguide, each of which” taps” pump light 111 that was not coupled
into the previous nonlinear optical element 120 in the arrangement sequence. In particular, F
IG. 4B illustrates a Photonic Integrated Circuit (PIC) in which a light emitting element 110
(e.g., a blue (approximately 440 nm) single mode laser 110’) is integrated with a linear (e.g.
, AlN) waveguide as the input coupling element 115. The input coupling element 115 is coupled to
one or more nonlinear optical elements 120 (e.g., AlN ring resonators) each having a respective
optical cavity 125, which generate SHG/SFG light 121’ in the far‐UVC (approximately 220 nm
) wavelength range. The far‐UVC light 121’ from each nonlinear optical element 120 is then coup
led into a respective linear (e.g., AlN) waveguide, each of which includes either a low refle
ctivity (with respect to the far‐UVC light 121’) exit facet or a grating as an output coupli
ng element 130. [0125] FIG. 4B thus illustrates (i) branching or splitting
of the pump light 111 into a plurality of SHG/SFG elements 120’, and (ii) configuring the d
esign of each subsequent SHG/SFG element 120’ to improve or optimize overall device performa
nce (as also shown in further detail in FIG. 14). As also shown in FIG. 4B, the nonlinear opti
cal elements 120 may not necessarily be identical to one another. For example, some embodim
ents may provide different coupling ratios for subsequent SHG/SFG ring cavities, as indic
ated by the final SHG/SFG ring (on the far right side of FIG. 4B) having different geometry.
More generally, respective ones of the nonlinear optical elements 120 may have different dim
ensions and/or even different materials, and the output coupling element 130 may include a p
lurality of output coupling elements 130 that are respectively configured to selectively outcou
ple the far‐UVC light 121’ as output light 131’ from the respective ones of the nonlinear opt
ical elements 120. [0126] FIG 4C1 is a schematic top view illustrating element
s of a UV light source 400c including a nonlinear optical element 120 coupled to an intra
cavity portion 105i of the light emitting element 110 according to some embodiments of the pre
sent disclosure. FIG 4C2 is a schematic top view illustrating an array 499 of UV light sour
ces 400c that respectively include a nonlinear optical element 120 coupled to an intra‐cavity port
ion 105i of a light emitting element 110 according to some embodiments of the present disclosu
re. [0127] As shown in FIGS. 4C1 and 4C2, a (ring) cavity res
onant SHG/SFG element 120’ is coupled to a pump laser 110’ on the same chip or
substrate 101 (e.g., a native substrate) in FIG. 4C1, with multiple chips in an array 499 on a subs
trate 101 (e.g., a native or non‐native substrate) in FIG. 4C2. Each nonlinear optical elem
ent 120 is arranged and configured to receive visible light 111’ from an intra‐cavity p
ortion 105i between first and second ends of a respective lasing cavity 105 (also referred to herein
as an “intra‐cavity‐tap” configuration), in contrast to the configurations shown in previous embo
diments where the nonlinear optical element 120(s) are arranged to receive light output
from an end of the lasing cavity 105 (also referred to herein as “external cavity‐tap” conf
igurations). That is, the terms intra‐cavity coupling or tapping and external‐cavity coupling or
tapping may be used herein to differentiate between relative positions of the nonlinear optical e
lements 120 with respect to the light emitting element 110, for light coupling into the no
nlinear optical elements 120. [0128] As shown in FIG. 4C1, in the intra‐cavity‐tap co
nfiguration, the light output from the laser 110’ or other light emitting element 110 may
only traverse one interface to be input to the optical cavity 125 of the nonlinear optical elem
ent 120, and thus, relatively high intensity intra‐cavity light 111’ may be in‐coupled to th
e nonlinear optical element 120. In the external cavity‐tap configuration, the light output from the
laser 110’ or other light emitting element 110 must pass through the end of the lasing cavity
105 (or other optical interface of the light emitting element 110), and then across a waveguide o
r other input coupling element 115 to be input to the optical cavity 125 of the nonlinear op
tical element 120. Because at least two optical interfaces between elements may be present in
the external cavity‐tap configuration (e.g., a waveguide having respective interfaces with
the lasing cavity 105 and the nonlinear optical element 120), the light input to the nonline
ar optical element 120 may be of lower intensity comparison to the intra‐cavity‐tap config
uration. Also, in the external cavity‐tap configuration, the light being tapped by the nonlinea
r optical element 120 propagates in a single direction (the direction of output from the l
aser 110’) relative to the nonlinear optical element 120. However, in the intra‐cavity‐tap co
nfiguration, the light propagates in two directions (between opposing ends of the lasing cavit
y 105), as indicated by the dual pointed arrows. [0129] FIG. 4C3 is a graph illustrating vernier frequency s
election for determining optical cavity 125 size (including height, width, and circumference/l
ength) of a ring‐shaped nonlinear optical element 120 according to some embodiments of the pre
sent disclosure. As shown in FIG. 4C3, the size of the optical cavity 125 may be tuned to
correspond to a free spectral range (FSR) with resonances that (only) match specific modes of the p
ump laser 110’. By increasing the FSR beyond the spectral width of the gain of the laser,
it may thus be possible for the nonlinear optical element 120 to modify the operation of the
pump laser 110’, and thereby force more (or up to all) of its intra‐cavity power to frequ
encies that are relevant to the SHG/SFG cavity, thereby increasing efficiency of the system. For
example, providing an SHG/SFG ring 120’ at the edge of the lasing cavity 105 of an otherwise
multimode laser 110’ may modify the laser 110’ into single mode operation. [0130] While the intra‐cavity‐tap configuration is describ
ed herein with respect to far‐UVC light 121’ generation where high SHG/SFG efficiency may b
e paramount, it may be used for light generation at other wavelengths. Also, it will be
understood that, although the nonlinear optical element 120 is coupled to the interior of t
he lasing cavity 105 in FIGS. 4C1 and 4C2, the SHG/SFG optical cavity 125 (ring) is a distinct opti
cal cavity 125 from the lasing cavity 105. In other words, the embodiments of FIGS. 4C1 and 4C2 i
llustrate two distinct or separate cavities per UV light source‐‐the lasing cavity 105 of th
e pump laser 110’, and the optical cavity 125 of
the nonlinear optical element 120. [0131] FIG. 5A is a schematic block diagram illustrating el
ements of a UV light source 500a in which the light emitting element 110 and the nonline
ar optical element 120 are provided in a same or shared optical cavity 125 according to som
e embodiments of the present disclosure. That is, the nonlinear optical element 120 may be i
ntegrated into the same, single cavity as the gain material of the pump laser 110’, such that t
he cavity of the laser 110’ (i.e., the lasing cav
ity 105) or other light emitting element 110 is shared
with the nonlinear optical element 120 that provides SHG/SFG. It will be understood that the
shared optical cavity 125 configuration (also referred to herein as an intra‐cavity‐SHG/SFG conf
iguration) shown in FIG. 5A is distinct from the “intra‐cavity‐tap” configuration where the
nonlinear optical element 120 is coupled to the interior portion of the lasing cavity 105, but the
optical cavity 125 of the nonlinear optical element 120 is distinct from the lasing cavity 105.
[0132] As shown in FIG. 5A, the shared optical cavity 12
5 may only be resonant at the pump wavelength, i.e., in a singly resonant configuration.
There may be no need for gain at the SHG/SFG wavelength, as it may be impossible for the
SHG/SFG wavelength to pass through the gain material (as the gain material is likely to be
absorbing and not transparent at the SHG/SFG wavelength). That is, the shared optical cavity 1
25 need not be doubly resonant at both the first (pump) and second (SHG/SFG) frequencies, as the
second harmonic frequency may be absorbed by the gain region and thus may not build
up intensity by optical resonance. Instead, the shared optical cavity 125 may provide the SHG/SF
G light 121’ to an output coupling element 130 that is configured to selectively outcoup
le the SHG/SFG photons as output light 131’ in a manner similar to that shown and descri
bed in other embodiments herein. [0133] Advantages of this approach may include allowing the
optical field of the fundamental (pump) wavelength to be far higher than that output
from the light emitting element 110. The shared optical cavity 125 may be designed or otherwi
se configured with as high Q as possible (e.g., with a reflectivity of the output coupling el
ement 130 of up to about 100%) at the pump wavelength in order to increase or maximize intracavi
ty field strength. However, in some embodiments the Q may be reduced (e.g., the output
coupling element 130 may have less than 100% reflectivity at the pump wavelength) in order t
o couple out some fraction of the pump wavelength for other purposes. [0134] FIGS. 5B1 and 5B2 are schematic perspective and top
views, respectively, illustrating elements of a UV light source 500b in a same or s
hared optical cavity 125 with an output coupling element 130 implemented as a reflective face
t configured for selective light extraction according to some embodiments of the present disclosu
re. In particular, FIGS. 5B1 and 5B2 illustrate an implementation of the elements shown in
FIG. 5A on a single chip, in a linear, horizontal geometry. [0135] As shown in FIGS. 5B1 and 5B2, the UV light sour
ce 500b includes a light emitting element 110 implemented as a laser diode 110’ conf
igured to generate light in the blue part of the visible spectrum near about 440 nm, a nonlinear
optical element 120 (monolithically integrated with the light emitting element 110) imple
mented as an AlN waveguide 120’ with nonlinear optical properties configured for generation
of frequency doubled light in the far‐UVC part of the visible spectrum near about 220 nm, a
first facet with high reflectivity for both approximately 440 nm and approximately 220 nm light,
and an output coupling element 130 implemented as a second facet with higher reflectivit
y for 440 nm light and lower reflectivity for 220 nm light (to provide selective outcoupling o
f the far‐UVC light 121’), in a horizontal linear arrangement. The first and second facets def
ine the shared optical cavity 125, which is resonant with respect to the 440 nm light (i.e., si
ngly resonant). An input coupling element 115 with low reflectivity for both approximately 440 nm
and approximately 220 nm light is provided between the light emitting element 110 and the nonli
near optical element 120. Optionally, semiconductor optical amplifier (SOA) with gain at ap
proximately 440 nm may amplify the output of the light emitting element 110 and provide
the resulting light to the input of the nonlinear optical element 120 in some embodiments. [0136] FIGS. 5C1 and 5C2 are schematic perspective and top
views, respectively, illustrating elements of a UV light source 500c in a same or s
hared optical cavity 125 with an output coupling element 130 implemented as an optical gratin
g configured for selective light extraction in a horizontal linear arrangement accordin
g to some embodiments of the present disclosure. The UV light source 500c may be simila
r to the UV light source 500b of FIGS. 5B1 and 5B2, but includes additional elements that may b
e used to implement a shared or an intra‐ cavity‐SHG/SFG configuration on a single chip, in a
linear, horizontal geometry. [0137] As shown in FIGS. 5C1 and 5C2, the UV light source
500c includes a light emitting element 110 (e.g., a laser diode 110’ configured t
o generate light in the blue part of the visible spectrum near about 440 nm), a nonlinear optical ele
ment 120 (e.g., an AlN waveguide 120’ with nonlinear optical properties monolithically integr
ated with the light emitting element 110 and configured for generation of frequency doubled li
ght in the far‐UVC part of the visible spectrum near about 220 nm), and a (optional) semico
nductor optical amplifier (with gain at approximately 440 nm) therebetween. A first facet (
with high reflectivity for both approximately 440 nm and 220 nm light) and a second
facet (with high reflectivity for at least the approximately 440 nm light, and in some embodime
nts for both the approximately 440 nm and 220 nm light) define the shared optical cavity
125, which is resonant with respect to at least at the 440 nm light (i.e., singly or doubly
resonant). The output coupling element 130 is implemented as second (or other order) grating, whi
ch is configured to selectively outcouple the 220nm light from the optical cavity 125, while
the 440 nm light (i.e., the pump wavelength) is highly contained. [0138] Further embodiments of the present disclosure may pro
vide both the nonlinear optical element 120 and the light emitting element 110 (e.g.
, the laser gain medium) inside the same or shared optical cavity 125, with the optical cavit
y 125 having a ring or other closed curve shape (also referred to herein as a “racetrack”
configuration, including non‐rotationally symmetric closed loops of any shape), with light pro
pagation in one or more directions (e.g., a single direction, or in opposite directions). [0139] FIG. 6A is a schematic block diagram illustrating el
ements of a UV light source 600a in which the light emitting element 110 and the nonline
ar optical element 120 are provided in a same or shared optical cavity 125 having a closed l
oop or racetrack (e.g., a rectangle with a semicircle at each end) configuration according to so
me embodiments of the present disclosure. [0140] As shown in FIG. 6A, the optical cavity 125 has a
closed curve shape that optically couples the laser gain medium and the SHG/SFG elemen
t 120’ therein, e.g., by curved waveguides with no reflective facets. The SHG/SFG
wavelength may be selectively outcoupled either as it is generated (e.g., along a length of
a portion of the optical cavity 125, also referred
to herein as distributed emission), or after it is
generated. This may be achieved, for example, by implementing the output coupling element 130 as a
partially reflecting mirror or facet, a distributed Bragg reflector (DBR), or a second order
diffraction grating to generate surface emission (e.g., in a direction orthogonal to the dir
ection(s) of light propagation around the closed loop). The fundamental wavelength, meanwhile,
can be confined completely within the optical cavity 125 with increased or maximum possible
quality factor (Q). [0141] FIGS. 6B1 and 6B2 are schematic perspective and top
views, respectively, illustrating elements of a UV light source 600b in a same or s
hared optical cavity 125 having a closed loop or racetrack configuration with an output coupling el
ement 130 implemented as an optical grating configured for selective light extraction acco
rding to some embodiments of the present disclosure. As shown in FIGS. 6B1 and 6B2, the UV
light source 600b includes a light emitting element 110 (e.g., a GaN laser diode 110’ configur
ed to generate light in the blue part of the visible spectrum near about 440 nm), and a nonlinear
optical element 120 (e.g., an AlN waveguide with nonlinear optical properties monolithica
lly integrated with the light emitting element 110 and configured for generation of frequenc
y doubled light in the far‐UVC part of the visible spectrum near about 220 nm), with curved
waveguides (e.g., AlN/GaN waveguides) that optically couple respective ends of the light e
mitting element 110 and the nonlinear optical element 120, with no reflective facets therebetween.
Avoiding or eliminating the use of reflective facets may allow for higher instantaneous
pulse intensity, by avoiding limits associated with catastrophic mirror damage. [0142] The output coupling element 130 is implemented as
second (or other order) grating, which is configured to selectively outcouple the 220n
m light from the optical cavity 125. For example, the output coupling element 130 may include
optical structures having a grating pitch that is configured based on the wavelength of the l
ight to be outcoupled. The output coupling element 130 may be configured to direct the SHG/SFG
wavelengths in a direction orthogonal to or otherwise out of a plane defined by the directio
n(s) of light propagation around the closed loop forming a surface emitting device), while the p
ump wavelength continues to propagate in the closed loop defined by the optical cavity 125.
In some embodiments, the output coupling element 130 may also be a nonlinear optical element
120 (e.g., an AlN element with optical structures at a desired grating pitch), so as to se
lectively outcouple the far‐UVC light 121’ as it
is generated. In some embodiments, the optical cavity
125 may further include a section that is configured to form a saturable absorber 1305 that is
configured to generate pulses of light of at the pump wavelength. [0143] In particular, while some embodiments have been descr
ibed by way of example with reference to continuous wave (CW) operation of the p
ump laser 110’, some embodiments may operate the pump laser 110’ in a pulsed mode, whi
ch can permit the operation of the devices at higher field intensities than CW. That is, the lig
ht emitting elements 110 in any of the embodiments described herein may be a laser diode 11
0’ that is configured to be driven in a continuous or pulsed manner. [0144] FIGS. 7A, 7B, and 7C illustrate example drive signal
s configured to provide pulsed light output from light emitting elements 110 in a UV lig
ht source according to some embodiments of the present disclosure. Referring to FIGS. 7A t
o 7C, SHG/SFG conversion may be higher when the (instantaneous) pump power is higher. Therefore t
he overall efficiency can be improved by maximizing the peak/average ratio of the pump laser
110’, in other words, by operating the pump laser 110’ to provide pulsed light output.
[0145] As shown in FIG. 7A, one method of obtaining pulsed
light output is by direct modulation, that is, by providing a pulsed drive sig
nal to the pump laser 110’, thereby activating and deactivating the laser diode 110’ to emit ligh
t for a desired pulse repetition frequency (PRF) and duration or duty factor (DF), e.g., with a puls
e width of about 1 to 5 ns, with higher intensity or pulse output power than would be accept
able if operated continuously. That is, some embodiments may realize pulsed operation by use
of an electrical drive circuit that provides short, high current pulses to a diode laser
110’. In this way, the laser 110’ may be “quasi CW” during the short period of high curre
nt application, but the low duty factor of the drive current may allow for higher transient operatio
n powers. [0146] As shown in FIG. 7B, another method of obtaining pu
lsed light output is to design or configure the laser diode 110’ to generate pulsed
output even when driven continuously. One method to achieve this is by configuring the pump w
avelength to be passively (or actively) mode locked, for example, by controlling the PRF and
the DF of the laser diode 110’ based on the optical cavity 125 length Lcavity. FIG. 7B ill
ustrates that a pulse “train” with pulse widths
of as short as about 1 ps (or less) may achieved, wit
h high pulse repetition frequency (e.g., c/Lcavity) to provide higher intensity or output puls
e power than may be achieved by CW. For example, some embodiments may provide a saturable abs
orber 1305 inside the lasing cavity 105 to achieve passive mode‐locking. Some embodime
nts may realize pulsed operation by active Q‐switching. [0147] As shown in FIG. 7C, the above or other pulsing st
rategies may be combined to realize even higher peak pulse powers and thus higher nonlin
ear conversion efficiency. In particular, FIG. 7C illustrates drive signals to provide a pulse
d light output based on a combination of the methods shown in FIG. 7A (direct modulation) and FIG
. 7B (continuous modulation), e.g., by using an electrical drive circuit to activate and de
activate the laser diode 110’ to emit light with
a desired or predetermined PRF and DF, in combinatio
n with a saturable absorber 1305 in the lasing cavity 105 to provide the pulse widths as sh
ort as about 1 ps (or less). Also, while illustrated herein with reference to particular lasing
cavities, it will be understood that other types of lasers may be used in any of the embodime
nts described herein. For example, a diffraction grating may be provided at ends of the
laser gain medium to form a Distributed Feedback Laser (DFB) emitting in a single longitudina
l mode. In some embodiments, a diffraction grating may be provided in a waveguide s
eparate from the laser gain region forming a Distributed Bragg Reflector (DBR) laser emitting in
a single longitudinal mode. More generally, embodiments of the present disclosure may
use multiple operating methods (e.g., continuous wave and/or pulsed, by various methods or
combinations thereof) and laser configurations (e.g., DFB, DBR) for the light emittin
g device, some of which may achieve higher SHG/SFG efficiency. [0148] In order to monolithically integrate the light emitti
ng element 110 with the nonlinear optical element 120, embodiments of the present discl
osure may utilize various fabrication techniques to combine different materials. For examp
le, some embodiments may utilize heterogeneous integration methods, such as microtransfe
r printing (MTP), to couple the laser 110’ and the nonlinear crystal if both are microsc
opic in size (e.g., with dimensions of about 0.5 µm to about 1000 µm). Microfabrication technique
s may allow direct, end‐to‐end coupling of two optical components without the use of extra opti
cal elements. [0149] In some embodiments, as an alternative to micro asse
mbly for the monolithic integration of two material sets into a single waveg
uide, epitaxial regrowth may be used on top of an existing waveguide that has been appropriately
patterned. An example of such a concept is shown in FIGS. 2A1 and 2A2, where the gain mate
rial of the laser diode 110’ (e.g., GaN or other group III‐nitride material) is formed, a sect
ion of the GaN is patterned and etched away, and an AlN layer or other nonlinear optical element
120 material is grown (e.g. by MOCVD, or MBE) such that a high quality optical interface is
realized and the waveguide material changes without modifying the physical cross‐section dimensio
ns of the waveguide. In some embodiments, some or all elements of UV light source
s described herein (e.g., the light emitting element 110, the nonlinear optical element 120, the
output coupling element 130, optical cavities 125, and one or more waveguides therebetween
) may be nitride‐based materials. [0150] Further embodiments may use MTP, pick and place, or
other assembly techniques to arrange distinct active and passive optical elements
on a non‐native substrate, also referred to herein as hybrid integration. For example, respectiv
e light emitting elements 110 of one material may be formed and optically coupled to nonl
inear optical elements 120 of a different material or material system on a non‐native substra
te, which is different from the source substrate of either the light emitting element 110 o
r the nonlinear optical element 120. [0151] FIG. 8 is a schematic perspective view illustrating
respective elements of a UV light source 800 arranged in a structure or configuration
on a common non‐native substrate 801 according to some embodiments of the present disclosu
re. As shown in FIG. 8, the UV light source 800 may be implemented as a Photonic Integrat
ed Circuit (PIC) that includes respective elements as described herein as discrete components t
hat are assembled onto a common substrate 801. In particular, the light emitting el
ement 110 (e.g., a laser diode 110’ including a gain material configured to generate light output in
the blue part of the visible spectrum with a wavelength of about 440 nm), the nonlinear optical e
lement 120 (e.g., an AlN, BBO, or lithium niobate‐based waveguide with nonlinear optical proper
ties configured for Second Harmonic Generation of frequency doubled light in the far UV
part of the visible spectrum near 220 nm), and the output coupling element 130 (e.g., a facet
with higher reflectivity for 440 nm light and lower reflectivity for 220 nm light to provide selec
tive outcoupling of the far‐UVC light 121’) may be fabricated as respective elements and arranged
on a non‐native substrate 801. An optional semiconductor optical amplifier (SOA) with ga
in at about 440 nm may amplify the output of the light emitting element 110 and provide
the resulting light to the input of the nonlinear optical element 120 in some embodiments.
[0152] In the “hybrid” integration example of FIG. 8, t
he optical coupling between the respective elements may be implemented by optical fib
er or free‐space propagation (with air interfaces therebetween). However, the UV light sour
ce 800 may be implemented by combinations of any of the fabrication techniques des
cribed herein, including (but not limited to) epitaxial regrowth, wafer bonding, microtransfer p
rinting, pick and place, lithography, etc. It will be understood that, while illustrated herein
with reference to monolithic integration of particular embodiments, any of these embodiments may
be assembled using hybrid configurations as described herein. That is, any of
the embodiments described herein may be assembled as unitary structures (with no air interfac
es between components) or hybrid structures (with air interfaces between two or more
components). [0153] Similarly, it will be understood that embodiments of
the present disclosure may include various types of optical cavities 125 and feedback s
tructures that can provide the high quality factor Q for efficient operation. Examples of possi
ble optical microcavities may include, but are not limited to a linear Fabry Perot cavity including
of polished facet end mirrors, a linear Fabry Perot cavity including distributed (dielectric) Bragg
reflector end mirrors, a linear optical Fabry Perot cavity including distributed feedback gratings,
a linear optical cavity 125 including various photonic crystal designs, a ring cavity fabricated by
a waveguide that closes on itself, and a ring cavity fabricated by a round or elliptical 2D or 3D
disk structure. [0154] Also, while embodiments herein have been primarily de
scribed with reference to optical second harmonic generation (SHG) from a pump laser 1
10’ (e.g., from blue light of about 400 nm to about 480 nm) to produce light emission of a
bout 200 nm to about 240 nm, it will be understood that embodiments of the present disclosure
may also include implementations in which higher order harmonic generation (e.g., third,
fourth, and/or fifth order harmonic generation) is applied to the pump laser 110’ or
other light 111 (including light of wavelengths appropriately higher than 400 nm to 480 nm) to prod
uce output light 121 of 200 nm to 240 nm. That is, it will be understood that the second harm
onic generation or frequency doubled light as described herein may be more generally be referre
d to as sum frequency (including harmonically multiplied) light generation, with the no
nlinear optical element 120 implementing an optical frequency multiplier or other nonlinear fr
equency conversion device, in any of the embodiments described herein. [0155] For some applications, more power in the far‐UVC w
avelength range may be desired than can be generated by a single UV light source.
In such cases, multiple UV light sources (e.g., arranged in an array) may be provided on a common
substrate 101 (native or non‐native). [0156] FIG. 9 is a schematic block diagram illustrating an
array 900 of UV light sources 100 that respectively include a light emitting element 110 and
a nonlinear optical element 120 according to some embodiments of the present disclosure. As
shown in FIG. 9, an array 900 includes a plurality of the light emitting element 110 and the
nonlinear optical element 120 on a common substrate 901. For example, some embodiments may ut
ilize heterogeneous integration methods, such as microtransfer printing (MTP), to eff
iciently fabricate large arrays of similar or identical UV light sources 100 on a non‐native sub
strate 901 in order to increase total optical power output from a single package. In other emb
odiments, rather than singulation and packaging of each UV light source separately, and su
bsequently aggregating the UV light sources on a common (i.e., non‐native) substrate at
the system level, arrays of identical (or non‐identical) devices may be fabricated on the sam
e chip (i.e., on the same native substrate) before packaging the chip. While illustrated as a
n array 900 of UV light sources 100 similar to that shown in FIG. 1B, it will be understood that
any of the UV light sources described herein may be implemented in array form. [0157] FIGS. 10A and 10B are schematic perspective and top
views, respectively, illustrating elements of a UV light source 1000 including optical
cavity enhancement with an output coupling element 130 configured to provide distributed
emission and selective light extraction according to some embodiments of the present disclosu
re. As used herein, distributed emission may refer to a configuration of the nonline
ar optical element 120 and the output coupling element 130 in which light (e.g., the far
UVC light 121’) is continuously or semi‐ continuously extracted as it is generated at multiple
positions along the length of the component, rather than from one specific point or po
sition. The distributed emission may or may not be collimated, and may or may not be coher
ent. [0158] As shown in FIGS. 10A and 10B, the UV light source
1000 includes a light emitting element 110 (e.g., a laser diode 110’ configured
to generate light in the blue part of the visible
spectrum near about 440 nm), and nonlinear optical e
lement 120 (e.g., an AlN waveguide with nonlinear optical properties monolithically integrated
with the light emitting element 110 and configured for generation of frequency doubled light
in the far‐UVC part of the visible spectrum near about 220 nm). An optional SOA amplifies an
d provides the output of the light emitting element 110 to the input of the nonlinear optical e
lement 120. In the embodiment of FIGS. 10A and 10B, the nonlinear optical element 120 and the
output coupling element 130 are integrated into an output element 120/130 that is configured to
outcouple the far‐UVC light 121’ at a plurality of positions or continuously along a length
thereof. In the example of FIGS. 10A and 10B, the output element 120/130 includes a waveguide
material with alternating sections or regions, over which the SHG/SFG light 121’ is alte
rnatingly generated (by the SFG/SHG section) and outcoupled (from the output coupling section).
More generally, the output element 120/130 includes a plurality of nonlinear optical ele
ment 120 and output coupling element 130 regions (e.g., in a periodic or other alternating ar
rangement), which are configured to provide continuous or semi‐continuous extraction or distribut
ed emission of the SHG/SFG light 121’ along a length of the output element 120/130 (also
referred to herein as a nonlinear optical output coupling element 120/130). [0159] Output elements 120/130 with multiple integrated nonli
near optical and output coupling sections may be advantageous in that typical
requirements or constraints with respect to phase matching between the SHG/SFG wavelength and
the fundamental wavelength may be relaxed or may not be necessary. By semi‐continuo
usly extracting and recovering the SHG/SFG light 121’ from the output element 120/130, it may
be possible to relax or eliminate constraints associated with phase matching between the SHG/SFG li
ght 121’ and the pump light 111 inside the waveguide, as the SHG/SFG wavelength is not expe
cted to co‐propagate with the fundamental wavelength. That is, the SHG/SFG field
intensity is not expected to accumulate within the waveguide; rather, the SHG light that is
generated is outcoupled (in some fraction) to the outside world as it is generated. The output li
ght 131 may thus primarily include the SHG/SFG light 121’, and in some instances may be
substantially free of the pump light 111. [0160] As such, the UV light source 1000 is free of phase
matching (i.e., is not configured to match a first phase of the visible light 111’ wit
h a second phase of the far‐UVC light 121’).
It will be understood that, although shown in the examples o
f FIGS. 10A and 10B with reference to a UV light source 1000 in a horizontal linear arrangem
ent, output elements 120/130 including integrated nonlinear optical and output coupling eleme
nts 130 (e.g., arranged in an alternating manner along a length thereof) can be used in one
or more other embodiments described herein, for example, embodiments including closed curv
e/racetrack optical cavity 125 configurations, spiral‐shaped waveguides, or other ou
tput coupling element 130 configurations. [0161] Also, while the example of FIGS. 10A and 10B illust
rates the output element 120/130 as including second order gratings that selectively coupl
e the SHG/SFG light 121’ out of the waveguide while confining the pump light 111 within
the waveguide, embodiments of the present disclosure may include other types of output
coupling elements 130 having wavelength‐dependent optical characteristics configured
for distributed emission or (semi‐ )continuous extraction of the SHG/SFG light 121’.
Some further output element 120/130 configurations for selective extraction of the SHG/SFG
light 121’ include, but are not limited to, dielectric material interfaces or stacks configured fo
r wavelength dependent transmission, curves or tapers in waveguide geometry to obtain wav
elength dependent effects. [0162] For example, in some embodiments, the output element
120/130 may be configured to provide periodic poling of the SHG/SFG material (e.g.
AlN) to accomplish “quasi” phase matching. In particular, the output element 120/130
may include alternating regions of AlN, each with different heights (relative to a substrate
101) and surface roughness. To address poor performance as a waveguide at λ 0 , a capping layer that is index matched at λ
0 may be provided on top of the alternating AlN regions.
That is, the output element 120/130 may include a plurality of periodically poled nonlinear o
ptical sections of a first material, and an index‐matched capping layer of a second material th
at is different than that of the nonlinear optical sections. Phase matching at the SHG/SFG fre
quency (e.g., λ 0 /2) may not be needed, as the SHG/SFG light 121’ may be scattered out of th
e output element 120/130 with this configuration (such that the need for phase matching
may be relaxed or obviated). [0163] More generally, output elements 120/130 configured to
provide distributed emission as described herein may include any optical structures
(or combinations thereof) that are configured to confine the light of the fundamental w
avelength (λ 0 ) output from the light emitting element 110 and radiate or outcouple the li
ght of the SFG/SHG wavelengths (e.g., λ 0 /2 ). As one example, in some embodiments, the output
element 120/130 may be a waveguide that includes nanopores or defects therein, which hav
e dimensions (or bandgaps) configured to affect only the SFG/SHG wavelengths (e.g., λ 0 /2 ) while leaving the fundamental wavelength
substantially unaffected. That is, the output coupli
ng element 130 may be implemented as a waveguide that includes nanopores or defects having d
imensions configured to be index‐ mismatched at the second (SHG/SFG) frequency of light
, but to not substantially affect propagation of the first (fundamental) frequency of l
ight. [0164] For example, in some embodiments, the output element
120/130 may be a waveguide that includes or incorporates two (or more) different
materials, having respective optical indexes that are matched at the fundamental wavelengt
h λ 0 , but are mismatched at the SHG/SFG wavelengths (e.g., λ 0 /2), and roughened (e.g., at an interface betwe
en the materials) so that the light of the SHG/SFG wavelengths is sca
ttered out of the output element 120/130, while the light of the fundamental wavelength is con
fined therein (also referred to herein as a confined mode). [0165] Conversely, in some embodiments, the output element 1
20/130 may be a waveguide that includes or incorporates two (or more) different
materials, whereby the two materials are index matched at the SHG/SFG wavelengths (e.g., λ 0 /2 ) but mismatched at the fundamental wavelength λ 0 , such that the first material provides a conf
ined mode for the fundamental wavelength λ 0 while the SHG/SFG wavelengths can occupy modes
in the second (optically “thicker”) material. Roughening or other scatteri
ng structures may be provided at a top of the second material (e.g., opposite an interface with the
first material) such that the SHG/SFG wavelengths ( e.g., λ 0 /2 ) are preferentially scattered out of the w
aveguide, while the fundamental wavelength λ 0 remains confined in the higher index material.
That is, the output coupling element 130 may be implemented as a wavegui
de including first and second materials having relative dispersion curves configured such that
first and second optical indexes thereof are matched at one of the first (fundamental) and s
econd (SHG/SFG) frequencies, but mismatched at the other. [0166] As yet another example, in some embodiments, rather
than combining the nonlinear optical element 120 with the scattering or output co
upling element 130 over the length of the output element 120/130, the output element 120/130 ma
y be a waveguide that includes distinct or separate nonlinear optical element 120 an
d output coupling element 130 sections. The SHG/SFG sections 120’ may be relatively short
(along the direction of propagation of the fundamental wavelength light 111) so that phase match
ing may not be required or necessary over the length of the optical element. A relative
ly long output coupling section 130 is provided after (relative to direction of propagation of the f
undamental wavelength light 111) each SHG/SFG section 120’, and may include different fir
st and second materials that are configured to scatter or outcouple the light of the SHG/SFG wa
velengths ( e.g., λ 0 /2 ) out of the waveguide while confining the light of the fundamental waveleng
th (λ 0 ). The sequence of alternating SHG/SFG sections 120’ and output coupling sections
130 may be repeated (e.g., periodically) along the direction of propagation of the fundamental
wavelength light 111 in order increase SHG/SFG. Respective materials for the SHG/SFG sectio
ns 120’ and output coupling sections 130 may be selected such that the optical index at the
fundamental wavelength (λ 0 ) is matched and confined across all periods of the structure. That
is, at the SHG/SFG wavelengths (e.g., λ 0 /2), the output element 120/130 may alternate between SHG/
SFG sections 120’ and output coupling sections 130 to extract the SHG/SFG waveleng
ths of light, while at the fundamental wavelength (λ 0 ), the output element 120/130 may be continuous
and may confine the light of the fundamental wavelength at all locations along the
propagation direction. [0167] Some photonic Integrated Circuits (PIC) as described
herein may be based upon the GaInAlN material system, may be scalable to high vol
umes, and can leverage the extensive growth and fabrication infrastructure that has been d
eployed for the manufacture of white LEDs . The PICs described herein may be configured
to emit an engineered monochromatic output at one or more wavelengths of choice between
200 ‐ 240 nm, which in some embodiments can eliminate the use of or need for an
optical filter, which may provide significant cost savings. The light emitting element
110 may be a laser 110’ that emits light in the 400 ‐ 480 nm (blue/violet) wavelength range, a
nd the nonlinear optical element 120 (which may be implemented as an engineered waveguide) may s
um or double the frequency of the light input from the laser 110’ based on SFG or
SHG, such that far‐UVC light 121’ is generated a
t the desired wavelength. The light is then coupled ou
t of the chip, in an out of plane direction (e.g., substantially normal to its surface plane) sim
ilar to the emission of a Vertical Cavity Surface Emitting Laser (VCSEL). Furthermore, the power
output can be increased beyond what one device is capable of, simply by designing the P
IC with a monolithic array of devices on a single chip analogous to a VCSEL array. [0168] Referring again to FIGS. 4C1 to 4C3, an example PIC
architecture according to some embodiments of the present disclosure includes a ligh
t emitting element 110 implemented as a linear single frequency pump laser diode 110’ (in
this example, an AlGaN laser diode) coupled to a resonator nonlinear optical element 120 (which,
in the example of FIG. 4C1, is ring‐shaped and formed from AlN), which is coupled to an output
coupling element 130 implemented as a waveguide (in this example, AlN) for extracting the
far‐UVC light 121’. Ring‐shaped nonlinear optical elements 120 may also be referred to herein
as ring resonators. However, it will be understood that the nonlinear optical elements 120 ne
ed not be ring‐shaped, and other nonlinear optical element 120 designs may be used in
embodiments described herein. [0169] As shown in FIG. 4C2, each emitter of an emitter a
rray 499 includes a laser 110’ (e.g., configured to emit 440 nm light; which may more gen
erally be referred to herein as input light) that builds high internal optical intensity and coupl
es a fraction of its light into the neighboring nonlinear optical element 120(AlN ring resonator), whi
ch is resonant at this mode. The coupling (shown by arrows in FIG. 4C1, indicating di
rections of laser light 111 propagation) may be in‐plane (i.e., along the plan view or x‐y a
xis in the figures) or vertical (i.e., perpendicular
to or out of the page), and in some embodiments, acros
s a gap (e.g., on the order of microns) between the laser 110’ and the nonlinear optical e
lement 120. The positive symbol represents the electrical anode and the negative symbol represen
ts the electrical cathode of the laser 110’. Respective mirrors or other low‐loss reflective elem
ents may be provided at opposite ends of the laser 110’. [0170] In particular embodiments, the laser 110’ may be i
mplemented with an integrated waveguide or other integrated lasing cavity 105 (show
n as bidirectional) in some embodiments. The pump (e.g., 440 nm) light that is coupled into
the AlN nonlinear optical element 120’ (ring resonator) generates far‐UVC (e.g., 220 nm) light b
ecause of the nonlinear response of AlN. This second harmonic generation (SHG) or sum frequency gen
eration (SFG) process builds far‐UVC (e.g., 220 nm) light more efficiently because of the
high Q of the cavity at the frequency of the pump light. The far‐UVC (e.g., 220 nm) light is
coupled (selectively) out of the nonlinear optical element 120’ (ring resonator) and into a neighbor
ing output waveguide, likewise across a gap (e.g., on the order of microns) therebetween in some
embodiments. The output waveguide may be formed of, for example, AlN, SiO 2 , or other materials, and may or may not be
linear in some embodiments. For example, a ring resonator 120
’ may have separate nonlinear and (selective) output coupling sections in some embodimen
ts. The far‐UVC light (or, more generally, SHG/SFG light 121’) may be output by re
spective output coupling elements 130 at opposing ends of the output waveguide, as output lig
ht 131’ propagating in a direction perpendicular to or otherwise out of the page, to p
rovide surface emission (similar to a VCSEL). The output waveguide may be narrower than the lasing
cavity 105 (e.g., may be less than 100 nm in width) in some embodiments. [0171] It should be noted that 440 nm light can couple in
two directions: from the laser 110’ into the ring resonator (forward coupling) and in re
verse (reverse coupling). The reverse coupling can provide benefits: by coupling the two o
ptical cavities 125 (the lasing cavity 105 and the SHG/SFG cavity), it can be ensured that the
single longitudinal lasing mode of the laser 110’ is the correct mode needed for pumping the S
HG process. This can ensure that the only frequency that is supported is matched to both reson
ators, as illustrated by the graphs shown in FIG. 4C3. The top graph illustrates resonances
of the laser 110’ (including 440 nm light); the middle graph illustrates resonances of the ring reson
ator (including the 440 nm light and 220 nm higher order light resulting from the second harm
onic generation or frequency doubling); and the bottom graph illustrates the synchronous reso
nances of the far UV output waveguide (including the 220 nm light). The physical sizes o
f the devices shown in herein are not to scale. The cross sectional dimensions of the respective elem
ents may be configured to provide the desired wavelength, mode requirements, phase matching,
etc. [0172] Further embodiments are described below with reference
to various configurations of a single emitter or UV light source. It will be und
erstood that, as noted above, any of the configurations described herein may be implemented in
an array including a plurality of emitters, which may or may not be of the same or
identical emitter configuration. That is, any of the UV light source configurations may be combine
d herein in any way. [0173] FIG. 11 is a schematic top view illustrating an exa
mple combination of various elements of a UV light source 1100 including a nonlinear opt
ical element 120 coupled to an intra‐cavity portion 105i of a light emitting element 110, in co
mbination with ring resonators at respective ends 105a, 105b of the light emitting element 110 a
nd an output coupling element 130 configured for selective light extraction according to
some embodiments of the present disclosure. In the example of FIG. 11, the light
emitting element 110 is implemented as a laser diode 110’, with one or more optical resonators 11
05 that at opposing first and second ends of the lasing cavity 105 (shown as including an AlGaN
waveguide). [0174] As shown in FIG. 11, the nonlinear optical element
120 is implemented as a SHG/SFG element 120’ having a ring‐shaped optical cavity
125, and is intra‐cavity‐tapped to the lasing cavity 105 of the laser diode 110’ as a pump las
er 110’. The longitudinal modes of the pump laser 110’ may be controlled by dual ring optical
resonators 1105 positioned at each end of the linear waveguide that provides the lasing cavity 105.
The optical resonators 1105 (shown as Si x N y dual ring resonators) are configured to reflec
t the light of the first (fundamental) frequency of the laser 110’, and thus, function as
wavelength‐selective “mirrors” (with dimensions that determines the specific longitudinal m
ode frequencies) for single frequency laser feedback without the use of facets, coatings,
diffraction gratings, and/or other reflective elements. The optical resonators 1105 or reflectors
are not nonlinear optical elements 120, but rather are elements of the active pump laser 110’
(e.g., configured for reflectivity for 400 nm to 480 nm light). That is, the laser diode 110’ incl
udes the double‐ring elements as wavelength selective mirrors to form the lasing cavity 105.
[0175] As shown in the graphs of FIG. 11B, because the SH
G/SFG optical cavity 125 also couples to the laser cavity, the vernier frequency effect pr
ovides for selection of only a subset of possible longitudinal modes supported by the laser 11
0’. It is noted that the SHG/SFG (e.g., 220 nm) light can couple back into the lasing cavity 10
5 from the nonlinear optical element 120 cavity (i.e., coupling may be in the forward/output
direction and in reverse), and that the optical resonators 1105 (the Si x N y mirrors) may not be 100% reflective, such tha
t there may be losses at the ends of the lasing cavity 105. [0176] Still referring to FIG. 11, the output coupling elem
ent 130 is implemented as a waveguide configured to provide a lower cutoff freque
ncy such that the fundamental wavelength (e.g., 440nm) light is confined within the
waveguide but the SHG/SFG wavelength (e.g., 220nm) light can escape. By selectively outc
oupling light from the nonlinear optical element 120, the Q of the ring‐shaped optical cavi
ty 125 at SHG/SFG wavelengths may be kept low. In some embodiments, the output coupling eleme
nt 130 may be configured to semi‐ continuously outcouple the SHG/SFG wavelength light, f
or example, where the efficiency of the output coupling element 130 is particularly high. I
n some embodiments, the output coupling element 130 may conformally extend along at least a
part of the nonlinear optical element 120 (e.g., may wrap partially around the ring), such tha
t the SHG/SFG wavelength light may be semi‐ continuously extracted, which can thereby allow for r
elaxed (or no) phase matching requirements. [0177] FIG. 12 is a schematic top view illustrating an exa
mple combination of various elements of a UV light source 1200 including nonlinear optica
l elements 120 coupled to an extra‐cavity portions of a light emitting element 110, in combina
tion with ring resonators 1105, tuning mechanisms 1225, output coupling elements 130 configur
ed for selective light extraction, and an output monitor element 1245 according to some emb
odiments of the present disclosure. In FIG. 12, the nonlinear optical element 120 is implem
ented by multiple (e.g., first and second) SHG/SFG elements 120’ (e.g., two AlN ring resonator
s) positioned adjacent respective first and second ends of a lasing cavity 105 (shown as includ
ing an AlGaN waveguide). [0178] The first and second SHG/SFG elements 120’ are arr
anged relative to the lasing cavity 105 in an external‐cavity‐tap configuration, where
the light of the fundamental wavelength (e.g., 440 nm) is output from the laser 110’ at
the respective first and second ends of the lasing
cavity 105, propagating in a single direction at eac
h end. As the laser 110’ is configured for ligh
t emission at both ends, the first and second SHG/SFG
elements 120’ may be identical (or similar). Providing the first and second SHG/SFG el
ements 120’ at respective ends 105a, 105b of the lasing cavity 105 may reduce coupling of the
SHG/SFG light 121’ back into the lasing cavity 105, such that a greater (or maximum) fractio
n of light (e.g. approaching 100%) can be coupled into the SHG/SFG elements 120’ (in comparis
on to the intra‐cavity‐tap configuration). [0179] The nonlinear optical elements 120 may be arranged a
djacent to the opposing ends of the lasing cavity 105 such that coupling between the
laser 110’ and the nonlinear optical elements 120 may be in‐plane (i.e., along one or
more directions parallel to a surface of a substrate 101), and/or may be at least partially sta
cked on the laser 110’ (e.g., at least partially
overlapping the ends of the lasing cavity 105 in th
e vertical direction, normal to the surface of the substrate 101) such that coupling between the la
ser 110’ and the nonlinear optical elements 120 may be in a vertical direction (i.e.,
perpendicular to the surface of the substrate 101). In such a vertically overlapping arrangement
, most or all of the fundamental wavelength light 111 that is not reflected back by the double
ring optical resonator elements is coupled into the nonlinear optical elements 120 (e.g., the AlN ri
ng resonators), which can improve overall device efficiency. Also, little to none of the SHG
/SFG light 121’ may be coupled back into the lasing cavity 105, which can also improve efficiency.
[0180] One or more mirror elements (shown as Si x N y optical resonators 1105) are provided at or near the ends of the lasing cavity 105. The S
HG/SFG light 121’ may be coupled (in‐plane or vertically) from the first and second SHG/SFG element
s 120’ into respective output waveguides, and output by respective output coupling elements 130
at respective ends of the output waveguides. The output coupling elements 130 may be
facets, gratings, or other optical elements configured to outcouple the SFG/SHG light 12
1’ in a direction that is substantially normal to the surface of the substrate 101 (or othe
rwise out of the plane shown in the illustrated plan view) to provide surface emission. [0181] FIG. 12 also illustrates at least one tuning mechani
sm 1225 that is configured to adjust one or more operating characteristics of the nonlinea
r element based on the light output from the light emitting element 110. The tuning mechanis
m(s) 1225 may include thermal heaters, electro‐optic (EO) devices, and/or other devices use
d to alter a thermal, electrical, and/or optical characteristics of the nonlinear optical eleme
nts 120 (e.g., the AlN ring resonators) to more closely match or correspond to the emission wav
elength of the light output from the light emitting element 110, which may vary or drift depend
ing on the operating environment (e.g., with changes in temperature) and/or manufacturing tole
rances. For example, a wavelength tuning mechanism 1225’ may be provided by a therma
l or electro‐optic element that is configured to adjust the resonance of the nonlinear
optical element 120 to correspond to the output of the laser 110’. In some embodiments, t
he wavelength tuning mechanism 1225’may be a gold or other thermally conductive metal plate
or element that is configured to alter the resonant wavelength responsive to heating. In anothe
r example, the wavelength tuning mechanism 1225’ may include one or more thermoelect
ric cooling (TEC) elements that are configured to alter the resonant wavelength responsive
to cooling. [0182] Such wavelength tuning mechanisms 1225 may be impleme
nted to allow for imperfections or variations in the output of the las
er 110’, for example, wavelength drift with changes in operating temperature, and may be similarl
y used in any of the embodiments described herein. Also, while shown by way of exam
ple as being used to adjust characteristics of the nonlinear optical elements 120, the tuning me
chanisms 1225 may be similarly used to adjust characteristics of other elements of UV light
sources (such as the one or more (Si x N y ) wavelength‐selective optical resonators 1105 and/or t
he light emitting element 110 itself) to more closely match or control the operating character
istics of the light emitting element 110 and the nonlinear optical elements 120. For example
, the wavelength‐selective optical resonators 1105 may include respective tuning mechanis
ms 1225 to be adjusted such that the reflected laser emission 111 matches the characteristi
cs of the nonlinear optical elements 120 for second harmonic or sum frequency generation. Wa
velength tuning (thermal or electro‐ optic) as described herein may also be used to comp
ensate for variations in the coupling gaps (e.g., between the laser 110’ and the nonlinear op
tical elements 120, and/or between the nonlinear optical elements 120 and the output wavegui
des), and/or the refractive index of one or more optical elements. That is, the tuning mech
anisms 1225 may be used with multiple optical elements described herein. [0183] FIG. 12 further illustrates a monitor element 1245 t
hat is configured to measure a property (e.g., detect a power level) of the output
light 131 and generate a feedback signal to a controller that is configured to operate the light e
mitting element 110 and/or the tuning mechanism 1225. For example, the monitor element 12
45 may be implemented as an integrated photodiode that is configured to detect
or monitor the field strength or power of the SHG/SFG light 121’ output in order to provide
feedback signals to other system components such as the laser drive and/or the tuning elements.
One or more monitor photodiodes 1245 may similarly be integrated at various positions in
the UV light source 1200 to monitor the light output from the light emitting element 110 and provi
de feedback signals to one or more controllers. Such monitor elements 1245 may be simi
larly used in any of the embodiments described herein. [0184] FIG. 13 is a schematic top view illustrating an exa
mple combination of various elements of a UV light source 1300 including nonlinear optica
l elements 120 coupled to an extra‐cavity portions of a light emitting element 110 including a
saturable absorber element 1305, in combination with tuning mechanisms 1225, output coupli
ng elements 130 configured for selective light extraction, and an output monitor ele
ment 1245 according to some embodiments of the present disclosure. It will be
understood that these and other illustrated implementation are by way of example, and that embod
iments of the present disclosure may include various other combinations of the illustrated
(and other) components. [0185] The UV light source 1300 of FIG. 13 may be similar
in some aspects to the UV light source shown in FIG. 12, and thus, description of s
imilar elements will not be repeated for brevity. However, in FIG. 13, the light emitting e
lement 110 (e.g., the AlGaN pump laser 110’) may include end‐mirror facets or other elements to
define the lasing cavity 105, without use of the dual ring reflectors. In addition, a saturable
absorber element 1305 is integrated or otherwise provided in the lasing cavity 105. The s
aturable absorber 1305 is configured to induce the laser 110’ to operate in a (passive or
active) mode‐locked manner or otherwise generate the light of the fundamental wavelength as
a plurality of light pulses. In particular the saturable absorber 1305 is configured to generate hig
her power pulses, which may increase SHG/SFG conversion efficiency (e.g., by exploiting qua
dratic nonlinearity in the SHG/SFG elements 120’). The saturable absorber 1305 may b
e substantially transparent at higher lasing intensities, which may cause the lasing cavity 105 t
o preferentially support modes in which the optical power is pulsed. It is understood that t
he embodiments here can include any type of saturable absorber material configured based on the d
esired output wavelengths of the light emitting element 110 (e.g., at the 400 – 480 nm
wavelengths), or may include artificial saturable absorbers such as Kerr Lens structures with
an aperture. [0186] As in other embodiments, the SHG/SFG elements 120’
are implemented as resonant cavities (shown in FIG. 13 as AlN‐based ring‐shap
ed cavities) that are overcoupled (e.g., near 100% collection) to the output of the pump laser 11
0’ at opposing ends of the lasing cavity 105 (i.e., in an external cavity‐tap configuration).
Respective wavelength tuning mechanisms 1225’ are used to adjust the operating characteristics of
the SHG/SFG elements 120’ (e.g., the AlN ring resonators) to match the emission wavelength of the
laser 110’. Also, one or more output monitors 1245 (e.g., photodiodes) provide feedback sig
nals (e.g. to drive controllers/electronics) to adjust the drive current a
nd/or the tuning elements based on the detected light output from the light emitting element
110 (i.e., the fundamental light) and/or the output coupling elements 130 (i.e., the SHG/SFG
light 121’). More generally, elements, mechanisms, arrangements, and/or other configurations d
escribed herein with respect to one embodiment may be combined with those of other embod
iments unless otherwise noted. [0187] Further embodiments of the present disclosure are des
cribed below with reference to one or more design features that are configured to
improve yield of emitters that provide far‐ UVC light output 131’ using cavity‐enhanced SHG.
These embodiments are described herein by way of example with reference to a light emitting e
lement 110 implemented as a laser diode 110’ that is configured to emit light having a pr
edetermined wavelength, e.g., 440 nm light. The configuration of the laser 110’ may be in accordan
ce with any of the embodiments described herein, including but not limited to ridge or buried
‐slab waveguides; mirror elements on ends thereof; distributed feedback (DFB) configurations; and
/or double ring reflective elements at respective ends 105a, 105b thereof. [0188] FIG. 14 is a schematic top view illustrating an exa
mple combination of various elements of a UV light source 1400 including a plurality of
different or non‐identical nonlinear optical elements 120 in ring cavity configurations with respe
ctive output coupling elements 130 according to some embodiments of the present disclosu
re. As shown in FIG. 14, light emission from the laser diodes 110’ is coupled directly int
o a linear (in these examples, unidirectional) input waveguide as the input coupling element 115 th
rough which the fundamental wavelength light 111 propagates in a single (or small number o
f) mode(s). The input coupling element 115 (at the laser output) may be a 440 nm input wavegu
ide or single mode fiber. Nonlinear optical elements 120 are provided by ring shaped SHG/SFG ele
ments 120’ (shown as a plurality of AlN ring resonators; but more generally, a plurality of
nonlinear optical elements 120) along an edge or side of the length of the input waveguide
115. The ring shaped SHG/SFG elements 120’ may be fabricated from various nonlinear optical mate
rials (in these examples, AlN, although other embodiments may use different materials). The
ring shaped SHG/SFG elements 120’ are configured to be doubly resonant at both the fundame
ntal and the higher order frequencies or wavelengths (e.g., 440 nm and 220 nm), and the ring
design may be configured to provide high Q for a single mode at both the fundamental frequen
cy ω 1 or wavelength (e.g., 440.0 nm) and the higher order frequencies ω 2 or wavelengths (e.g., 220.0 nm). [0189] In particular embodiments, it may be critical that t
he waveguide is configured to support modes at wavelengths of the two modes suppor
ted that are related by a factor of 2.0 exactly, in order to yield improved or best possible
SHG conversion efficiency. The doubly resonant cavities of the ring shaped SHG/SFG elements
120’ may be configured to have a gap between edges thereof and the edge of the adjacent
input waveguide, where the size of the gap may be configured and carefully controlled to im
prove or maximize coupling of the 440 nm light from the input waveguide into the ring shaped
SHG elements. In addition, on the other side (as shown, on the bottom) of the ring shaped
SHG elements, one or more linear output waveguides are fabricated and configured to collect 2
20 nm light outcoupled therefrom. In some embodiments, the respective output waveguides (in
the illustrated embodiment, one per ring shaped SHG/SFG element 120’) are configured su
ch that only the desired wavelength of light (the SHG/SFG frequencies or wavelengths, e.g.,
220nm) is supported (and not the fundamental frequency or wavelength, e.g., 440nm) beca
use the output waveguide’s critical frequency is greater than the fundamental frequency.
[0190] Without wishing to be bound by theory, some advantag
es of providing a plurality of nonlinear optical elements 120 may be explained as f
ollows. Although the design of the ring shaped nonlinear optical elements 120 may be such th
at a single or respective cavity can deliver very high SHG conversion efficiency, the ma
nufacturing tolerances for the radius, the coupling gap(s) and the losses of each ring shaped
nonlinear optical elements 120 may be difficult to maintain from one emitter to another.
Accordingly, multiple, nominally identical (or substantially similar) nonlinear optical elements 120
along the edge of the waveguide may be provided for redundancy, where each of the nonlinear
optical elements 120 may have slightly different dimensions (e.g., radius R, supported freque
ncy ω, gap to waveguide, etc.). As such, even if only a single one of the plurality of nonl
inear optical elements 120 provides sufficient SHG efficiency, the overall device (laser + waveguide
(s) + nonlinear optical element 120) may operate as intended. [0191] That is, as fabrication tolerances of structures or
elements of the UV light source 1400 may present challenges, some embodiments may include
a plurality of variants of SHG/SFG elements 120’ to address differences in manufacturin
g tolerances. In particular, as the pump light 111 is coupled out of the laser 110’ and p
ropagates down the input waveguide, the geometry (including dimensions and shapes) of one or
more (or each) of the SHG/SFG elements 120’ may be different, either by design or due to
manufacturing variations. With different geometries, each SHG/SFG element 120’ may have a d
ifferent amount or level of coupling to the waveguide, or may have different resonant frequen
cies, such that one or more of the plurality of SHG/SFG elements 120’ may be particula
rly well matched (in terms of wavelengths of operation) to the pump laser 110’, while one o
r more others of the plurality of SHG/SFG elements 120’ may not be as well matched to the
pump laser 110’. As such, only a subset of the plurality of SHG/SFG elements 120’ may contribu
te the majority of the SHG/SFG light 121’ generated by the UV light source 1400. Alternativel
y, with changes in temperature or operating conditions, one or more SHG/SFG elements 120’ may
come into and out of ideal matching with the pump wavelength (i.e., due to variations in oper
ating characteristics over the time or duration of operation of the UV light source. Over
all, redundancy provided by the plurality of variants of the SHG/SFG elements 120’ may contribut
e to the robustness of the overall performance of the UV light source 1400. [0192] In other words, to address challenges with respect t
o manufacturing tolerances, a plurality of nonlinear optical elements 120 may be i
ntentionally fabricated (e.g., along a length of an input coupling element 115 or lasing cavity 1
05) with one or more different dimensions, shapes, or even materials, such that at least one o
f the nonlinear optical elements 120 might have the desired dimensions to yield high conversion
efficiency with respect to the fundamental wavelength of the light output from the
light emitting element 110. The remaining (i.e., non‐conforming) nonlinear optical el
ements 120 may be unused if they have either poor coupling to the input waveguide or they
do not provide sufficiently high SHG/SFG efficiency, and any fundamental wavelength light 111
that is coupled into the non‐conforming ring shaped nonlinear optical elements 120 may be re
turned or outcoupled back into the input waveguide. Or the non‐conforming nonlinear optica
l elements 120 may be intentionally removed, destroyed or disabled by some additional pro
cess step after fabrication. [0193] Because the overall area or footprint of the UV lig
ht source 1400 may be dictated by the length of the laser 110’ and the width (i.e., alo
ng a direction perpendicular to the length of the laser 110’ or waveguide) of a respective nonlinear
optical element 120, the inclusion of “extra” or unused nonlinear optical elements 120 of nominally
the same or similar configuration may not require substantially more area on the chip. T
hat is, the use of multiple nonlinear optical elements 120 may have relatively little cost, particu
larly when the SHG/SFG element 120’ sizes are small (e.g., a fraction of the length) in compa
rison to the dimensions of the pump laser 110’ and/or waveguide. However, including the additional
nonlinear optical elements 120 can increase the likelihood of desired device functionalit
y and/or achieving high conversion efficiency. In this way, providing a plurality of
ring shaped or other nonlinear optical elements 120 with one or more different dimensions, shapes, a
nd/or materials along a length of the waveguide can lead to higher device yields than may
conventionally be possible, with little to no size penalty. [0194] More generally, in the example of FIG. 14, respectiv
e ones of the nonlinear optical elements 120 may have different dimensions, shapes, a
nd/or materials, and the output coupling element 130 may include a plurality of outp
ut coupling elements 130 that are respectively configured to selectively outcouple far‐
UVC light 121’ as output light 131’ from the respective ones of the nonlinear optical elements 120
. As noted, variations of the embodiments described herein may be implemented with
a similar layout or configuration, but including (a) different types of laser 110’ that i
s used to pump the system or the material used for the laser 110’, such as GaN, AlGaN, InGaN, et
c.; (b) different materials used for the nonlinear optical elements 120, such as AlN, Li Niob
ate, BBO, and/or other nonlinear materials; (c) different shapes or designs of the waveguides th
emselves (e.g., some embodiments may use ridge waveguides with flat edges while others may us
e slabs of high index material with the guiding provided by a lateral variation in the cladd
ing, etc.); (d) different substrates such as AlN, GaN, Si, Sapphire or other materials; (e) d
ifferent methods of fabrication in order to heterogeneously integrate the various components (e.g.,
monolithic fabrication using epi regrowth, wafer bonding, and/or microtransfer printing)
. Any or all of the various combinations of embodiments surrounding the concepts d
escribed herein are included in the above embodiment using multiple nonlinear optical elem
ents 120 along the same waveguide. [0195] Further embodiments of the present disclosure may lik
ewise provide multiple nonlinear optical elements 120 along one side or opposing side
s of an input waveguide (at the laser output), but may remove or omit the respective outpu
t waveguides (shown in FIG. 14 as providing the SHG/SFG light 121’ from the nonlinear
optical element 120 to the output coupling elements 130). Rather, the SHG/SFG light 121’ gen
erated by the nonlinear optical elements 120 may escape by radiative bending losses from the
ring‐shaped nonlinear optical elements 120. In general, the smaller the bending radius, t
he higher the radiative losses from a ring shaped waveguide. The ring‐shaped nonlinear optical
elements 120 may thus be configured to account for the radiative bending losses when conside
ring the radius thereof. However, in contrast to the embodiment shown in FIG. 14 (which
maintains low radiative losses relative to the coupling of SHG/SFG light 121’ out through the
bottom output waveguides as output light 131’), further embodiments may be configured to emi
t some or up to all of the SHG/SFG light 121’ as output light 131’ via the bending radius
of the respective ring‐shaped nonlinear optical elements 120. Without wishing to be bound by theor
y, some advantages of removing the respective output waveguides and relying on radiative
bending emission to collect the SHG/SFG light 121’ may include reduced manufacturing complex
ity, by removing at least one component of the overall device (e.g., the 220 nm output wave
guide(s)) whose dimensions may otherwise require rigorous precision with respect to dimensions
and positioning relative to the nonlinear optical element 120(s). [0196] That is, while FIG. 14 illustrates the use of multi
ple SHG cavities in an “external‐cavity‐ tapped” configuration, and with output waveguides pr
ovide to couple the SHG light out, other embodiments may vary by providing “intra‐cavity‐t
apping” of the pump laser 110’, and/or radiative or scattering output of the SHG/SFG light
121’ (instead of the use of a waveguide). Furthermore, these embodiments may use nonlinear optic
al elements 120 that are not ring cavities, and instead are some other kind of whisper
ing gallery mode cavity or even some kind of linear cavity. [0197] The omission of the output waveguide element(s) for
collection of the SHG/SFG light 121’ outcoupled from the nonlinear optical elements
120 indicates that the output light 131 may be emitted laterally (i.e., in‐plane light emis
sion with respect to the above plan view), rather than surface emission in a direction perpendic
ular to the plane in the plan view shown above (or otherwise with some out of plane component
). As such, in some embodiments, one or more angled reflectors can be integrated around t
he exterior (e.g., the circumference) and/or interior of the ring. Such angled reflectors
(not shown) may extend around and/or within a circumference or perimeter of the nonlinear
optical elements 120 in plan view. For example, in cross sectional view, the angled reflecto
r may be triangular‐shaped reflectors on opposing sides of the nonlinear optical elements 120.
It will be understood that some out‐of‐ plane emission components might also be incorporated
into the SHG/SFG light 121’ Poynting vector (propagation direction) by the shape of the s
ide walls of the ring shaped nonlinear optical elements 120 (which may be trapezoidal in cr
oss section). That is, the sidewalls of the nonlinear optical elements 120 may have a substantial
angle in cross section, such that the light may be emitted with some upward/out of plane compone
nt. In other words, the nonlinear optical element 120 waveguide sidewall angle may be
controlled or otherwise configured to optimize desired light emission in some embodiments.
[0198] It will be understood that the concept of SHG/SFG e
mission via bending losses as described in the above embodiment may be similarly i
mplemented in any of the embodiments described herein. For example, any of the embodimen
ts including ring shaped nonlinear optical elements 120 as described herein may be configured s
uch that the light generated by the nonlinear optical elements 120 may escape by radiativ
e bending losses from the ring‐shaped nonlinear optical elements 120 (e.g., by omitting the
output coupler(s) and using one or more angled reflectors for light extraction in the desired
direction). [0199] Further embodiments of the present disclosure may lik
ewise provide multiple nonlinear optical elements 120, but in an intra‐cavity config
uration to receive input light directly from the lasing cavity 105 at one side thereof, in combinatio
n with one or more output waveguides (e.g., one per nonlinear optical element 120) along the oth
er side thereof. In particular, further embodiments may similarly include the SHG/SFG light 1
21’ output waveguides for outcoupling the SHG/SFG light 121’ (e.g., at 220 nm) from eac
h ring resonator as shown in FIG. 14, but instead of introducing the fundamental wavelength ligh
t 111 to the nonlinear optical elements 120 by means of a unidirectional wave from a laser
110’, the nonlinear optical elements 120 may be arranged alongside a waveguide that provides
the lasing cavity 105 of the laser 110’. That is, while shown in FIG. 14 as including an in
put coupling element 115 implemented as an input waveguide, the input coupling element 115 may
be omitted and light from the light emitting element 110 can be coupled directly into th
e nonlinear optical elements 120 from within the cavity of the (active) laser 110’ itsel
f, rather than from a separate passive input waveguide. Such embodiments may be more complex wit
h respect to design of the laser 110’ itself (the presence of one or more rings represents
a loss term in the energy balance of the laser 110’) but may be advantageous in that higher
intensity pump power (e.g., at 440 nm) may be provided. Remaining aspects of this embodiment m
ay be the same as or similar to previous embodiments, including variations with respect to mate
rials, substrates, and/or and fabrication methods as noted herein. [0200] Further embodiments of the present disclosure may lik
ewise provide multiple nonlinear optical elements 120 configured to receive input ligh
t at one side thereof in combination with one or more output waveguides (e.g., one per nonline
ar optical element 120) along the other side thereof. However, the nonlinear optical element
120 may be implemented with shapes other than rings (for example, other rotationally sym
metric shapes, such as disk (e.g., microdisks) or sphere (e.g., microspheres) shapes).
The modes supported by such structures may be different than that of a ring‐shaped nonlin
ear optical element 120, but the overall concept and benefits of use of a plurality of reson
ators remain the same as previous embodiments. [0201] Further embodiments of the present disclosure may sim
ilarly include multiple nonlinear optical elements 120 configured to receive input ligh
t at one side thereof in combination with one or more output waveguides (e.g., one per nonline
ar optical element 120) along the other side thereof, with the nonlinear optical elements 120
(e.g., ring resonators) coupled directly to the intra‐cavity region 105i of the lasing cavity
105 of the laser diode 110’. However, the laser
diode 110’ (to which the plurality of ring‐shaped
nonlinear optical elements 120 is coupled) may be implemented as a ring laser, rather than a
linear ridge laser. Coupling to a ring laser may be advantageous in terms of the type of mode t
o which coupling can be realized. Coupling to a ring laser as the light emitting element 110
may also be configured for coupling only to modes that propagate in a single direction (as oppos
ed to multiple directions, as may be obtained from a linear laser which contains a standi
ng wave). [0202] Further embodiments of the present disclosure may inc
lude at least one nonlinear optical element 120 configured to receive input light
at one side thereof, in combination with at least one output waveguides (e.g., one per nonlinear
optical element 120) along the other side thereof. However, optical coupling between elements
may be realized at least in part by lateral overlap of the nonlinear optical element 120(s) with
the components for input and/or output light coupling, such that at least two of the compo
nents (e.g., the light emitting element 110, the input coupling element 115, the nonlinear optical
element 120, or the output coupling element 130) are not in the same plane, also referr
ed to herein as multi‐layer integration. [0203] FIG. 15 is a schematic top view illustrating an exa
mple combination of various elements of a UV light source 1500 in physically overlapping
configurations for multi‐layer integration according to some embodiments of the present disclosu
re. In FIG. 15, the geometry of coupling differs from some embodiments described herein in tha
t respective portions of the SHG/SFG element 120’ physically overlap the input and outpu
t waveguides, but are not arranged on a same plane or coplanar surface (i.e., the elements a
re offset in the vertical or Z direction (in or out of plane) relative to one another). [0204] Advantages conferred by the multi‐layer integration
arrangement shown in FIG. 15 include that the critical dimensions through which op
tical coupling occurs are in the vertical or Z‐direction (i.e., the out of plane direction in t
he illustrated plan view), which can be easier to control by many fabrication methods. It will be un
derstood that the vertical overlap of two or more components as shown in the embodiment of FIG.
15 may include any and all methods of integrating or manufacturing multiple components such
that two or more do not share the same plane. Example methods may include wafer bondi
ng, microtransfer printing, microassembly, self‐assembly, and/or polishing and ep
itaxial regrowth. Additional variations of vertically overlapping elements beyond that shown in
FIG. 15 may also be used for coupling. Such coupling configurations may include, but are not
limited to, vertical overlap evanescent wave coupling, lateral evanescent coupling, use of gr
atings to direct light in or out of plane of the waveguide, butt (end‐to‐end) coupling, and/or
use of tapered element shapes (e.g., waveguides with non‐uniform thicknesses) to enhance
coupling. More generally, UV light sources as described herein may include two or more
of the light emitting element 110, the input coupling element 115, the nonlinear optical ele
ment 120, and the output coupling element 130 overlapping in a direction that is perpe
ndicular to a surface of a substrate 101 having the light emitting element 110, the nonlinear
optical element 120, and the output coupling element 130 thereon. [0205] Further embodiments of the present disclosure may inc
lude various coupled ring configurations, which may extend the vernier frequency
selection strategy through the use of an intermediate cavity (e.g., a ring‐ or other‐sh
aped cavity between the lasing cavity 105 and the optical cavity 125 of the nonlinear optical elem
ent 120) to select only a single mode over an even larger free spectral range (FSR). Doing so ma
y help concentrate energy from the pump laser into a single mode that is doubled and thus
increase efficiency. [0206] Some embodiments may be configured to provide the ab
ility to switch coupling on‐and‐ off by providing an electro‐optic or thermo‐optica
l material between two rings (to shift the index electrically or thermally). For example, the
electro‐optic or thermo‐optical material may be provided between a ring laser and a ring‐shaped
nonlinear optical element. [0207] Some embodiments may include a saturable absorber in
the ring laser to induce pulsed modes. [0208] Some embodiments may include one or more secondary r
ings as ‘filters’ to provide wider free spectral range and matching specific ring
mode to specific SHG/SFG ring mode. For example, at least one passive oscillator may be prov
ided as a secondary ring that receives light outcoupled from the ring laser and outcouples a subs
et of the light to a nonlinear optical element for secondary harmonic generation. [0209] Some embodiments may include multiple nonlinear optica
l elements that are arranged partially or substantially around a periphery of one
laser. For example, multiple ring‐shaped nonlinear optical elements 120 are provided around a
circumference of a single ring laser. [0210] Some embodiments may include one or more secondary r
ings as filters that are arranged partially or substantially around a periphery
of one laser, within a larger nonlinear optical element. For example, a large radius ring
shaped nonlinear optical element extends around a ring laser, which may be filtered in some
embodiments by one or more ring ‐ filter oscillators to allow mode selection from SHG ring wh
ich has high mode density. [0211] Some embodiments may include multiple ring lasers per
nonlinear optical element. For example, multiple ring lasers may be arranged around
a periphery (or circumference) of a ring‐ shaped nonlinear optical element. Due to coupling b
etween rings, the ring lasers may all be forced to same phase or mode. In some embodiments,
the ring lasers may be turned on or activated sequentially, allowing the first ring laser
to set the phase for the remaining ring lasers.
Some embodiments may include a wavelength tuning mech
anism configured to provide localized temperature or electric field tuning of eac
h ring laser independently, which can provide another degree of freedom. [0212] As noted above, while some conventional designs of U
V light sources may be handicapped by coupling losses between the active and
passive components and/or conversion efficiencies, embodiments of the present disclosure ma
y provide higher nonlinear conversion efficiencies by use of optical cavities 125 to incre
ase the number of passes that the pump laser 110’ makes through the material (effectively recycli
ng unconverted pump light 111). These benefits have been demonstrated at other wavelengths.
For example, optical microresonators fabricated from AlN have demonstrated over 17,000%/W
SHG/SFG conversion efficiency (up to 10% absolute conversion efficiency for 10 mW input)
and 180%/W2 third harmonic conversion efficiency, albeit using a 1540 nm fundamental wavele
ngth [9‐12]. These results demonstrate that AlN has a sufficiently high nonlinear response
(4 pm/V vs. 7 pm/V) to deliver very high conversion efficiency, in particular when cavity‐enha
ncement is also used to increase or maximize the intensity of the fundamental wavelength.
[0213] Further embodiments of the present disclosure may pro
vide ways of controlling the far field pattern of the output light 131 that is outco
upled from nonlinear optical elements 120 described herein (e.g., the far‐UVC light 121’).
For some applications, control of the spatial distribution of irradiance over an area (or equivalen
tly, the angular distribution of radiant intensity over some field) of illumination may be cr
itical to performance. Indeed, visible illumination products support an entire industry dedic
ated to shaping and sculpting the pattern of illumination. For UV applications, there may be
similar need for control of this “far field pattern”, for example, to provide germicidal effic
acy for the far‐UVC output light 131’, which may depend on spreading the germicidal UV across a
region of application in an optimal manner. [0214] Some embodiments the present disclosure may include a
n output coupling element 130 implemented as a second order diffraction grating tha
t is configured to couple the light out of an in‐plane PIC and project it over a range of a
ngles surrounding the normal surface vector. The design or configuration of the diffraction gratin
g may provide some ability to modify how wide of an angle the light is spread over as well
as the uniformity of the radiant intensity within the range of emission angles. [0215] In further embodiments, the photonic integrated circui
t may include structures that are configured to divide the light generated on the chip
into multiple channels, each of which has its own output coupling element 130 which may or ma
y not include a second order diffraction grating. FIG. 16 is a schematic top view illustr
ating output coupling element configurations 1600 of a UV light source configured to provide a
desired far field emission pattern according to some embodiments of the present disclosure. [0216] In FIG. 16, the output coupling element 130 is impl
emented as a plurality of output coupling elements 130‐1, 130‐2, 130‐3, 130‐4 t
hat are configured to outcouple the far‐UVC light
121’ as output light 131 in respective directions
(e.g., at various angles in‐plane or out of the page or substrate 101 as depicted), to provide the
output light 131 with a desired far field pattern. One or more waveguides 160 may be coupled
between the nonlinear optical element 120 and the respective output coupling element 130,
and the output light 131 may be a split beam by respective waveguides and output coupling ele
ments 130 to provide respective channels for light output. The multiple output coup
ling elements 130 may be configured to direct the light upward (e.g., away from a surface
or substrate 101), but in directions that are not necessarily or perfectly normal (e.g., not necess
arily at 90 degrees) to the surface or substrate 101. By centering or providing the emissi
on pattern of the termination of each channel to provide the output light 131 (e.g., the
far‐UVC light 121’) into different directions (coherently or incoherently), a combined overall light
pattern can be designed to the requirements of a desired application. The number o
f channels for which this configuration may be used could be as few as one or two, but t
he upper bound is limited only by practicality of PIC design (e.g., thousands or more). [0217] Two specific subclasses of multiple output channel co
nfigurations as described herein include arrays of UV light sources, and coherent lig
ht combination. In an array of UV light sources, the light output of any one UV light sourc
e may or may not be divided into multiple output couplers. However, because the devices are m
eant to be operated as part of a larger array of nominally identical devices, the output coup
ling element 130(s) of each individual UV light source within the array may be individually mo
dified such that the combined far field pattern of the overall array meets a desired specifi
cation. [0218] In coherent combination of light, the individual UV
light sources may have their respective light output divided into at least two di
fferent channels. Because the output light 131 (e.g., the far‐UVC light 121’) emitted from
each of two or more channels originate from the same coherent light source (e.g. the visible light 1
11’ output from the laser 110’), the output light 131 from the respective channels may maintain
a fixed phase relationship. As such, the far field emission pattern generated by the respective em
ission channels (one per UV light source) may be subject to coherent effects (similar to that
used for optical beam steering). In other words, some embodiments may take advantage of coheren
t combinations of output from respective channels of multiple individual UV light s
ources in order to obtain a desired emission pattern. When multiple UV light sources are opera
ted as a very large array or as an array with distinct or different output coupling element configur
ations, however, it may be unlikely that phase coherence can be maintained between the UV lig
ht sources, so far field patterns generated by the collective light output across an a
rray may include an incoherent combination of optical fields. [0219] Embodiments of the present disclosure may differ from
some conventional designs in several ways. For example, some embodiments of th
e present disclosure integrate active and passive components on the same chip (e.g., using com
ponents of the same material systems, such as nitride based materials) such that the optic
al losses between devices are reduced or minimized. In addition, some embodiments of the prese
nt disclosure may specifically target conversion from 440 nm to 220 nm with a focus on
conversion efficiency, in contrast to designs that may attempt to fabricate coherent, polarized las
er (beams) with narrow linewidth, which may not be necessary for some applications. Also, in
contrast with previous demonstrations of SHG to generate 220 nm light, PICs in accordance wi
th some embodiments of the present disclosure leverage resonant cavity enhancement to inc
rease or maximize the intensity of the fundamental wave and thus increase or maximize effici
ency. The output light provided by embodiments of the present disclosure may be collimat
ed or non‐collimated, coherent or incoherent, and emitted as a beam or as distributed
emission. Some embodiments may use (but are not limited to) one or more of the follow
ing technology elements, in various combinations: wavelength conversion using nonlinear opt
ics (SHG/SFG); use of AlN‐based nonlinear optical elements 120 for wavelength conversi
on; selective outcoupling of far‐UVC wavelengths; use of waveguides, including AlN‐based
waveguides or PICs; use of optically resonant microcavities; monolithic integration of activ
e and passive components; and light output that is free of the fundamental wavelength of
the light emitting element 110. [0220] Embodiments of the present disclosure as described he
rein may thereby reduce cost and increase the (power) efficiency for producing far
‐UVC light 121’, which may be advantageous in providing a cost‐competitive source
of disinfecting light that can be widely deployed to combat airborne (and surface) pathogens.
Moreover, by providing UV light emission in the far‐UVC range (e.g., from about 20
0 nm to about 240 nm), embodiments of the present disclosure can be used to actively eliminate
pathogens from the air while people are present, in contrast to conventional use of UV wavel
engths for disinfectant purposes in wavelength ranges that are harmful to humans (e.g..
from greater than about 240 nm to about 400 nm). [0221] Further embodiments of the present disclosure provide
devices configured to generate electromagnetic radiation in the far‐UVC spectrum to
provide germicidal effects, while also complying with human safety regulations and requiremen
ts. Germicidal light sources configured to operate in the far‐UVC wavelength ran
ge may be advantageous in that (i) the rate of disinfection of pathogens may be higher, and (ii)
from a human safety perspective, acceptable levels of irradiation may be higher (and
perhaps infinite or limitless) as compared to the remainder of the wavelengths in the UV spectrum.
[0222] In light of safety regulations and/or concerns, it m
ay be advantageous to operate GUV light sources only when necessary and/or at power le
vels, duty cycles, and/or spatial illumination patterns that are optimized for minimizin
g risk of airborne pathogen transmission. Achieving such operation may require detection of ope
rating conditions and/or other information in real time. [0223] Embodiments of the present disclosure described herein
can provide real‐time, actionable information to a GUV light source by inte
grating sensors into the GUV system operation, either physically or by way of communicati
on networks. In particular, some embodiments of the present disclosure provide a senso
r feedback‐based “smart” illumination device that includes a GUV light source communicative
ly coupled to sensors of various types, which are configured to feedback information to a co
ntroller of the GUV light source to allow for algorithmic decision making and optimized operatio
n. [0224] Integration of sensor(s) and GUV light sources into
a single device may be advantageous in terms of the capability and scope of operation o
f GUV illumination products, allowing detected operating conditions to be provided to a co
ntroller in real time, allowing for control of the operation of the GUV light source in accordance
with the detected operating conditions. GUV irradiation and illumination may t
hereby be optimized, i.e., with respect to increasing or maximizing the effectiveness of the GUV
in terms of ability to disinfect while reducing or minimizing any overall GUV optical output
in the interest of remaining within safety limits, prolonging GUV lifetime, and reducing or mini
mizing impact of UV light on the surrounding environment. [0225] In contrast, some conventional GUV systems may not b
e configured to detect or control operations based on existing operating conditions. R
ather, such conventional GUV systems may be operated with a limited, small number of sta
tes, typically “on” or “off” irradiation states. Moreover, such conventional GUV systems may
require manual intervention in order to modify the operating condition of the GUV light sour
ce. Some GUV systems are driven by autonomous robots that are used to disinfect surfaces
inside an enclosed room. While these autonomous robots may employ sensors in conjunction w
ith the operation of the UV light, the sensors are typically directed to controlling the ope
ration of the autonomous robot, rather than optimization of the GUV illumination in a dynamic en
vironment. [0226] Also, while sensors may be conventionally used in co
mbination with typical visible lighting, embodiments of the present disclosure are d
irected to operation in the UV spectrum where (a) the availability and cost of illumination
is scarce and (b) concerns regarding human safety are particularly high. For GUV applications,
the types of sensors used and reasons for employing them may be distinct from those of general
(visible) lighting applications. For example, sensors that may guide use of GUV lighting
may include various forms of air quality sensors (aerosol detectors, pathogen detectors, etc.)
in order to judge the degree of need for or effectiveness of GUV illumination including the relati
ve intensity with which the illumination fixtures should be operated. Alternatively or in ad
dition to the use of these sensors, 3D time of flight cameras or other positional sensors that can
both detect movement and quantify occupancy levels in a given space may be used to m
oderate the amount of GUV illumination provided in order to stay within regulatory limits.
In either of these cases the distinction from the kind and sophistication of any sensors that are
integrated in general lighting is great. [0227] Some elements of embodiments of the present disclosur
e may integrate a sensor suite with a GUV illumination devices as described herein.
FIG. 17 is a schematic block diagram illustrating components of a sensor feedback‐based
smart” illumination device that includes a germicidal UV (GUV) light source communicatively coupl
ed to sensors 1750 that are configured to feedback information to a controller 1701 of the
GUV light source according to some embodiments of the present disclosure. [0228] In particular, FIG. 17 illustrates an illumination de
vice 1700 including a GUV light source 100’ configured to generate and emit electromagnetic
radiation in the germicidal region of the UV spectrum. In some embodiments, the GUV illuminat
ion device 1700 may be configured to provide light emission in the far‐UVC spectrum, fro
m about 200 nm to about 240nm (e.g. at about 222 nm). The GUV illumination device 1700 ma
y include a UV light source (such as the UV light source 100’), a controller 1701, and one
or more sensors 1750 configured to detect real‐time conditions in an operating environment of
the UV light source 100’, and to provide detection signals indicating the real‐time conditions
to the controller 1701. The controller 1701 is configured to control operation of the light emit
ting element 110 of the GUV light source 100’ based on the detection signals. [0229] The GUV light source 100’ may be implemented using
solid state systems for generating coherent or non‐coherent, electromagnetic, non‐ioniz
ing radiation in the far‐UVC wavelength band, based on nonlinear optical processes and using
photonic integrated circuits (PIC), as described above in “Nonlinear Solid State Devices F
or Optical Radiation In Far‐UVC Spectrum” to Fisher, et al., the disclosure of which is incorpora
ted by reference herein. Alternatively, the GUV light source 100’ may be implemented by any o
f the UV light sources (e.g., 100, 200, 300, etc.) or arrays (e.g., 499, 900) described herein.
For example, the GUV light source 100’ may include a light emitting element 110 implemented by
a pump laser 110’ (e.g., a Group‐III nitride‐based laser diode, such as a blue pump las
er diode) or light emitting diode (LED) configured to generate visible light 111’, and a
nonlinear optical element 120 (e.g., a nonlinear optical crystal) that is configured to receive the v
isible light 111’ from the light emitting element
110 and generate far‐UVC light 121’ of a second
frequency based on the visible light 111’ of the
first frequency (e.g., based on SHG or SFG). The
nonlinear optical element 120 may be optically transparent to wavelengths at or below the desired o
utput wavelength (e.g., the far‐UVC wavelength range). An input coupling element 115 (e
.g., a continuous waveguide that connects radiation from the pump laser 110’ or LED to the
nonlinear optical crystal) may be configured to couple light from the pump laser 110’ into the no
nlinear optical element 120. In some embodiments, phase matching may be provided between t
he SHG/SFG light 121’ and the fundamental (pump) wavelength light 111’. An outpu
t coupling element 130 is configured to outcouple the SHG/SFG light 121’ from the nonlinear
optical element 120, either selectively or in combination with the visible light 111’ (that i
s, such the light output includes the far‐UVC light 131’ alone, or the far‐UVC light 131’ of
the second frequency alone, or in combination with
the visible light 111’ of the first (fundamental)
frequency) as output light 131’. However, it will
be understood that embodiments of the present disclos
ure may be used for sensor feedback‐ based control of other GUV light sources. [0230] Still referring to FIG. 17, a sensor suite including
one or more sensors 1750 of various types are configured to detect real‐time conditions
in the operating environment of the GUV light source. Communication between the sensor suite
and the GUV light source 100’ may be provided by a controller 1701 and/or other communicat
ive coupling. The communication/controller 1701 is configured to provide
information obtained by the sensors 1750 back to the GUV device 1700 in order to contr
ol the operation of the GUV light source 100’ for light generation 131. [0231] The sensors 1750 are thereby configured to provide i
nformation feedback to improve or optimize the operation of the GUV illuminator 1700 f
or a desired application. The sensors 1750 may also be configured to detect and communicate inf
ormation for purposes other than operation of the GUV illuminator 1700. Examples of
possible sensors 1750 include, but are not limited to, air quality sensors (such as humidity, t
emperature, VOC, chemical sensors (CO2, CO, etc.), particular matter sensors, and aerosol sensors;
biological sensors such as virus or pathogen detectors, etc.; radar sensors, e.g., for as
sessing distance to objects; 2D camera sensors, e.g., for assessing conditions inside the ar
ea of operation including personnel and occupancy; 3D cameras or lidar systems e.g., for m
easuring distances to objects, occupancy, motion, etc.; irradiation sensors, e.g., for assessing
the intensity of GUV irradiation within a field of view over the course of time; and/or passi
ve infrared (IR) or other motion sensors. [0232] The communication channel 1702 between the sensors 17
50 and the controller 1701 may be bi‐directional, so that information from the
GUV light source 100’ can be shared with the sensor suite 1750 in order to obtain more accur
ate measurements of the environment. That is, the controller 1701 may be configured to c
ontrol operation of the GUV light source 100’ based on the information or data output from the se
nsors 1750, and/or to control operation of the sensors 1750 based on the operation and/or light
output 131 of the GUV light source 100’. [0233] Furthermore, the components (e.g., 110’, 1701, 1750)
of the GUV illumination device 1700 may or may not be integrated within a same ho
using. For example, it will be understood that one or more sensors 1750 of the sensor suite
and GUV light source 110’ need not be contained within the same physical housing, and/or ne
ed not even be collocated. More generally, embodiments of the present disclosure may
include any configuration whereby the sensor information can be communicated with a GUV li
ght source to control operation of the GUV light source based on the sensor information.
It will be understood that the GUV light source may be a UV light source (e.g., 100, etc.)
as described herein, or may be another light source (e.g., a non‐solid state light source, such
as an excimer lamp or other conventional UV light source). That is, the operations and componen
ts of FIG. 17 may be used with any UV light source, including (but not limited to) the UV light
sources described herein. [0234] Benefits of embodiments of the present disclosure may
include overall optimization of the operation of GUV illumination systems, including
maximization of pathogen disinfection per unit cost. Cost can be reduced, for example, by
more effectively operating the illumination devices (e.g., operating the GUV light source at hig
her intensities for short periods of time), operating the GUV light source when the sensors indi
cate that value is maximized, and/or by utilizing fewer units to cover a given space (thus
reducing cost). Cost can also be reduced by reducing or minimizing the overall time that a given
GUV light source is on, i.e., effectively reducing the duty factor. This can extend the li
fetime of the GUV light source and thus reduce overall operating cost. Beneficiaries of such improv
ed operation and/or optimization may include both customers, system operators, and also an
y persons who come into contact with the disinfection technology. [0235] Commercial applications for far‐UVC illumination in
accordance with embodiments of the present disclosure can include elimination of pat
hogens from air and/or surfaces in any indoor spaces where humans congregate (e.g., airports,
schools, hospitals, inpatient care centers, workplaces, etc.), as well as in transport
ation vehicles (e.g., subway cars, trains, taxis, airplanes) and agricultural settings (e.g., animal pro
duction facilities, meatpacking facilities, indoor greenhouses, etc.). [0236] Additionally the generation of far‐UVC light 121’
may have numerous applications beyond germicidal use, which may include (but are no
t limited to) spectroscopy, optical sensing, detection, etc. In particular, UV light so
urces configured to provide far‐UVC illumination in accordance with embodiments of the pr
esent disclosure can be used the detection of trace chemical or biological species in
various field environments (air, water, etc.), in which UV fluorescence and Raman spectroscopy are
widely used and developed. The use of extremely short wavelength (e.g., in the far‐UVC wa
velength range) excitation for such applications may be beneficial to each in different
ways. For example, the efficiency of Raman scattering may scale inversely with excitation wavelen
gth to the fourth power (1/λ 4 ). For fluorescence applications, moving the excitation wavele
ngth further into the UV range can open up a wider spectral range of possible emission,
with reduced or minimal background from the excitation wavelength or background light. [0237] Some existing light sources used to generate these (
far) UV wavelengths may be expensive, large, may not achieve the required wavele
ngths, and/or may not be human safe. In contrast, UV light sources in accordance with embodim
ents of the present disclosure may provide several attributes that may be particularly u
seful for Raman and/or UV spectroscopy applications, including (but not limited to) (a) smal
l size per unit optical output, (b) low cost, (c) ability to operate in the solar blind region of
the visible spectrum (i.e., with emission wavelengths in a spectral range that is free of bac
kground noise from the sun), and (d) emission in human safe wavelength ranges. Embodiments describ
ed herein can thereby provide new ways of deploying fluorescence and Raman spectroscopy
into low cost handheld devices or low cost wall mountable devices that monitor environments
in which people are persistently present. [0238] UV light sources according to embodiments of the pre
sent disclosure may further generate output light 131 (e.g., SHG/SFG light 121’
)over a very narrow bandwidth (e.g., with an emission linewidth or bandwidth of less than about 1
nm, for example, less than about 0.5 nm, or less than about 0.1 nm ). In some embodiment
s, the output light 131 may be emitted from an edge of the output coupling element 130, for exa
mple, as a coherent beam. That is, in addition to providing output light 131 in the far‐
UVC wavelength range (about 200‐240nm), the linewidth of the emission from some embodiments of o
ur invention may be, for example, less than about 0.1 nm, which is far narrower than some
conventional light sources. Raman spectroscopy applications, in particular, may benefit
from an extremely narrow spectral width for the light source. [0239] Various embodiments have been described herein with r
eference to the accompanying drawings in which example embodiments are shown. Th
ese embodiments may, however, be embodied in different forms and should not be constr
ued as limited to the embodiments set forth herein. Rather, these embodiments are provided
so that this disclosure is thorough and complete and fully conveys the inventive concept to
those skilled in the art. Various modifications to the example embodiments and the gene
ric principles and features described herein will be readily apparent. In the drawings, th
e sizes and relative sizes of layers and regions are not shown to scale, and in some instances may
be exaggerated for clarity. [0240] The example embodiments are mainly described in terms
of particular methods and devices provided in particular implementations. However
, the methods and devices may operate effectively in other implementations. Phrases
such as "example embodiment", "one embodiment" and "another embodiment" may refer to the
same or different embodiments as well as to multiple embodiments. The embodiments will
be described with respect to systems and/or devices having certain components. However, the
systems and/or devices may include fewer or additional components than those shown, and
variations in the arrangement and type of the components may be made without departing from
the scope of the inventive concepts. [0241] The example embodiments will also be described in th
e context of particular methods having certain steps or operations. However, the meth
ods and devices may operate effectively for other methods having different and/or additional
steps/operations and steps/operations in different orders that are not inconsistent with the
example embodiments. Thus, the present inventive concepts are not intended to be limited to
the embodiments shown, but are to be accorded the widest scope consistent with the princip
les and features described herein. [0242] It will be understood that when an element is refer
red to or illustrated as being "on," "connected," or "coupled" to another element, it can
be directly on, connected, or coupled to the other element, or intervening elements may be pr
esent. In contrast, when an element is referred to as being "directly on," "directly connect
ed," or "directly coupled" to another element, there are no intervening elements present.
[0243] It will also be understood that, although the terms
first, second, etc. may be used herein to describe various elements, these elements should n
ot be limited by these terms. These terms are only used to distinguish one element from
another. For example, a first element could be termed a second element, and, similarly, a
second element could be termed a first element, without departing from the scope of the pre
sent disclosure. [0244] Furthermore, relative terms, such as "lower" or "bott
om" and "upper" or "top," may be used herein to describe one element's relationship to
another element as illustrated in the Figures. It will be understood that relative terms
are intended to encompass different orientations of the device in addition to the orient
ation depicted in the Figures. For example, if the device in one of the figures is turned over, e
lements described as being on the "lower" side of other elements would then be oriented on "upper"
sides of the other elements. The exemplary term "lower", can therefore, encompasses bot
h an orientation of "lower" and "upper," depending of the particular orientation of t
he figure. Similarly, if the device in one of the figures is turned over, elements described as "b
elow" or "beneath" other elements would then be oriented "above" the other elements. The e
xemplary terms "below" or "beneath" can, therefore, encompass both an orientation of above and
below. [0245] The terminology used in the description of the inven
tion herein is for the purpose of describing particular embodiments only and is not int
ended to be limiting of the invention. As used in the description of the invention and the ap
pended claims, the singular forms “a”, “an” and “the” are intended to include the plural for
ms as well, unless the context clearly indicates otherwise. [0246] It will also be understood that the term "and/or" a
s used herein refers to and encompasses any and all possible combinations of one
or more of the associated listed items. It will be further understood that the terms “incl
ude,” “including,” "comprises," and/or "comprising," when used in this specification, specify
the presence of stated features, integers, steps, operations, elements, and/or components, but do
not preclude the presence or addition of one or more other features, integers, steps, oper
ations, elements, components, and/or groups thereof. [0247] Embodiments of the invention are described herein wit
h reference to illustrations that are schematic illustrations of idealized embodiments (
and intermediate structures) of the invention. As such, variations from the shapes of
the illustrations as a result, for example, of manufacturing techniques and/or tolerances, are to be
expected. Thus, the regions illustrated in the figures are schematic in nature and their sh
apes are not intended to illustrate the actual shape of a region of a device and are not intended
to limit the scope of the invention. [0248] Unless otherwise defined, all terms used in disclosin
g embodiments of the invention, including technical and scientific terms, have the sa
me meaning as commonly understood by one of ordinary skill in the art to which this inv
ention belongs, and are not necessarily limited to the specific definitions known at the time of the p
resent disclosure being described. Accordingly, these terms can include equivalent terms
that are created after such time. It will be further understood that terms, such as those defi
ned in commonly used dictionaries, should be interpreted as having a meaning that is consisten
t with their meaning in the present specification and in the context of the relevant art
and will not be interpreted in an idealized or overly formal sense unless expressly so defined herei
n. All publications, patent applications, patents, and other references mentioned herein are in
corporated by reference in their entireties. [0249] Many different embodiments have been disclosed herein,
in connection with the above description and the drawings. It will be understood
that it would be unduly repetitious and obfuscating to literally describe and illustrate every
combination and subcombination of these embodiments. Accordingly, the present specification,
including the drawings, shall be construed to constitute a complete written description
of all combinations and subcombinations of the embodiments of the present dis
closure described herein, and of the manner and process of making and using them, and sh
all support claims to any such combination or subcombination. [0250] Although the invention has been described herein with
reference to various embodiments, it will be appreciated that further vari
ations and modifications may be made within the scope and spirit of the principles of th
e invention as set forth in the following claims.