The present invention relates to a photomixer for generating terahertz electromagnetic radiation in response to illumination by a time-modulated optical signal. The photomixer comprises a carrier substrate with a plurality of quantum dots arranged in an emission region of the carrier substrate. A laser cavity is arranged around the emission region of the carrier substrate and an incident light passage is adapted to directing the time-modulated optical signal to the emission region. Stimulated emission via the laser cavity may be applied to the emission region such that recombina- tion of trapped electrons and holes in the plurality of quantum dots is accelerated to efficiently deplete the trap states.

BACKGROUND OF THE INVENTION

The general operating principles of a photomixer includes illuminating an intrinsic semiconductor material with a time-modulated laser light output, where time modulation of laser intensity lies in a terahertz (THz) frequency range. This can for example be achieved by dual-wavelength laser output where two laser modes are detuned by a frequency of e.g. 1 THz or by ultrafast pulse laser operation where an optical bandwidth of each laser pulse lies in the THz range. When incident onto the semi- conductor material, the laser light illumination results in conductivity of the semiconductor material as free electrons and holes are generated. If the semiconductor material exhibits an ultra-short free carrier lifetime, i.e. photo-carriers are free for a time comparable to, or shorter than, the optical excitation duration, the induced semiconductor photoconductivity becomes essentially a replica of the temporal intensity pro- file of the laser light output. The laser light temporal intensity profile changes on picosecond or sub-picosecond timescale corresponds to a THz frequency bandwidth. When a bias voltage is applied to the semiconductor material externally, a photocurrent will flow in the semiconductor material. This photocurrent will be time-modulated according to the photoconductivity dynamics and will therefore reflect the ultrafast dynamics of the optical illumination or excitation of the laser light output. This results in an ac current: a quasi-continuous wave (at the dual-wavelength laser light excitation), or pulsed signal (at the pulsed ultrafast laser light excitation). The ac photocurrent, modulated at sub-picosecond rate, will radiate into free-space as terahertz (THz) electromagnetic radiation according to the basic rules of electrodynamics (in similar fashion as radio signals are generated but at much faster rate, i.e. terahertz rates). This mechanism results in the free-space THz electromagnetic radiation which may be out-coupled or transmitted efficiently in numerous ways for example using metallic micro-antennas.

THz electromagnetic radiation is highly useful in numerous applications such as ultra-high speed wireless communication systems and components, THz spectroscopy and metrology, sensing for non-destructive evaluation and security, etc.

Existing photomixers have an operational speed limited by their recovery speeds. For this reason only a single broadband pulse of THz radiation can be efficiently emitted by existing photomixers. After the emission of the initial or first broadband pulse internal relaxation processes within the photomixer, following the photoexcita- tion, prevent efficient THz pulse generation for a certain time interval. In conventional photomixers the repetition rate, the frequency at which a train of isolated terahertz pulses of maximal amplitude can be efficiently emitted, is practically limited to about 10 GHz. This low frequency or pulse repetition rate of conventional photomixers in particular prevents their efficient use in numerous applications such as ultra-high speed wireless data communications in the THz frequency range. Such ultra-high speed wireless data communications require efficient conversion of very high bit rate optical signals (e.g. Terabit-per-second, Tbit/s) into THz electromagnetic radiation. The slow recovery of conventional photomixers is caused by a long (> 100 picoseconds) spontaneous recombination time of spatially separated electrons and holes in the semiconductor material or the electrons and holes trapped onto the material defects.

Consequently, it would be desirable to provide a photomixer capable of operating at higher recovery speeds, hence providing optical pulse conversion rates leading to higher bit-rates than existing photomixers. The present invention achieves this goal and many others by efficient trapping of the free electrons and holes into a plurality of quantum dots (QDs) located in a matrix material. Stimulated emission is applied to the QD semiconductor material to accelerate recombination of the trapped electrons and holes, thereby depleting or emptying the QDs which are the trap states. These processes lead to a significant reduction of the recovery time of the carrier material and leads to much higher achievable operational speeds of the photomixer.

US 201 1/0149368 A1 describes a photomixer module generating THz waves. The photomixer module comprises a semiconductor optical amplifier integrated together with a photomixer. An excited light signal is generated by beating two laser outputs having different wavelengths to generate a continuous THz wave. The exciting light signal is amplified by the optical amplifier before being applied to the photomixer. SUMMARY OF THE INVENTION

A first aspect of the invention relates to a photomixer for generating terahertz electromagnetic radiation in response to illumination by a time-modulated optical signal. The photomixer comprising a carrier substrate comprising a first bias terminal and a second bias terminal arranged at a predetermined distance apart from each other on or at a first surface of the carrier substrate. The first and second bias terminals are electrically coupled to respective portions of the carrier substrate to apply an electrical field to an emission region of the carrier substrate situated in-between the first and second bias terminals. A plurality of quantum dots are arranged in the emission region of the carrier substrate and a laser cavity having first and second optically reflective surfaces are arranged around the emission region of the carrier substrate. An incident light passage is adapted to directing the modulated optical signal to the emission region.

In accordance with the present invention, free carriers such as electrons and holes generated in the emission region of the carrier substrate in response to the illumination by the time-modulated optical signal, i.e. the photoexcitation signal, are displaced toward, and trapped in, the quantum dots of the emission region of the carrier substrate. The trapping of the free carriers in the quantum dots or QDs is a very efficient mechanism for free-carrier lifetime reduction in the carrier substrate. This trapping further contributes to the generated photocurrent, j, as expressed by equation 2 of Fig. 1 b), and to the accompanying emission of terahertz electromagnetic radiation, E _THz . Accordingly, the applied photoexcitation for example at 1 μηη wavelength leads to creation of the free carriers in two-dimensional wetting layers of the quantum dots, followed by an ultrafast, preferably on a time scale of 1 ps or shorter, free carrier trapping into strongly localized quantum-confined states of the QDs. Therefore, a short-lived in-plane conductivity of the emission region/ two- dimensional wetting layers will be achieved. Thus, abrupt switch-off or interruption of drift current in the wetting layers will be achieved, accompanied by a spike of the displacement current in the same direction as the free carriers populate the strongly polarized states within the QDs. This create a net in-plane displacement current in the wetting layers oscillating at the photoexcitation signal such as a beat frequency of a 2-color pump laser, e.g. at 1 THz. This terahertz oscillating AC displacement current or photocurrent will naturally create the desired terahertz electromagnetic radiation radiating to the free space at the same terahertz frequency, e.g. at 1 THz. However, while the carrier lifetime in the QDs is known to be long, typically a few hundreds of picoseconds, this carrier life time, which ordinarily prevents fast recovery of the trapped states, is significantly reduced by the provision of the laser cavity around the emission region holding the QDs in accordance with the present inven- tion. The laser cavity allows for the application of stimulated optical radiation to the QDs such that stimulated emission is used to empty the trapped states at a speed much faster than the spontaneous recombination time.

In one embodiment, the photomixer further comprises a reflective mirror arranged below a second surface of the carrier substrate arranged oppositely to the first surface, said reflective mirror being adapted to reflect impinging terahertz electromagnetic radiation towards the first surface. The reflective mirror may comprise a highly doped GaAs layer for example deposited or grown on top of a base, or intrinsic semiconductor material of a standard wafer such as a 3 inch or 5 inch GaAs wafer. The carrier substrate preferably comprises a layer of semiconductor material such as GaAs, InGaAs, GaAsP, GaAIAs, InGaAINAsPSb, GaN and InAIGaN.

The laser cavity is preferably configured to generate stimulated optical radiation at a wavelength between 0.5 μηη and 2.0 μηη such as a wavelength selected from a group of {1.05 μηη, 1.3 μηη, 1 .55 μηη}. The wavelength of the stimulated optical radiation is larger than the wavelengths of the applied photoexcitation signal in order for the applied excitation to create gain at the wavelength of the photoexcitation signal. The incident light passage may be arranged at various locations of the present photomixer depending on its physical layout or structure. In one embodiment, the inci- dent light passage is arranged at an opposite surface to the first surface of the carrier substrate which preferably holds the first and second bias terminals. In this manner, the photoexcitation signal can be applied from a back-side (i.e. the side opposite to the side radiating the terahertz electromagnetic radiation) of the photomixer which is convenient because the back-side can be coupled or attached to mating surface of an optical waveguide.

The photomixer may comprise a conductive, preferably metallic, antenna to optimize radiation of the generated terahertz electromagnetic radiation for example a metallic micro-antenna such as a micro-dipole or a bow-tie, a square-spiral antenna configured to out-couple the terahertz electromagnetic radiation. The metallic micro- antenna is preferably arranged above the emission region of the carrier substrate at the first side of the carrier substrate. The emission region of the carrier substrate preferably comprises a quantum dot density between 10 ¹² and 10 ¹⁴ dots/mm ³ . This density range is advantageous because it will provide a sufficient number of traps in combination with sufficient gain to achieve lasing at a low inversion. In one embodiment of the photomixer, the first and second optically reflective surfaces of the laser cavity comprise a pair of oppositely arranged high-reflector (HR) coated side surfaces of the carrier substrate. Each of the HR coated surfaces may possess a reflectivity larger than 95 % such as larger than 99 % to ensure good lasing properties of the laser cavity. In one embodiment, the photomixer additionally comprises first and second cladding layers arranged above and below, respectively, the carrier substrate to further retain optical radiation within the laser cavity. Each of the upper and lower cladding layers may comprise a layer of AIGaAs with a lower refraction index than the base substrate and the emission region of the carrier substrate, i.e. the layer with embedded quantum dots.

A second aspect of the invention relates to a method of generating terahertz electromagnetic radiation comprising steps of:

- providing an carrier substrate having a plurality of quantum dots arranged in an emission region thereof, - applying an electrical field to the emission region of the carrier substrate via first and second bias voltage terminals,

- illuminating the emission region by a time-modulated optical signal at a first predetermined central wavelength to generate photocurrent in the emission region, - displacing and trapping free electrons and holes in the plurality of quantum dots to produce a temporary conduction current and a displacement current in the carrier substrate,

- applying stimulated optical emission at a second predetermined wavelength to the quantum dots to accelerate recombination of the trapped electrons and holes and thereby deplete or empty the trapped states.

A DC bias voltage, such as voltage between 100 and 250 volt may be applied between the first and second bias voltage terminals to establish the electrical field across the emission region of the carrier substrate. However, the DC bias voltage is preferably adjusted such that the electrical field strength in the emission region lies between 1 and 300 kV/cm such as between 5 and 100 kV/cm. As previously mentioned, the first predetermined central wavelength is preferably smaller than the second predetermined wavelength. As previously explained, the photo generated displacement current creates the desired terahertz electromagnetic radiation which radiates to free space at the same terahertz frequency e.g. 1 THz as the frequency of the time-modulated optical excitation signal.

In one embodiment, the method comprises a further step of:

- placing a terahertz (THz) radiation or reflective mirror below a second surface of the carrier substrate to reflect terahertz electromagnetic radiation incident thereon towards an opposite surface of the carrier substrate. In one such embodiment, the reflective mirror is arranged below the carrier substrate in opposite direction to the intended emission direction of the terahertz electromagnetic radiation. In this manner, a terahertz electromagnetic radiation component seeking to propagate into a base substrate below the carrier substrate is reflected. Thus, the majority of generated terahertz electromagnetic radiation is emitted in the intended direction. Moreover, if a thickness or height of the carrier substrate is small compared to a wavelength of the terahertz electromagnetic radiation, the reflected and a directly emitted component of the terahertz electromagnetic radiation will be substantially in-phase and interfere constructively. In one embodiment, the modulated optical signal propagates through an incident light passage extending through the radiation mirror.

The time-modulated optical signal preferably comprises at least one of:

- dual wavelength laser light separated by a predetermined frequency span such as between 0.1 and 10 THz,

- a stream of laser pulses having pulse widths smaller than 1 ps.

As previously-mentioned, an incoming optical excitation signal in form of a dual wavelength laser light may be generated as a beat-frequency optical signal supplied by a two-colour pump laser. The predetermined frequency span is set by a wavelength difference between individual wavelengths of the two-colour laser light.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred embodiments of the invention will be described in more detail in connection with the appended drawings, in which:

Fig. 1 a) is a schematic illustration of bandgaps and free electrons and holes in a carrier substrate of a photomixer,

Fig. 1 b) is a schematic illustration of a photomixer carrier substrate under illumina- tion by femtosecond pulses of laser light,

Fig. 2 is a series of schematic illustrations schematically showing three steps of the displacement, trapping and recombination of free electrons and holes in quantum dots of the carrier substrate; and

Fig. 3 illustrates schematically a photomixer in accordance with a first embodiment of the invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Fig. 1 b) is a schematic illustration of a prior art photomixer 10 under illumination by femtosecond pulses of laser light for the purpose of illustrating general operating principles of existing photomixers. A photomixer semiconductor substrate 12 may be formed in bulk semiconductor for example GaAs. The photomixer semiconductor substrate 12 comprises a first electrode 16 and a second electrode 20 both deposited on a surface of semiconductor substrate 12 and separated by a predetermined distance. A DC voltage source 8 is connected between the first electrode 16 and a second electrode 20 to provide a DC voltage difference between the electrodes which leads to the generation of an electrical field, E, in the semiconductor substrate 12 between the first electrode 16 and a second electrode 20. A single broad band laser pulse 14 is applied to the photomixer semiconductor substrate 12 with a temporal light intensity profile as illustrated which means the light intensity changes on a sub-picosecond timescale which in turn corresponds to a THz frequency bandwidth. The single broad band laser pulse 14 leads to a photocurrent, j, in the semiconductor substrate 12. The photocurrent, j, leads to the emission of a single broadband pulse of THz electromagnetic radiation 18 with a temporal profile corresponding to the time derivative of the photocurrent, j, according to equation 2 for E _THz in Fig. 1 a). However, the prior art photomixer 10 is unable to recover fast enough to immediately thereafter emit further broadband pulses of THz electromagnetic radiation. This makes it impossible to efficiently generate a high-repetition rate, continuous train of broadband pulses. The slow recovery speed of the traps in the photomixer semiconductor substrate 12 limits its operational speed. For this reason only a single broadband pulse 18 of THz radiation can be efficiently emitted by the existing type of photomixer. After the emission of the initial or first broadband pulse 18, internal relaxation processes within the photomixer 10 prevent efficient THz pulse genera- tion for a certain time interval. This time interval is typically longer than 100 ps (100 ^* 10 ^"12 s) caused by the spontaneous recombination time of spatially separated electrons and holes in the semiconductor material, or the electrons and holes trapped onto the material defects of the semiconductor substrate 12. Fig. 2 is a series of schematic illustrations showing three steps of the displacement, trapping and recombination of free electrons and holes in quantum dots 205 of the carrier substrate 310 of the photomixer 300 (refer to Fig. 3) in accordance with a preferred embodiment of the present invention. In step 1 , free carriers in form of a free electron 204 and a hole 202 are created by light illumination of the photomixer 300 (refer to Fig. 3) and subsequently displaced toward respective quantum dots 205 of the carrier substrate 310. In step 2, the free electron 204 and the hole 202 are caught or trapped in their respective quantum dots. The trapping of the carriers into the quantum dots 205 or QDs is a very efficient mechanism for free-carrier lifetime reduction in the carrier substrate. This trapping leads to generation of a photo- current as expressed by equation 2 of Fig. 1 a). However, after the trapping of the free carriers 204, 202 the traps are occupied preventing immobilization of the next hole and electron pair. Even though spontaneous recombination may empty the trapped states, this process is very slow as previously explained in connection with the Figs. 1 a) and 1 b) leading to a slow recovery of the photomixer. However, this slow recovery problem is solved in the present photomixer embodiment as illustrated by step 3 where stimulated emission is applied to the QDs with the trapped states. This is schematically illustrated in step 3 by an incoming photon 206 which accelerates recombination of the trapped electrons and holes 204, 202 so as deplet- ing or emptying the trapped states of the QDs 205. The depletion of a trapped state is accompanied by the emission of a pair of photons 208, 210 as illustrated. The depletion process leads to significant reduction of the recovery time of the photomixer and enables much higher achievable operational speeds of the photomixer such that continuous emission of broadband pulses of THz electromagnetic radia- tion is possible.

Fig. 3 is schematic cross-sectional view of a photomixer 300 in accordance with a first embodiment of the invention. The photomixer 300 comprises a base substrate 302 on which the photomixer structure in grown upon. The base substrate 302 may comprise base material of a standard GaAs wafer such as a 3 inch or 5 inch wafer. The photomixer 300 comprises a carrier substrate 310 in which a plurality of quantum dots (not shown) are arranged in an emission region 308 thereof. A first bias terminal 316 and a second bias terminal 320 are deposited on an upper surface of the carrier substrate 310 arranged a distance apart from each other. The emission region 308 is preferably electrically non-conductive in an unexcited state and will preferably only be electrically conductive when illuminated during optical excitation. This optical excitation creates free carriers inside the emission region as discussed in detail above. Each of the first and second bias terminals 316, 320, respectively, preferably comprises a suitable electrical conducting material such as titanium-gold or other metals compatible with wire bonding processes. A layer of AIGaAs 312 is applied on top of the base substrate 302, above the emission region 308, thereof extending between the first and second bias terminals 316, 320, respectively. The layer of AIGaAs 312 has a lower refraction index than the base substrate 302 and therefore serves as a cladding layer retaining optical radiation of a laser cavity sur- rounding the base substrate 302 as explained in detail below. A corresponding lower layer of AIGaAs 304 is arranged below the carrier substrate 310 for the same purpose as the upper layer of AIGaAs 312. A layer 309 of highly n-doped GaAs is finally arranged below the lower layer of of AIGaAs 304 and above the base substrate 302. The highly n-doped GaAs layer 309 acts like a reflective mirror reflecting impinging terahertz electromagnetic radiation, i.e. terahertz electromagnetic radiation propagating into the base substrate 302, towards the uppermost layer of AIGaAs 312 such that the majority of generated terahertz electromagnetic radiation is emitted in direction of the arrow 305. Since the thickness or height of the carrier substrate 310 and the lower layer of of AIGaAs 304 is small compared to a wavelength of the terahertz electromagnetic radiation, the reflected component and a directly emitted component of the terahertz electromagnetic radiation will be substantially in-phase and propagate as indicated by field arrow 305. In the present embodiment, a width of the emission region 308 of the carrier substrate 310 preferably lies between 25 and 75 μηη such as about 50 μηη. The electrical field strength in the emission region may lie between 1 and 300 kV/cm such as between 5 and 100 kV/cm. In practice, the height of the base substrate 302 may lie between 200 and 500 μηη such that a majority of the height dimension of the photo- mixer 300 is consumed by the base substrate. The residual layers of the photomixer structure discussed above may have a combined height of about 2-3 μηη. A depth dimension (extending in direction of dotted arrow 320) of the photomixer 300 may lie between 0.5 mm and 2.0 mm such as about 1 .0 mm. The photomixer 300 comprises a laser cavity (not shown) which surrounds the carrier substrate 310 holding the plurality of quantum dots inside the emission region 308. The laser cavity comprises a first and a second optically reflective surface arranged around a proximate and distal side surface of the emission region 308 as a HR coated layer deposited on each of these surfaces of the carrier substrate 310. In this manner a laser cavity is formed between the upper and lower cladding layers of AIGaAs 312, 304, respectively, and the proximate and distal HR coated side surfaces. For example, the laser cavity can be adapted for QD ground state 1.3 μηη lasing, driven by the incoming excitation by the optical signal of the wavelength around 1.0 μηη. This incoming signal can be created by a two-colour pump laser, and will be time-modulated at a THz frequency corresponding to a detuning between the two colors.

In operation, a DC bias voltage, such as a voltage between 100 and 250 volt, is ap- plied between the first and second bias terminals 316, 320, respectively, to establish an electrical field across the emission region 308. The incoming excitation light, schematically indicated by arrow 303, follows an incident light passage extending through the base substrate 302 up to the emission region 308 of the carrier substrate. The incoming excitation light 303 is accordingly applied through a back-side of the photomixer 300 in the present embodiment which serves as a convenient coupling surface and entry port for an optical fibre or optical medium coupled to the back-side surface. Lenses may be used to shape the excitation to the laser cavity of the photomixer. During operation of the photomixer 300, the photoexcitation at e.g. 1 micron wavelength will lead to the creation of the free carriers in the two- dimensional wetting layers of the QDs followed by an ultrafast (on the time scale of 1 ps and shorter) carrier trapping into the strongly localized quantum-confined states of the QDs arranged in the emission region 308. Therefore, the short-lived in-plane conductivity of the emission region 308 will be achieved. The trapping of the free carriers into the QDs is a very efficient mechanism for free carrier lifetime reduction. Thus, an abrupt switch-off of the drift current in the wetting layers will be achieved, accompanied by the spike of the displacement current in the same direction as the free carriers populate the strongly polarized states within the QDs. This will create a net in-plane photocurrent in the wetting layers, oscillating at the beat frequency of the 2-color pump laser, e.g. at 1 THz. This THz-oscillating ac current will, naturally, radiate to the free space at the same frequency of 1 THz.