WO2008054846A2 - Mélangeur optique permettant de créer un rayonnement térahertzien cohérent et de détecter le rayonnement - Google Patents

Mélangeur optique permettant de créer un rayonnement térahertzien cohérent et de détecter le rayonnement Download PDF

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WO2008054846A2
WO2008054846A2 PCT/US2007/065475 US2007065475W WO2008054846A2 WO 2008054846 A2 WO2008054846 A2 WO 2008054846A2 US 2007065475 W US2007065475 W US 2007065475W WO 2008054846 A2 WO2008054846 A2 WO 2008054846A2
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photomixer
layer
interdigitated
gaas
absorbing layer
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WO2008054846A3 (fr
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Subrahmanyam Pilla
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The Regents Of The University Of California
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    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L31/00—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof
    • H01L31/08—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof in which radiation controls flow of current through the device, e.g. photoresistors
    • H01L31/09—Devices sensitive to infrared, visible or ultraviolet radiation
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01S—DEVICES USING THE PROCESS OF LIGHT AMPLIFICATION BY STIMULATED EMISSION OF RADIATION [LASER] TO AMPLIFY OR GENERATE LIGHT; DEVICES USING STIMULATED EMISSION OF ELECTROMAGNETIC RADIATION IN WAVE RANGES OTHER THAN OPTICAL
    • H01S1/00—Masers, i.e. devices using stimulated emission of electromagnetic radiation in the microwave range
    • H01S1/02—Masers, i.e. devices using stimulated emission of electromagnetic radiation in the microwave range solid

Definitions

  • the present invention relates to a device for generation of continuous, coherent microwave and terahertz (T-Ray) radiation.
  • Optical to electrical down conversion is achieved by mixing the optical radiation in a suitable photoconductive medium such as a photo diode or a planar MSM device also known as an Auston switch.
  • a suitable photoconductive medium such as a photo diode or a planar MSM device also known as an Auston switch.
  • tunable optical radiation source using heterodyne techniques with two fundamental optical lines whose beat frequency (or difference frequency) is in 0.1-10 THz range
  • high efficiency photomixer with subpicosecond carrier recombination time to convert the optical difference frequency signal to a corresponding electrical signal
  • the photomixer is formed from a single layer of metal-semiconductor-metal
  • MCM multi-digitated Electrode
  • LT-GaAs low temperature- grown GaAs
  • Optical radiation with difference frequencies from few MHz to THz are generated based on well known optical heterodyne techniques, by using highly coherent, tunable, and affordable external cavity stabilized diode lasers (at infrared and optical wavelengths), with output power up to 150 mW.
  • Sub-Hertz relative frequency stabilization of two lasers has been demonstrated in 1992, using Pound-Drever-Hall technique where, the two lasers are coupled to different axial modes of an external high finesse ( ⁇ 22 000) Fabry-Perot ("FP") cavity to produce the discriminator signal for servo control.
  • This optical radiation is mixed in the above photomixer to generate the desired electrical signal at the difference frequency in the interdigitated region.
  • a well designed planar antenna coupled to the interdigitated electrodes allows T-rays to be released into the free space. Planar antenna designs used for emitting the THz radiation are also well characterized and optimized.
  • FIGs. Ia- Id illustrate a prior art conventional Metal-Semiconductor-Metal (MSM) photomixer design for T-ray generation.
  • the active volume 102 of the device comprises of a 1.5 ⁇ m thick LT-GaAs layer deposited on a high-resistivity GaAs substrate 104 using MBE techniques. This layer is covered by interdigitated electrodes 106 fabricated using submicron e-beam lithography. When this region is illuminated by radiation with photon energy larger than the semiconductor's band gap, electron-hole pairs 108 are generated for each incident photon absorbed in the photoconductor, shown in FIG. Ic.
  • MSM Metal-Semiconductor-Metal
  • the electrons and holes are accelerated away in the opposite directions by the strong electric fields 110 generated by the DC biased interdigitated electrodes. Due to these high electric fields, the carriers attain drift velocity in a relatively short time when compared to their average transit time, i.e., the time required for a carrier to reach an electrode and contribute to the external current. As the incident optical radiation intensity varies at the beat frequency, the pair density also varies at the same rate provided the carrier recombination time is short when compared to the inverse of the difference frequency.
  • Typical dimensions of the active device are 10 ⁇ 10 x l.5 ⁇ m.
  • the devices optimized for THz generation have 100 nm thick metal lines that are typically 200 nm wide, ⁇ 10 ⁇ m long, with a pitch of 0.8 to 1 ⁇ m.
  • K(k') finger period where ⁇ r is the relative dielectric constant of the photoconductor, A is the area of the interdigitated region, and
  • FIG. Id provides the equivalent circuit diagram of the prior art photomixer.
  • the millimeter wave output power is then given by
  • R L , Rs are the load resistance and small internal resistance of the photomixer respectively.
  • R L is in 72-200 ⁇ range and Rs « R L -
  • Equation 4 Based on photomixer theory, it can be shown that for the prior art design illustrated in FIGS. Ia- Id, i v is given in Equation 4.
  • the parameter ⁇ is a product of carrier generation factor which is a measure of the fraction of incident power absorbed in the active volume and a collection factor which is a measure of the fraction of carriers generated that contribute to current in external circuit.
  • large increase in output power should result from modest increases in ⁇ if ⁇ and C are minimized with the better choice of photoconductor material and electrode design.
  • LT-GaAs is not suitable for optical communications applications where the spectrum of choice is at 1.55 ⁇ m.
  • this area has produced a wide range of commercially available products, such as lasers, detectors, modulators, and optical amplifiers. Many of these components are based on the compound semiconductor material system Ino.53Gao.47As/Ino.52Alo.4gAs on InP substrates. Only recently LT-InGaAs as well as ErAs/InGaAs materials are being explored for THz photomixer applications at 1.55 ⁇ m. Much work is still desired in improving the crystalline quality and other photoconductor properties of these materials to match the success of LTGaAs in THz photomixer applications.
  • the increased intensity, thereby photocarrier density near the top surface, should increase the photomixer efficiency while lowering the noise with decrease in effective path length of the carriers.
  • the estimated THz output power for a cavity device with 5.2 x 5.2 x 1.5 ⁇ m active volume is estimated to be ⁇ 6.5 and 2 ⁇ W respectively at 0.65 and 1.5 THz. This is ⁇ 7.2x improvement in the output power when compared to a noncavity device.
  • the metal lines act as optical stop barriers thereby limiting the amount of incident optical power that is effectively used in photocarrier generation.
  • the present invention is directed to increasing the efficiency and long time stability of this most crucial component of terahertz generators.
  • the present invention is directed to the enhancement of the photomixer performance in generating higher THz power for a given incident optical power and DC bias voltage across the device.
  • the physical basis for the inventive photomixer is electron-hole pair generation by photon absorption in LT-GaAs, InGaAs or GaN.
  • the LT-GaAs has recently displayed the remarkable photoconductive properties of subpicosecond electron-hole recombination time ( ⁇ ) and high DC breakdown field (E B > 4 X IO 7 V/m). In addition, it displays high photocarrier mobility ( ⁇ ⁇ 200 Cm 2 V 1 S 1 ) relative to semiconductors having a comparable recombination time.
  • a photomixer in one aspect of the invention, includes a Fabry-Perot cavity comprising a top distributed Bragg reflector, a bottom distributed Bragg reflector and a semiconductor absorbing layer disposed between the top distributed Bragg reflector and the bottom distributed Bragg reflector; and a plurality of layers of interdigitated conductive lines embedded at different heights within the absorbing layer, each of the plurality of layers having a pair of leads extending therefrom for connecting to a DC bias source and/or to other layers of lines.
  • the electrodes are formed during epitaxial growth of the semiconductor absorbing layer.
  • the photomixer of the present invention has an active volume comprising a semiconductor absorbing layer having a top and a bottom; an antireflective cap layer abutting the top of the absorbing layer; a free space disposed above the cap layer; a buried mirror abutting the bottom of the absorbing layer; and a plurality of interdigitated electrodes embedded in the absorbing layer at different positions along a z-axis, each electrode having a lead for connection to a DC bias source and/or to other electrodes of the plurality.
  • the electrodes are formed during epitaxial growth of the semiconductor absorbing layer.
  • the improvement provided by the invention is increased output power ( ⁇ 2.8 mW at 1 THz and 31.8 ⁇ W at 5 THz) of the coherent, CW, T-ray source in the 0.1 to 10 THz frequency range.
  • Present research in this area is focused on increasing the output power of the source to > 10 ⁇ W to be useful for practical applications.
  • the maximum power available from the present continuously tunable CW sources is ⁇ 10 ⁇ W at ITHz.
  • the present invention offers tunability in the entire spectral range and very low linewidth of the emitted radiation.
  • the inventive photomixer offers high quantum efficiency, low noise, and high speed when used as an optical detector.
  • Present day detectors used in applications ranging from long haul high bit-rate fiber optic communication systems, microscopy and astronomy offer up to 500 GHz gain-bandwidth product.
  • the present invention offers close to unity quantum efficiency and detection rate > 5 THz at unity gain.
  • FIGs. Ia- Id are diagrammatic views of a prior art photomixer, where FIG. Ia is a cross-sectional view of the photomixer, FIG. Ib is a schematic drawing of the interdigitated metal lines, FIG. Ic illustrates the photo generated carriers accelerated along the electrostatic field lines, and FIG. Id is a schematic view of the equivalent circuit diagram of the photomixer.
  • FIG. 2a illustrates the FP cavity and FIG. 2b illustrates the stacked inter digitated metal line.
  • FIG. 3 is a contour plot of the electric field amplitude.
  • FIG. 4a is a contour plot of field strength and FIG. 4b is a plot of the static electric field strength distribution.
  • FIG. 5 is a plot of the carrier transit time distribution.
  • FIG. 6 is a plot of the calculated THz output power Pf.
  • FIG. 7 is a schematic drawing of the modified interdigitated electrode structure.
  • FIG. 8a illustrates the photomixer design and
  • FIG. 8b is a contour plot of the electric field amplitude.
  • FIG. 9a is a contour plot of field strength and FIG. 9b is a plot of the static electric field strength distribution.
  • FIG. 10a illustrates the photomixer design and FIG. 10b is a contour plot of the electric field amplitude.
  • FIG. 1 Ia is a contour plot of field strength and FIG. 1 Ib is a plot of the static electric field strength distribution.
  • the present invention is directed to the enhancement of the photomixer performance in generating higher THz power for a given incident optical power and DC bias voltage across the device.
  • a similar device when used as an optical detector, offers unprecedented speed with high gain.
  • the physical basis for the proposed photomixer is electron-hole pair generation by photon absorption in LT-GaAs, InGaAs or GaN.
  • the LT-GaAs has recently displayed the remarkable photoconductive properties of subpicosecond electron-hole recombination time ( ⁇ ) and high DC breakdown field (E B > 4 X IO 7 V/m).
  • the optical microwave signals are generated by heterodyning the optical spectral lines of a mode locked laser or of two or more semiconductor diode lasers whose difference frequency is stabilized using external high finesse ( ⁇ 22 000) Fabry- Perot (FP) cavity.
  • the existing art in generating the optical microwaves is well developed over a wide range of optical frequencies including far infrared, infrared, and visible spectrum. Depending on the photoconductive medium used for the down conversion, a suitable commercially available.
  • Ti- Sapphire or DBR lasers operating at 800 to 860 nm spectral range are most appropriate as the photoconductive medium used for the photomixer described next is Low Temperature grown GaAs (LT-GaAs).
  • LT-GaAs Low Temperature grown GaAs
  • photon energies in this spectral range are just above the semiconductor band gap in the 77 to 300 K range to allow efficient absorption of the incident radiation.
  • the semiconductor used for photomixer is InP, Ini_ x Ga x As y Pi_ y lattice matched to InP substrate, Ini_ x Ga x As, InAs, GaN, AlGaN, or InGaN, DBR lasers operating in different spectral regions suitable for each material in the 340 to 1600 nm spectral range are also commercially available.
  • the inventive multilayer photomixer design increases the carrier collection efficiency by decreasing the carrier transit time t Xv , increases the average electrostatic field throughout active volume at lower bias voltage Vb , and increases the carrier generation efficiency by effectively absorbing most of the incident optical radiation.
  • Thin interdigitated metal lines are embedded in the host semiconductor matrix grown by suitable epitaxial techniques such as MBE, LPE (Liquid Phase Epitaxy), and
  • the carrier recombination time ⁇ can be as large as 4 ps. This is a significant advancement because present designs requiring ⁇ ⁇ 0.1 ps set severe constraints on the material choice and its growth conditions. In general the low temperature growth conditions required for obtaining low ⁇ degrade the material conductivity and breakdown field limit. Until now only in LT-GaAs all the photomixer requirements were sufficiently met but the present design allows most of the semiconductors to be suitable for THz power generation.
  • FIGS. 2a and 2b illustrate the FP cavity and stacked interdigitated metal line parameters used in FDTD computation.
  • the active volume is terminated by 15 cell Uniaxial Perfectly Matched (UPM) layers 208 in FDTD implementation to mimic real world structure.
  • the parameters in brackets show the refractive index and thickness of each layer.
  • FIG. 1 Uniaxial Perfectly Matched
  • FIG. 2b illustrates the geometry of the interdigitated metal lines 210 placed at various z locations within the LT-GaAs absorbing layer 206.
  • the two 200 nm wide lines 212, placed along the x axis are connected to either signal transmission lines or planar antenna fabricated on top of the LT-GaAs layer (not shown).
  • Through layer vias electrically connect the leads 214 of multiple layers of interdigitated lines 210.
  • the overall dimensions of the active structure (excluding DBRs 202, 204 and via leads 214) are given by L x , L y , and D whereas the metal line thickness, z location (measured from the Air- TiO 2 interface), line pitch, width and lengths of the interdigitated lines 210 are given by d, Z 1 , p, w, l ⁇ , and h as shown.
  • FDTD Finite Difference Time-Domain
  • UPML Uniaxial Perfectly Matched Layer
  • the active volume consists of a few hundred nm of free space 201 followed by an antireflective cap layer 202 with two or three layer quarter wave stack, photomixer region 206 with complex structure of normal metal electrodes 210, and a four pair DBR (Distributed Bragg Reflector) stack 204 for the buried mirror (FIG. 2a).
  • FDTD Finite Difference Time-Domain
  • UPML Uniaxial Perfectly Matched Layer
  • the top antireflective coating 202 along with the bottom DBR stack 204 thus forms the FP cavity for plane waves propagating along z-axis.
  • at least 1500 nm of computational space 220 is left between the UPML slabs 208 and photomixer volume.
  • the photomixer cross sectional area is 7000 x 2500 nm (i.e., area covered by metal lines in x-y plane)
  • the active area sandwiched between UPML slabs along x or y axis is at least 10000 x 5500 nm.
  • the 15 cell UPML region is polynomially graded for optimal results.
  • the DBR and antireflective layer parameters i.e., index and thickness
  • total field/scattered field formulation is adopted.
  • the scattered field at the source plane goes to zero when the cavity is fully excited, therefore this source condition is adequate, provided the simulation is not run for too long.
  • the standing wave thus formed will have antinodes at the interfaces of LT-GaAs with top and bottom DBRs as well as at the center of LT-GaAs layer.
  • a direct comparison of the field amplitudes at these antinodes showed that the FDTD calculations for this cavity are in excellent agreement with the above matrix calculations.
  • the noise resulting from reflections from the UPML slabs is observed to be ⁇ 0.5%.
  • a second-order material model such as a frequency-domain Lorentz dispersion model is adapted for metals with negative dielectric constant in the frequency of interest such as W or Pt.
  • This model can be designed to give the correct refractive index for any material at a single frequency.
  • the dielectric constant and refractive index of a metal in Lorentz model is given by Equation 6;
  • the material parameters can be very sensitive to impurities and film's morphology resulting in variation of the dielectric constant from +ve to -ve or vice versa (note: the tabulated values are for pure bulk metal and its dielectric constant changes sign in the vicinity of 1.4 eV), therefore, the Lorentz model parameters were varied over a wide range in the FDTD computation.
  • the near field enhancement is insensitive to such model parameter variations indicating that enhancement results as long as the material response is not instantaneous (as in a perfect metal), and the host matrix dielectric constant is large when compared to the normal metal used.
  • FIG. 3 shows the FDTD results of a three layer stack of interdigitated W lines embedded in LT-GaAs absorbing layer of a photomixer design of FIG. 2a and 2b.
  • the metal lines are 10 nm thick with the same interdigitated pattern of FIG. 2b in each layer of the stack.
  • the FD results were cross checked with analytical formulae for capacitances in each case (such as Equation 1).
  • the FD computed values are in excellent agreement with the theoretical values in each case.
  • the internal electric fields and capacitances were then computed for the stacked electrode geometry of FIG. 2a and 2b for different L x , L y , d, Z 1 , p, w, l ⁇ , h parameter values.
  • the FD volume included the DBRs with the appropriate static dielectric constant values for each layer available in literature and sufficient buffer volume between outermost computational boundaries and the active volume to mimic real world device.
  • the metal electrodes are DC biased ⁇ 1.0 V.
  • the plots show that unlike the traditional MSM structure of FIG. 1 , the field strength in this design is well above the critical field ( ⁇ 5 kV/cm) required to accelerate the photocarriers generated throughout the absorbing layer volume.
  • bias voltage Vb > 40 V is required to accelerate carriers generated deep inside the photomixer. At these high bias voltages, the field close to the electrodes exceeds the breakdown field for
  • the internal resistance of the electrodes is estimated to be 2.4 ⁇ based on the resistivity of thin annealed or epitaxial W films ( ⁇ 5 ⁇ ⁇ -cm).
  • the highest electric field inside the device is about four times lower than the breakdown voltage; therefore device failure due to electric breakdown is unlikely at this V b .
  • the grey rectangular regions in the plot show the positions of 10 nm thick interdigitated W lines with/?
  • the carrier collection efficiency and photo current /p by first computing the carrier transit time distribution for the entire absorbing volume. This distribution is obtained by dividing the total volume into smaller volume elements and then calculating the number of carriers generated in each volume element as well as their transit time to the nearest electrodes.
  • the transit time ( ttr» 1 ps) of the carriers is on an average » ⁇ and the carriers travel at equilibrium drift velocity.
  • one can exploit the nonequilibrium effects such as velocity overshoot and ballistic transport of carriers that takes place when the electric field is uniformly high along the carrier's trajectory.
  • THz photomixer based on qasiballistic transport in LT-GaAs and GaAs nano-pin-diodes has been recently proposed to take advantage of the nonequilibrium transport in semiconductors.
  • the field dependent, transient velocity of electrons and holes has been studied extensively using Monte-Carlo techniques. From these studies, it is now well known that under high fields, electrons can achieve peak velocities 8 to 10 times their equilibrium saturation drift velocity in these materials.
  • the overshoot effects can only be exploited over a limited range of transit distances before polar optical emission and intervalley scattering lead to negative differential mobility of the carriers.
  • ballistic transport is mostly applicable to electrons generated close to the +ve electrodes and holes generated close to -ve electrodes. These carriers generated predominantly in the near field enhancement region, transit through non-uniform DC fields in the 5 to 90 kV/cm range.
  • t onset 100 fs is considerably lower than the theoretical limit of ballistic motion in GaAs for this field strength range.
  • FIG. 5 shows the carrier transit time distribution «tr(/tr) for electrons and holes in the photomixer design of FIG. 2 obtained with the above approximations for carrier velocities.
  • n tr has peaks at t tr ⁇ 65 and 70 fs for electrons and holes respectively (solid and dotted curves) for the device parameters of FIG. 3 and FIG. 4.
  • a sharp peak followed by several satellite peaks and a long tail of the n Xx distribution can be understood from the fact that most of the carriers are generated close to the electrodes in the near field enhancement regions where the static fields are also strong.
  • the satellite peaks result from periodicity of the electrode structure and specific choice of the velocity distribution. It is important to note that in the above distribution, number of carriers with t Xx > 1 ps is very small therefore, for this device structure ⁇ is not the critical factor in determining the photomixer performance at THz frequencies.
  • FIG. 5 illustrates the carrier transit time distribution r ⁇ tr(ttr) for electrons (e) and holes (h) in the photomixer design of FIG. 2.
  • For the dashed and short dashed curves (L x , p, w) are changed to (7560, 300, 60) and (7480, 320, 120) nm respectively while rest of the parameters remaining unaltered from those of FIGS. 3 and 4.
  • the number of carrier pairs generated in the above photomixer in 1 THz cycle will be -6 x 10 16 cm 3 .
  • space charge effects begin to degrade the device performance at this carrier density because the devices are transit time limited.
  • the conventional designs require ultra short carrier recombination time , ⁇ typically in sub picosecond range.
  • the steady state carrier density will be not high even when ⁇ is of the order of a few ps.
  • the number of electrons captured in the metal electrodes at time t is given by Equation 7.
  • n g e en (t) + cos(2;r / 1)] is the number of electrons generated in LT-GaAs layer per second by incident laser power P 1 modulated at a THz frequency
  • t c the carrier creation time, electron transit time distribution (see FIG. 5), and electron recombination time.
  • Similar expressions can be written for number of holes captured n c h ap (t) and the carriers available for conduction in the photomixer n a e vl it) and n m h l (t) .
  • Equations 3 and 4 show that for a well optimized photomixer, Pf ⁇ f 2 77 in the 0.5 to 6.5 THz range.
  • Equations 3 and 4 result in/ 4 roll-off of THz power.
  • a recent n-i-p-n-i-p photomixer concept is shown to have/ 2 roll-off for/ ⁇ 1.5 THz. Therefore, the design of FIG. 2 offers significant improvement over the existing photomixer designs.
  • Table 1 lists Pf of FIG. 6 at a few representative frequencies.
  • the design parameters of FIGS. 3 and 4 results in highest THz power throughout the 0.1 to 10 THz range.
  • Recombination time ⁇ is 4 ps for designs Sl to S6 where as for S7 ⁇ is 1 ps. Parameters for Sl are same as in FIGS.
  • FIG. 7 is a schematic drawing of the modified interdigitated electrode structure of FIG. 2b for doubling THz output power. Two interdigitated patterns 704, 706 similar to that shown in FIG. 2b are connected in series with the center electrode 710 grounded.
  • Through layer vias electrically connect the leads 708 of multiple layers of interdigitated lines.
  • the resulting structure is excited by a laser beam with two quasielliptical spots 712 as shown.
  • Such a radiation pattern is achievable by choosing the proper laser propagation mode (such as TEMlO mode).
  • the spacing between the two metal patterns 704, 706 is adjusted to match the beam pattern of the specific laser system being used.
  • photoconductor properties such as recombination time ⁇ are not that important for the inventive photomixer.
  • Photoconductors such as InP, Ini_ x Ga x As y Pi_ y , Ini_ x Ga x As, InAs lattice matched to InP substrate, GaN, AlGaN, and InGaN grown on sapphire, SiC, CVD diamond, or Si are other alternatives for the proposed photomixer. All these materials can readily offer ⁇ ⁇ 4 ps with excellent high field, nonequilibrium carrier mobilities (> 10 7 cm/s) however, due to the difficulty in lowering ⁇ to subpicosecond level while retaining high crystalline quality and breakdown field limits in these materials similar to LT- GaAs; they were not previously considered for THz power generation using conventional design of FIG. 1.
  • Ini_ x Ga x As and to some extent Ini_ x Ga x As y Pi_ y and InP are the materials of choice for optical communication devices and components because fiber-optic communication frequencies match material's band gap at ⁇ ⁇ 1550 nm.
  • the band gap of LT-GaAs at ⁇ o 850 nm is rather too large to be useful for optical communications applications.
  • ⁇ o 1550 nm.
  • having a thin layer ( ⁇ 2nm) of InAlAs to raise the effective Schottky barrier height on InGaAs is also possible for reducing the dark current of the device.
  • the active volume is terminated by 15 cell UPML layers 808 in FDTD implementation to mimic real world structure.
  • the breakdown field is roughly 5 times lower than LT-GaAs but fortuitously the strong overshoot effects also manifest at relatively lower fields. Therefore, obtaining uniformly strong fields ⁇ 100 kV/cm is all the more important in this case.
  • FIG. 9b illustrates the static electric field strength distribution in the 220 nm LT-InGaAs absorbing layer of FIG. 9a plotted as a volume fraction.
  • a broad distribution with a peak at -42 kV/cm shows that throughout the absorbing volume the field is well below the breakdown field, yet strong enough for exploiting ballistic and quasiballistic carrier motion.
  • the volume fraction plot of FIG. 9b demonstrates that the above electrode design satisfies this requirement fairly well. Increasing the bias voltage moves the distribution in FIG. 9b to higher field values without significantly altering its shape, therefore the design allows sufficient bias voltage tuning suitable for different absorbing materials in the 1060 to 1550 nm range.
  • the device capacitance C and internal resistance R s for the structure of FIG. 8a are -3.01 fF and 5.31 ⁇ respectively.
  • Increase in R s is due to the decrease in number of metal fingers and layers to 8 pairs and 2 layers respectively.
  • GaN 's large peak velocity in the overshoot regime makes it an important candidate for high frequency applications such as photomixing.
  • the transit distance in GaN and GaAs are approximately equal (-120 nm) when the accelerating fields are 300 and 30 kV/cm in GaN and GaAs respectively.
  • the distance traversed in GaN is significantly higher than in GaAs due to larger peak velocity (> 8 ⁇ 10 7 cm/s) in GaN.
  • the breakdown field in GaN (2MV/cm) is about 4 times lager than in GaAs.
  • Various types of GaN devices have been demonstrated including Schottky barriers detectors, p-n junctions, p-i-n structures, MSM photodetectors, and AlGaN/GaN heterostructure field effect transistors (HFETS). Recently, the carrier recombination time ⁇ as low as 720 fs was demonstrated in LT grown GaN with 200 kV/cm breakdown field.
  • GaN has not been used to date for THz photomixing applications mainly due to the difficulty in lowering ⁇ to subpicosecond level while having large breakdown field limit. If near field enhancement can be achieved in GaN similar to LT-GaAs photomixer, one only needs ⁇ > 4 ps thereby providing a material with high breakdown field limit and good crystalline quality.
  • FIG. 10a shows the FP cavity parameters of a photomixer based on GaN absorbing layer optimized for maximum absorption efficiency (> 97%) with strong near field enhancement when the cavity is illuminated with 363 nm radiation.
  • Standard epitaxial growth techniques with GaN growth temperature -650 to 750 0 C are adequate to produce a GaN film with ⁇ > 4 ps and high breakdown field limit.
  • FIG. 10b shows the near field enhancement in the GaN photomixer with the DBR parameters of FIG. 10a.
  • platinum Pt
  • interdigitated Pt line width w, pitch/? , vertical positions Z 1 , and thickness d are varied in the FDTD computation to optimize the near field enhancement and absorption efficiency.
  • w > 100 nm the low static field region between the layers (FIG.
  • the grey rectangular regions in the plot show the positions of 8 nm thick inter
  • the THz output power from GaN based device should be either comparable or higher than the power generated by a LT-GaAs photomixer. GaN, with higher thermal conductivity and higher operating temperature capability should therefore be a better choice for THz power generation if other material's issues discussed earlier are resolved.
  • the same device structure with little modification can be used for wideband, ultrafast optical signal detection as well. Due to the excellent signal-to-noise ratio offered by these photoconductors, the invention offers near unity quantum efficiency at detection rates exceeding 5 THz. Simultaneous generation of continuous spectrum of THz signals from an MSM photomixer excited by a femtosecond optical pulse is a well known technique in coherent time domain THz spectroscopy. These high power optical pulses can generate THz waves with several mW of peak power.
  • the power in higher frequency (> ITHz) spectral components is very weak due to long t ti in single layer MSM photomixer discussed earlier.
  • the present invention can significantly improve the power in the high frequency spectral components similar to FIG. 6 due to significantly reduced t b .
  • the present invention is directed to increasing the efficiency and long time stability of this most crucial component in generating T-rays as summarized in Table 2.
  • Photomixer of the present invention include: 1) All solid state, semiconductor based 0.1 to > 5THz local oscillator for use in optical communications.
  • the local oscillator is one of the most crucial components in modulating and demodulating rf signals carried by WDM channels of the present day long haul fiber optic communications systems.
  • the current state-of-the-art local oscillators used in these applications do not have the capability to go beyond -190 GHz.
  • Monolithic, vibration proof, light weight, preferably solid state oscillators with low power consumption are in great demand for satellite communications and spectrometry. Due to the lack of photomixers with sufficient output power, presently bulky frequency multipliers with relatively high power consumption are used in space based applications.
  • THz imaging in physical, chemical and biological sciences is an emerging field with potential applications in chemical analysis and identification such as trace explosive detection, water and oil content analysis; noninvasive imaging of metal objects hidden in baggage; biological tissue analysis such as identification bad fat tissue from good tissue, cancer detection. Recently, skin cancer was detected using THz imaging. Semiconductor chip inspection, industrial process control, environmental monitoring, food inspection are few other important applications based on THz spectroscopy.

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  • Condensed Matter Physics & Semiconductors (AREA)
  • Electromagnetism (AREA)
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Abstract

L'invention concerne un mélangeur optique permettant de créer un rayonnement térahertzien et micro-onde, ou qui est utilisé dans un détecteur optique ayant un volume actif contenant une couche absorbante semi-conductrice dotée d'une surface supérieure et d'un fond, d'une couche de recouvrement anti-réfléchissante attenante à la surface supérieure de la couche absorbante, d'un espace libre situé au-dessus de la couche de recouvrement, d'un miroir encastré attenant au fond de la couche absorbante, et d'une pluralité d'électrodes interdigitées incorporées dans la couche absorbante à différents emplacements le long d'un axe z, où chaque électrode contient un plomb pour se connecter aux autres électrodes de la pluralité d'électrodes. Les électrodes sont formées au cours de la croissance épitaxiale de la couche absorbante semi-conductrice.
PCT/US2007/065475 2006-03-29 2007-03-29 Mélangeur optique permettant de créer un rayonnement térahertzien cohérent et de détecter le rayonnement WO2008054846A2 (fr)

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DE102008031751B3 (de) * 2008-07-04 2009-08-06 Batop Gmbh Photoleitende Antenne zur Abstrahlung oder zum Empfang von Terahertz-Strahlung
EP2466686A1 (fr) 2010-12-15 2012-06-20 Philipps-Universität Marburg Antenne d'émission et de réception de rayonnement GHz et/ou THz ayant une caractéristique de fréquence optimisée
GB2493193A (en) * 2011-07-27 2013-01-30 Thales Holdings Uk Plc Semiconducting optoelectronic switch for THz operation using undoped InGaAs with defects created by N-ion implantation.
US8809092B2 (en) 2009-07-17 2014-08-19 Edmund Linfield Generating and detecting radiation
DE102013020216A1 (de) * 2013-12-12 2015-07-02 Christopher Matheisen Auslegermikrostrukturbauelement zur optischen Erzeugung von elekromagnetischen Signalen im Terahertzfrequenzbereich
WO2016144779A1 (fr) * 2015-03-06 2016-09-15 Massachusetts Institute Of Technology Systèmes, procédés et appareils de détection de rayonnement
RU2657306C2 (ru) * 2016-10-07 2018-06-13 Федеральное государственное бюджетное учреждение науки Институт сверхвысокочастотной полупроводниковой электроники Российской академии наук (ИСВЧПЭ РАН) Материал на основе InGaAs на подложках InP для фотопроводящих антенн
WO2018160858A1 (fr) * 2017-03-01 2018-09-07 Phase Sensitive Innovations, Inc. Photodiodes à dos de diamant, photodiodes à prise en sandwich de diamant, systèmes de photodiode et procédés de fabrication associés
CN113904208A (zh) * 2021-09-18 2022-01-07 江苏师范大学 一种高纯度拉盖尔高斯光束产生系统及其产生方法

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Cited By (15)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
DE102008031751B3 (de) * 2008-07-04 2009-08-06 Batop Gmbh Photoleitende Antenne zur Abstrahlung oder zum Empfang von Terahertz-Strahlung
US8809092B2 (en) 2009-07-17 2014-08-19 Edmund Linfield Generating and detecting radiation
EP2466686A1 (fr) 2010-12-15 2012-06-20 Philipps-Universität Marburg Antenne d'émission et de réception de rayonnement GHz et/ou THz ayant une caractéristique de fréquence optimisée
WO2012080105A1 (fr) 2010-12-15 2012-06-21 Philipps Universität Marburg Antenne servant à émettre et à recevoir un rayonnement en ghz et/ou thz à caractéristique de fréquence optimisée
GB2493193B (en) * 2011-07-27 2015-07-08 Thales Holdings Uk Plc Semiconductor device for optoelectric switching
GB2493193A (en) * 2011-07-27 2013-01-30 Thales Holdings Uk Plc Semiconducting optoelectronic switch for THz operation using undoped InGaAs with defects created by N-ion implantation.
DE102013020216A1 (de) * 2013-12-12 2015-07-02 Christopher Matheisen Auslegermikrostrukturbauelement zur optischen Erzeugung von elekromagnetischen Signalen im Terahertzfrequenzbereich
WO2016144779A1 (fr) * 2015-03-06 2016-09-15 Massachusetts Institute Of Technology Systèmes, procédés et appareils de détection de rayonnement
US9810578B2 (en) 2015-03-06 2017-11-07 Massachusetts Institute Of Technology Systems, methods, and apparatus for radiation detection
RU2657306C2 (ru) * 2016-10-07 2018-06-13 Федеральное государственное бюджетное учреждение науки Институт сверхвысокочастотной полупроводниковой электроники Российской академии наук (ИСВЧПЭ РАН) Материал на основе InGaAs на подложках InP для фотопроводящих антенн
WO2018160858A1 (fr) * 2017-03-01 2018-09-07 Phase Sensitive Innovations, Inc. Photodiodes à dos de diamant, photodiodes à prise en sandwich de diamant, systèmes de photodiode et procédés de fabrication associés
US10686084B2 (en) 2017-03-01 2020-06-16 Phase Sensitive Innovations, Inc. Diamond-backed photodiodes, diamond-sandwiched photodiodes, photodiode systems and related methods of manufacture
US11004989B2 (en) 2017-03-01 2021-05-11 Phase Sensitive Innovations, Inc. Photodiodes formed on a thermally conductive layer and, photodiode systems
CN113904208A (zh) * 2021-09-18 2022-01-07 江苏师范大学 一种高纯度拉盖尔高斯光束产生系统及其产生方法
CN113904208B (zh) * 2021-09-18 2023-07-14 江苏师范大学 一种高纯度拉盖尔高斯光束产生系统及其产生方法

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