FIELD OF THE INVENTION

The present invention relates to an illumination device, and in particular to an illumination device comprising a plasmonic antenna array. BACKGROUND OF THE INVENTION

For light emitting diodes (LEDs) for use in lighting applications, it is desirable to provide essentially white light having a color temperature approximately comparable to that produced by incandescent lighting.

White light from LEDs is commonly provided by using a pn-diode emitting blue light, having a wavelength around 450 nm, where part of the blue light is converted to longer wavelengths using one or more wavelength converting materials arranged on top of or in the vicinity of the diode. By combining the converted light with the unabsorbed blue light, a reasonably broadband spectrum which is perceived as white light can be obtained.

Currently, in most commercial applications, the wavelength converting material is applied directly on the LED. Furthermore, the wavelength converting material is often scattering in order to obtain a low variation in color over angle. This means that blue light will also be scattered back into the diode which leads to absorption losses in the LED. Moreover, the active component of the wavelength converting material, commonly phosphor, is an isotropic emitter, meaning that the same amount of wavelength converted light is emitted in all directions. This leads to further losses as only a portion of the light escapes through the output surface of the light emitting device. Further, in etendue limited

applications only part of the generated light can be used, limiting the system efficiency.

The problem of reducing losses has for example been addressed by using a phosphor which is less scattering to reduce the amount of blue light which is backscattered and absorbed by the diode. However, the isotropic emission from the phosphor remains.

The amount of light leaving the light emitting device may also be increased by introducing a photonic band gap material in which the emission direction can be modified. However, to be able to control the emission direction, a photonic band gap material needs to be made from materials having a high refractive index contrast, high aspect ratio holes or pillars must be patterned and formed, the size control is very strict and the material must be luminescent which will incur scattering losses. Furthermore, a photonic band gap material is only really effective in the plane perpendicular to the surface of the material, i.e. in a direction parallel to the holes or pillars.

Accordingly, the suggested approaches for increasing the emission efficiency of a light emitting device suffer from inherent drawbacks which are hard to overcome.

SUMMARY OF THE INVENTION

In view of the above-mentioned desired properties of a light emitting device, and the above-mentioned and other drawbacks of the prior art, it is an object of the present invention to provide an improved light emitting device.

According to a first aspect of the invention, an illumination device is provided comprising: a light source configured to emit light of a first wavelength; a wavelength converting layer comprising a wavelength converting material configured to receive light from the light source, and further configured to convert light from the first wavelength to a second wavelength; a periodic plasmonic antenna array, arranged embedded within the wavelength converting layer, and comprising a plurality of individual antenna elements arranged in an antenna array plane, the plasmonic antenna array being configured to support a first mode of surface lattice resonances at the second wavelength, arising from diffractive coupling of localized surface plasmon resonances in the individual antenna elements, wherein the plasmonic antenna array is configured to comprise plasmon resonance modes such that light emitted from the plasmonic antenna array has an anisotropic angle distribution; and wherein the light source is further arranged to emit light in the form of a plane wave having an angle of incidence in relation to the antenna array plane such that an electric field intensity in the antenna elements, resulting from the plasmon resonance modes, is minimized.

The field of plasmonics refers to the interaction of small conducting structures, typically metal structures, with light, whereby the size of the metal structures is similar to the wavelength of the light. The conduction electrons in the metal respond to an external electric field and the electron cloud oscillates at the driving optical frequency, leaving behind a more positive charged area, which pulls back the electrons. Due the small size of the metal structures, the resonances can reach the frequencies of visible light. As a result, a metal structure can have a large scatter cross-section which allows a strong interaction with any light that is incident on the metal particles or with any light that is generated in close proximity to the metal particles. It has been found that regular arrays exhibit of such metal particles, herein also referred to as antenna elements, exhibit strong enhancement in directionality of the emission which is attributed to hybrid coupled Localized Surface Plasmon Resonance (LSPR ) and photonic modes, also referred to as hybrid lattice plasmonic photonic modes, or plasmonic- photonic lattice resonances. The directionality enhancement of the emission is herein referred to as anisotropic emission, i.e. non-Lambertian emission.

Ordered arrays of optical antennas support collective resonances and when the wavelength of the radiation is in the order of the periodicity of the array, a diffracted order can radiate in the plane of the array. In this way the localized surface plasmon polaritons sustained by the individual particles may couple via diffraction leading to collective, lattice- induced, hybrid photonic-plasmonic resonances known as surface lattice resonances (SLRs). These delocalized modes extend over several unit cells, making it possible to obtain a collective enhancement of the emission from emitters distributed over large volumes as is required in solid-state lighting.

Here, use is made of periodic arrays of nanoparticles that behave as collective nano-antennas. These arrays sustain collective plasmonic resonances. On the one hand, metallic nanoparticles have large scattering cross sections that allow resonant excitation of phosphors in the wavelength converting material, thereby enhancing the converting of light. On the other hand, collective plasmonic resonances enable shaping the angular pattern of the emission, beaming most of the light into a narrow angular range in a defined direction at a certain wavelength range. Therefore, the directional enhancement is explained as the combination of an increased efficiency in the excitation of the wavelength converting medium and an enhancement of the out-coupling efficiency of the emission of the phosphors to extended plasmonic-photonic modes in the array and the subsequent out-coupling to free- space radiation.

A more detailed description of the function and configuration of a plasmonic antenna array can be found in WO2012/098487, and in unpublished European patent application EP13179374.

The combination of a plasmonic antenna array and a wavelength converting material in a wavelength converting layer may also be referred to as a plasmonic-based phosphor, and the resulting illumination device including a light source is sometimes referred to as a LED-phosphor system.

The present invention is based on the realization that a desirable anisotropic light distribution can be achieved by configuring a plasmonic antenna array such that it supports plasmonic-photonic lattice resonances. However, when using a plasmonic antenna array comprising metallic antenna elements, losses in the metal lead to a reduced quantum efficiency, also referred to as external photoluminescence quantum yield (EQY), of the whole LED-phosphor system Plasmonic-based phosphors are thus able to enhance the emission in defined directions but at the expense of reduced overall efficiency (for integrated emission over all the angles).

In view of this, the present invention is further based on the realization that losses in the metal particles can be minimized by providing the incoming light, also known as pump light, in the form of a plane wave and by selecting the angle of incidence of the incoming light such that an electric field intensity in the antenna elements, resulting from the plasmon resonance modes, is minimized.

By controlling the way in which the plasmonic antenna array is illuminated, and in particular by controlling the incident angle of an incoming plane wave, the external quantum efficiency of a plasmonic-based wavelength converting material can be largely improved. In particular, it is desirable to minimize the electric field intensity close to and in the metallic particles, and to ensure instead that field maxima are located at a distance from the metallic particles, thereby reducing losses in the metallic particles.

As the field distribution of the lattice resonance modes is far from homogeneous, and as the field distribution is determined both by the configuration of the antenna array and on the wavelength and direction of incoming light, the resulting electric field distribution in the wavelength converting layer can be determined for a known antenna array and for known characteristics of the incoming light. Conversely, starting from a desirable field distribution, simulations can provide an antenna array configuration and pump light parameters for incoming light resulting in minimal losses in the metallic particles, thereby improving the overall external photoluminescence quantum yield in the illumination device. Accordingly, the overall efficiency of the device can be improved by correlating the incident angle of pump light with the properties of the plasmonic antenna array.

According to one embodiment of the invention, the angle of incidence may advantageously be selected based on the configuration of the antenna array. The resulting field distribution from the plasmonic antenna array depends on the parameters of the antenna array such as the two-dimensional lattice of the array, i.e. square, rhombic, hexagonal etc. and the pitch, or lattice constant, of the array. Thereby, the angle of incidence to achieve the optimal EQY can be simulated based on a known antenna array geometry. Moreover, the angle of incidence may advantageously be selected based on the geometry of the antenna element. The resulting field distribution is also sensitive to the specific geometry of the antenna elements. Thus, the shape and dimensions of the antenna element may be tailored to achieve a preferred field distribution, for example to avoid that field maxima are located at the locations of the antenna elements.

Furthermore, the angle of incidence may also be selected based on the first wavelength, i.e. the wavelength of the incoming light, also referred to as pump light, since the field distribution depends also on the wavelength in combination with the antenna array. Thus, different starting points may be used to achieve the optimal EQY for a given set of circumstances. For example, the starting point may be the wavelength of pump light, coming from a known light source such as a semiconductor laser. In that case the plasmonic antenna array is configured to convert light to the desired wavelength together with the wavelength converting material, while the angle of incidence is selected based on the wavelength and array configuration.

According to one embodiment of the invention, the light source and/or the wavelength converting layer may advantageously be configured such that said angle of incidence is tunable within a predetermined angle range. The angle of incidence could for example be tunable by combining the periodic antenna array with active materials that permit to actively control the separation between the particles in the array such as in stretchable or swellable elastomers. A tunable angle of incidence is advantageous as it allows for optimization of the angle for different wavelengths. An illumination device having a controllable angle of incidence could thus be combined with a light source having a tunable wavelength to the effect that the illumination device is capable of emitting a range of wavelength with a minimum of losses in the metallic particles. A tunable incident angle may also be achieved by means of suitably arranged controllable optics arranged between the light source and the plasmonic antenna array. Furthermore, the tunable incident angle may be achieved by a mechanical arrangement where the orientation or tilt of the light source can be adjusted in relation to the plasmonic antenna array.

According to one embodiment of the invention, the light source may be a resonant cavity light emitting diode, a photonic crystal light emitting diode in combination with a resonant cavity light emitting diode, or a plasmonic light emitting diode. Photonic crystal-based diffracting layers show an improvement in the light extraction of LEDs and in the control of the radiation pattern. Such dielectric periodic nanostructures are directly applied over the quantum well, so no other external optical element would be required to control the directionality of the LED emission.

A resonant cavity LED restricts the amount of waveguide modes, and therefore has a preferential emission under certain angles. For an optimal efficiency, angular emission and extraction, photonic crystal patterns are combined with resonant cavity LEDs to ensure high mode overlap. The combination will give a LED pump light source with a desired emission angle which corresponds to the optimal pumping angle of the plasmonic antenna array.

In one embodiment of the invention, the light source may comprise a

Lambertian light emitter and collimating optics configured to achieve a plane wave. In applications where it is impractical or not possible to use a light source directly producing a plane wave, light in the form of a plane wave may equally well be provided by collimating optics.

In one embodiment of the invention, the wavelength converting material may advantageously be selected from the group comprising rare earth ions, dye molecules and quantum dots. The wavelength converting material may be a material that comprises different types of dyes and phosphors known by the person skilled in the art. Furthermore, the wavelength converting medium may also comprise a line emitter in the form of an ion of a rare earth element. It should also be understood that the wavelength converting materials may also be referred to as fluorescent materials, phosphors or dyes, and in general as photon emitters.

In one embodiment of the invention, the wavelength converting layer may comprise a quantum well structure. A quantum well structure can be arranged on top of the first layer, and both the optical properties and physical thickness can be controlled to achieve the desire properties. The illumination device may also be formed by first epitaxially growing e.g. a III-V QW structure and applying the plasmonics on top of the QW structure.

According to one embodiment of the invention, the first wavelength may be 448 nm, said antenna array is a square antenna array of metallic nanoparticles having a lattice constant of 400 nm and said angle of incidence is θ=4° and φ=0° in polar coordinates. The first wavelength may be selected to be 448 nm which is a common wavelength for commercially available blue lasers. The polar coordinates comprising the radius Θ and the polar angle φ can also be explained in Cartesian coordinates as x=cos9 coscj), y=cos9 · sincj).

In one embodiment of the invention, the illumination device may further comprise a cover layer, arranged on the wavelength converting layer, the cover layer having a refractive index equal to the refractive index of the wavelength converting layer. The spectral position of the lattice modes for a given antenna geometry depends on the total thickness of the layer or layers arranged to cover the antennas, and on the refractive index of such layer/layers. Therefore, a cover layer may be employed on the wavelength converting layer to achieve a targeted total thickness of the wavelength conversion layer.

In one embodiment of the invention, the illumination device may further comprise a cover layer, arranged on the wavelength converting layer, the cover layer having a refractive index higher than the refractive index of the wavelength converting layer. Arrays of metal nanoparticles can support delocalized plasmonic-photonic hybrid states due to localized surface plasmon polaritons (LSPPs) as described above coupled to diffracted or refractive-index guided modes. In order to support the refractive-index guided modes, the refractive index of the layer in which the antenna array is arranged should be higher than that of the substrate. By providing an additional layer, i.e. a cover layer, having a higher refractive index than the wavelength converting layer, controlled refractive-index guided modes are further facilitated.

According to one embodiment of the invention, the illumination device may further comprise a first cover layer, arranged on the wavelength conversion layer, the first cover layer having a refractive index equal to the refractive index of the wavelength converting layer, and a second cover layer, arranged on the first cover layer, the second cover layer having a refractive index higher than the refractive index of the wavelength converting layer.

According to one embodiment of the invention, the antenna array may comprise a plurality of truncated pyramidal antenna elements having a top side in the range of 110 to 130 nm, a bottom side in the range of 135 to 155 nm, and a height in the range of 140 to 160 nm, and wherein the antenna elements are arranged in a square array having a lattice constant in the range of 300 to 450 nm, or in a hexagonal lattice having a lattice constant in the range of 300 to 500nm. In embodiments the antenna elements are made from Aluminum. The sides are defined as the length of the sides of a square or rectangle or triangle.

Further features of, and advantages with, the present invention will become apparent when studying the appended claims and the following description. In principle, several light sources emitting light of different wavelengths may be used, for example in combination with several antenna arrays arranged in different planes. The skilled person realize that different features of the present invention may be combined to create embodiments other than those described in the following, without departing from the scope of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

This and other aspects of the present invention will now be described in more detail, with reference to the appended drawings showing embodiments of the invention.

Fig. 1 is a schematic illustration of an illumination device according to an embodiment of the invention;

Fig. 2 is a schematic illustration of an illumination device according to an embodiment of the invention;

Fig. 3 is a schematic illustration of an illumination device according to an embodiment of the invention;

Figs. 4a-f illustrate numerical simulations of illumination devices according to embodiments of the invention; and

Figs. 5a-d illustrate numerical simulations and measurement results of illumination devices according to embodiments of the invention.

DETAILED DESCRIPTION

The present invention will now be described more fully hereinafter with reference to the accompanying drawings, in which exemplary embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided for thoroughness and completeness, and fully convey the scope of the invention to the skilled person. Like reference characters refer to like elements throughout.

Fig. 1 is a schematic illustration of an illumination device 100 comprising a wavelength converting layer 104 comprising a wavelength converting material in the form of a plurality of wavelength converting particles configured to convert light from a first wavelength to a second wavelength. A plurality of antenna elements 108 is arranged in the wavelength converting layer 104 to form an antenna array. The antenna array is configured to support lattice resonances at the second wavelength, emitted by the wavelength converting material, arising from diffractive coupling of localized surface plasmon resonances in the individual antenna elements such that light emitted from the plasmonic antenna array has an anisotropic angle distribution 110 illustrated in Fig. 1. By tuning the configuration of the antenna array, the angle distribution of emitted light from the light emitting surface can be controlled, illustrated by the angle 112, such that light is emitted within a predetermined angle range, i.e. an anisotropic distribution of the emitted light.

The wavelength converting particles, i.e. photon emitters, may for example be dye molecules configured to convert blue light into light having longer wavelengths. Suitable dye molecules may be provided in a polymer to provide a desired dye concentration in a polymer-based wavelength converting layer 104. Typically, it is desirable to achieve white- light by wavelength converting of blue or UV light from InGaN-based LEDs by suitable color converters, known as phosphors. In general, the wavelength converting particles may be excited through addition of any type of energy such as photons, heat, electrons, x-rays etc. In Fig. 1, the wavelength converting particles are illustrated as being homogeneously distributed in the wavelength converting layer 104. However, the wavelength converting particles may equally well have a non-uniform distribution in the wavelength converting layer 104. The wavelength converting particles may for example be arranged within a region in a plane at a predetermined distance from the antenna array. The illumination device 100 is illustrated as receiving light from a light source 102, where the light source provides light in the form of a plane wave, which has a predetermined angle of incidence in relation to the plane of the antenna array.

Fig. 2 further illustrates a periodic plasmonic antenna array comprising a plurality of individual antenna elements 108 arranged in an antenna array plane. The antenna elements 108 are here illustrated as having a square cross section as seen from above.

However, the antenna elements 108 may equally well have a polygonal or circular cross section, and they may or may not be truncated. The antenna array is here arranged within the wavelength converting layer 104.

Fig. 3 illustrates an illumination device 300 where an additional layer 302, i.e. a cover layer or top layer, is arranged on the wavelength converting layer 302. By providing the cover layer 302 in a material having a refractive index lower than the refractive index of the wavelength converting layer 104, the formation of refractive-index guided modes are facilitated. A waveguide is not per-se required, as surface lattice modes or other hybrid modes can be achieved without a waveguide layer. However from calculations and experiments it has been observed that hybrid lattice-waveguide modes are efficient.

Numerical simulations have been employed to study the influence of the metal nanoparticle array on how the optical pump light is absorbed in the plasmonic-based phosphor. In order to calculate the spatial distribution of the total electric field, simulations consider a plane wave with a wavelength λ=448 nm. The fraction of the incident light absorbed by the phosphor (A _em ) can be calculated according to the following expression:

where Ω is the solid angle associated to the elevation and the azimuthal angle of illumination (θ, φ). Ε τ, λ, Ω) is the local field at the wavelength of excitation λ and at the position r where each emitter is located. V _em is the volume over which the emitters are distributed. e _em is the dielectric permittivity of the emitter layer and e ₀ is the vacuum dielectric permittivity. c represents the speed of light in vacuum. / ₀ corresponds to the incident intensity.

Analogously, the fraction of light absorbed by the metal A _metal can be determined as:

1 1

7 ^e 0 Im Ametal )^nc J \E(f, λ, ) \ ² dV

Δ ( i — ' metal Λ

> 0

V-metai is ^me volume occupied by the metal nanoparticles and e _metal is the dielectric permittivity of the metal. A _em and A _metal depend on the one hand on the optical properties of the phosphor and the metal, respectively. On the other, A _m and A _metal scale with the spatial distribution of the electric field intensity which in turn depends on the angle Ω. We indicate the fraction of the incident light absorbed by the plasmonic-based phosphor as A _total = A - A

Figs. 4a-f schematically illustrate results of numerical simulations for a square array of aluminum nanoparticles with a lattice constant of 400 nm, deposited over a substrate with a refractive index of 1.46, covered by a 600 nm-thick layer of material with a refractive index of 1.608+0.004i. The simulations consider a plane wave incident to the array with a wavelength of λ=448 at an angle defined by (θ, φ).

Fig. 4a illustrates the absorption in the dye layer A _em and Fig. 4b illustrates absorption in the metal, A _metal . Results are plotted in polar coordinates, being Θ the radius and φ the polar angle. Fig. 4a shows bands of high absorption in the emitter layer that are associated to lattice modes (LMs). Specifically, they correspond to the fundamental (m=0) TM-and TE-waveguide-plasmon polaritons, calculated at λ=448 nm, assuming that the wavelength converting layer acts as a waveguide. The coupling of the pump light to LMs leads to a large electric field intensity in the regions of the space where the emitters are distributed, resulting in an increased A _em .

Figs. 4c-f show numerical simulations of spatial distributions of the total electrical field intensity (E) normalized to the incident field intensity (Eg) in a unit cell of the array for a plane wave incident at two different angles relative to the surface of the plasmonic-based phosphor. The field intensity enhancement is shown in a plane intersecting the nanoparticles in a unit cell of the array. These simulations reveal that the complex photonic environment forming the sample gives rise to an inhomogeneous spatial distribution of the local field intensities around the metal nanoparticles. Specifically, Figs. 4c-d and Figs. 4e-f display the field intensity associated to two different angles of illumination: θ=4° and φ=0° and 0=13° and φ=45°, respectively. As a result of the interaction between the modes supported by the array and the optical pump field, a distinct field intensity distribution is found in the regions where the emitters and the metal are distributed. A large intensity enhancement is found in the emitter layer at θ=4° and φ=0° as shown in Fig. 4c. In contrast, when the same structure is illuminated at 0=13° and φ=45° illustrated in Fig. 4e, the electric field intensity is reduced in the layer whereas it increases in the metal nanoparticle.

Consequently, it is possible to engineer the illumination of the plasmonic-based phosphor such that the inherent losses associated with the presence of the metal are minimized whereas the light conversion is optimized.

Numerical simulations have been verified by fabricating plasmonic-based phosphors and measuring the A _total and the EQY as a function of the illumination angle using a λ=448 nm laser source.

The array of aluminum nanoparticles is fabricated over a silica substrate using a nanoimprint lithography technique, called substrate conformal imprint lithography (SCIL), in combination with reactive ion etching (RIE). Over the array, a 600 nm-thick dye-doped polystyrene layer is deposited. The same dye layer is also deposited over the flat silica substrate which serves as a reference. The rest of the substrate is not covered by the dye and it is sand-blasted on the back side. In order to measure the angular dependence of the A _total and the EQY, an integrating sphere, in which the illumination angle (θ, φ) can be controlled, is employed.

Figs. 5a-b illustrates measured (top panels) and simulated (bottom panels) Atotal ^{as a} function of Θ and with φ=0° (Fig. 5a) and φ=45° (Fig. 5b) for a square array of aluminum nanoparticles with a lattice constant of 400 nm, deposited over a substrate with a refractive index of 1.46, covered by a 600 nm-thick layer of material with a refractive index of 1.608+0.004i. The measurements confirm the numerical predictions.

In Figs. 5c-d, the upper panels display the measured EQY as a function of Θ and φ=0° and φ=45°, respectively. In order to compare with the numerical simulations, the ratio A _em /A _total is calculated and plotted as a function of the illumination angle in the bottom panels of Figs. 5c-d. From Figs. 5c-d it can be seen that the EQY fluctuates around EQY-0.4 at φ=0° and EQY-0.3 for φ=45°.

The combined experimental and numerical characterization of the light absorbed in plasmonic-based phosphors suggests that a careful design of the illumination of the phosphor leads to a significant improvement on the EQY.

Additionally, variations to the disclosed embodiments can be understood and effected by the skilled person in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims. The plasmonic antenna array may be arranged and configured in different ways to support resonance modes for light of different wavelengths and at different incident angles.

In the claims, the word "comprising" does not exclude other elements or steps, and the indefinite article "a" or "an" does not exclude a plurality. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measured cannot be used to advantage.