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Advancements in Photomixing and Photoconductive Switching for THz Spectroscopy and Imaging E.R. Brown Wright State University, Dayton OH 45324 Physical Domains, LLC, Glendale, CA, 91214, Dayton, OH 45324 ABSTRACT This paper reviews the design methodology and some of the applications space of standard photomixers and photoconductive switches. The methodology falls into three categories: (1) photoelectrostatics, (2) terahertz (THz) electromagnetics, and (3) laser coupling and thermal management. The applications space of ultrafast photoconductive devices, as for any device technology, is the best measure of their utility. At present photomixers are being used worldwide in at least these two instruments: (1) broadly tunable sweep oscillators for THz diagnostics, and (2) broadly tunable coherent transceivers for high-resolution THz spectroscopy. Photoconductive switches are being used in at least these two systems applications: (1) time-domain spectrometers, and (2) illuminators for THz impulse radars. Each of these applications will be addressed in turn, and some commercialization challenges facing ultrafast photoconductive devices will be addressed. Keywords: ultrafast photoconductors, photoconductive switches, photomixers, photoelectrostatics, terahertz, THz, electromagnetics, spectroscopy, frequency-domain spectrometer, time-domain spectrometer, impulse radar. I. INTRODUCTION The THz portion of the electromagnetic spectrum occupies the spectral range from 300 GHz to 3 THz (or beyond, depending on who is defining it) and has long been the realm of gas-phase molecular spectroscopy and astrophysics and, to a lesser extent, earth sensing and materials science. This situation has changed dramatically in the past decade with the heightened interest in concealed weapon and contraband detection for homeland security, biological-agent detection, and biomedical imaging. Along with these world-event-related interests have come heightened scientific interests in molecular chemistry, biochemistry, and biology. A second factor in the recent advancement of the THz field is the maturation and commercialization of the fields of high-speed electronics and optoelectronics, photonics, and materials science, many of which are now being “pulled” by industrial applications in broadband wireless and fiber-optic communications. Two examples are the

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  • 1.Advancements in Photomixing and Photoconductive Switching forTHz Spectroscopy and ImagingE.R. BrownWright State University, Dayton OH 45324Physical Domains, LLC, Glendale, CA, 91214, Dayton, OH 45324 ABSTRACTThis paper reviews the design methodology and some of the applications space of standard photomixers andphotoconductive switches. The methodology falls into three categories: (1) photoelectrostatics, (2) terahertz (THz)electromagnetics, and (3) laser coupling and thermal management. The applications space of ultrafast photoconductivedevices, as for any device technology, is the best measure of their utility. At present photomixers are being usedworldwide in at least these two instruments: (1) broadly tunable sweep oscillators for THz diagnostics, and (2) broadlytunable coherent transceivers for high-resolution THz spectroscopy. Photoconductive switches are being used in at leastthese two systems applications: (1) time-domain spectrometers, and (2) illuminators for THz impulse radars. Each ofthese applications will be addressed in turn, and some commercialization challenges facing ultrafast photoconductivedevices will be addressed.Keywords: ultrafast photoconductors, photoconductive switches, photomixers, photoelectrostatics, terahertz, THz,electromagnetics, spectroscopy, frequency-domain spectrometer, time-domain spectrometer, impulse radar.I. INTRODUCTION The THz portion of the electromagnetic spectrum occupies the spectral range from 300 GHz to 3 THz (orbeyond, depending on who is defining it) and has long been the realm of gas-phase molecular spectroscopy andastrophysics and, to a lesser extent, earth sensing and materials science. This situation has changed dramatically in thepast decade with the heightened interest in concealed weapon and contraband detection for homeland security,biological-agent detection, and biomedical imaging.Along with these world-event-related interests have comeheightened scientific interests in molecular chemistry, biochemistry, and biology. A second factor in the recent advancement of the THz field is the maturation and commercialization of thefields of high-speed electronics and optoelectronics, photonics, and materials science, many of which are now beingpulled by industrial applications in broadband wireless and fiber-optic communications.Two examples are the

2. engineering of nanostructures by molecular-beam epitaxy, and deep-submicron lithography to fabricate devices havingTHz speeds. Another example is the commercial availability of high-performance sources, such as near-infrared single-frequency semiconductor and solid-state lasers and optical amplifiers.This is particularly true of optical-fibercomponents and amplifiers in the telecommunications band around 1550 nm. A third factor is the advent and rapid development of ultrafast photoconductive devices. Arguably their impactduring the past two decades has been on par with Schottky diodes as a building block for new THz components andsystems. Photoconductive switches have become the workhorse in time-domain systems, and photomixers have beenwidely implemented in high-resolution frequency-domain systems of various types. The primary photoconductivematerial has been low-temperature-grown gallium arsenide (GaAs). More recently, this has been rivaled by erbiumarsenide-gallium arsenide (ErAs-GaAs): a nanocomposite consisting of ErAs nanoparticles embedded in a GaAs matrix.ErAs-GaAs photomixers have produced very useful THz output power levels between 1.0 and 10.0 microwatts whenpumped by low-cost distributed feedback (DFB) lasers operating around 780 nm. ErAs-GaAs photoconductive switcheshave produced average output power up to ~1 mW, and peak power exceeding 10 W when pumped by frequency-doubled fiber mode-locked lasers. Device performance is always important, but system applications is another matter. To be useful in systems,devices must have unique capabilities, and be reliable and affordable. Without a doubt, the unique feature of ultrafastphotoconductive devices is bandwidth. Photomixers are continuously tunable over at least 1.0 THz, usually limited bythe drive lasers (when using DFBs or similar laser diodes).Photoconductive switches generally have a hugeinstantaneous bandwidth of 0.5 THz or more depending on the pulse width of the mode-locked laser driver and theimpulse response of the photoconductive switch in its THz embedding circuit. Bandwidth is important in THzspectroscopic instruments of all sorts since spectral signatures from interesting materials, such as explosives or toxicgases, can be spread over a decade of frequency or more. Its also important in impulse radar where instantaneousbandwidth determines the pulsewidth in the time-domain, which, in-turn, defines the range resolution. II. BACKGROUND ON THz PHOTOCONDUCTIVE DEVICES As is now well understood, photomixing (short for photoconductive mixing) entails the driving of an ultrafastphotoconductive two-terminal structure with two single-frequency, frequency-offset lasers. The result is a highly-tunable, continuous-wave (cw), coherent source of radiation contained in a single spatial mode, either in a transmissionline or free space. Fig. 1 shows a microphotograph of a typical THz photomixer used today. In contrast,photoconductive (PC) switching entails the pumping of an ultrafast photoconductive two-terminal structure with a singlemode-locked laser. The result is a train of subpicosecond pulses whose power spectrum is a comb peaked in the sub-THz region, but still produces useful power well beyond 1.0 THz. Most of the photomixer and PC-switch research overthe past decade has been carried out on devices made from low-temperature-grown (LTG) GaAs, or ErAs:GaAs. 3. Active Region, 9x9 MicronDesired Polarization Fig. 1. Top view of a typical GaAs photomixer showing the interdigitated-electrode active region at the driving gap of a square spiral antenna. Photomixers and PC switches have become a very useful and successful THz device technology during the pastdecade. They are now being used worldwide and have been integrated into commercial systems in both the UnitedStates [1, 2] and Europe [3,4]. The two devices complement each other to a large extent. The PC switch is well suited totime-domain THz spectroscopy with modest resolution requirements, ~10 GHz, but very broad spectral coverage, upto 3 THz or greater. The photomixer is well suited to high-resolution ( < 1 GHz) spectroscopy over a more modestspectral range of ~2 THz.PC switches generally have greater spectral coverage than photomixers because of theirlower capacitance and lower RC time constant under laser operating conditions. A big difference between photomixers and PC switches is average power. In devices fabricated from the samematerial and coupled to the same planar antenna or transmission line, the photomixer is limited to just a few W below1.0 THz. The corresponding optical-to-THz conversion efficiency is less than 10-4 [5]. The PC switch typicallyproduces ~100x higher average power than a photomixer, and a similar margin in optical-to-THz conversion efficiency[6]. After analysis and large-signal equivalent-circuit modeling, the difference can be primarily attributed to impedancematching. Both devices have very high dark differential resistance, photomixers between 107 and 108 Ohms, and PCswitches between 108 and 109 Ohms at their respective bias voltages. Under illumination, however, the PC switchsinstantaneous resistance will drop to 100 Ohms or even less because of the high peak power that mode-locked laserstypically provide. In contrast, the photomixer will drop to a minimum of 10 k, depending on the laser drive power,which is usually taken from single-frequency distributed feedback (DFB) semiconductor lasers. Attempts to reduce thisresistance further by increasing the laser power usually leads to device burnout. As such, photomixers generally presenta poor impedance match to their THz load circuits, which has a major impact on the THz delivered power and theoptical-to-electrical conversion efficiency. Given these issues, great care must be exercised in the design and fabrication of THz photoconductive devices,particularly photomixers. The first and foremost issue is the choice of ultrafast material. Unfortunately, 20 years afterthe advent of LTG-GaAs and more than a decade after ErAs:GaAs, these materials are still rather exotic and difficult to 4. (a)Interdigital +ElectrodeInterdigital GapElectrodesContacts Ultrafast ~1 mPhotoconductor Decreasing Field Magnitude Semi-Insulating InP Substrate(a)(b)Fig. 2. (a) Top view of interdigital electrode structure commonly used in THz ultrafast photoconductive devices. (b)Cross-sectional view of active region along dashed line shown in (a). The electric lines of force are represented by thecurved loci with the largest magnitude of electric field occurring at the top air-semiconductor interface.obtain. There are several reasons for this, not the least of which is the unusual growth materials or conditions required todo the molecular beam epitaxy (MBE). Because MBE challenges are difficult to overcome, this paper will focus onimportant issues that the THz engineer or scientist has more control over, which are: (1) photoelectrostatics, (2) THzelectromagnetics, and (3) laser coupling and thermal management.II.A. PhotoelectrostaticsAs the name suggests, ultrafast photoconductivity is a balancing act between the internal photoelectric effectand the collection of photogenerated carriers by drift and diffusion between two electrodes under bias. The internalphotoelectric effect produces more carriers as the thickness of the semiconductor increases, which in-turn reduces thecollection efficiency, increases the device capacitance, or both. This tradeoff is captured by the following expression forthe maximum difference-frequency power Pdiff generated from photomixers (below the frequencies where rolloff starts tooccur): 21 2 eg Pdiff i RL P1 P2(1)2 h where i is the difference-frequency photocurrent amplitude, is the external quantum efficiency (i.e., the fraction ofincident photons that produce photoelectrons or photoholes in the active region), and g is the photoconductive gain.From the Shockley-Ramo theorem of device electrostatics, the photoconductive gain is the mean distance an electron orhole drifts in the dc bias field before recombination, divided by the physical distance between the electrodes. 5. 240 modified Bookers Impedance [Ohms] Realresistance1509-micron9-microngapsarms00 Imaginary-150ActiveBias 0.2 0.6 1.0 1.4 1.8 2.2 2.6 3.0RegionLead Frequency [THz] (a) (b) Fig. 3. (a) Square spiral antenna used for THz photomixers and PC switches. (b) Real and imaginary parts of driving-point impedance of square-spiral antenna on left. A better qualitative understanding can be had by inspecting the popular interdigital-electrode THzphotoconductor structure shown in Fig. 2(a). Its popularity rests on simplicity of fabrication, low capacitance, and veryshort interconnects to balanced planar antennas and coplanar transmission lines of all sorts. The photoconductivetradeoff becomes clearer in the cross-sectional view of Fig. 2(b) which shows the elliptical electric lines of forcebetween two adjacent electrodes. The highest magnitude of electric field occurs at the top of the structure at thesemiconductor-air interface, and drops monotonically with depth in the photoconductive epitaxial layer. This means thatphotons absorbed near the surface will have the most contribution to the THz photocurrent in the structure, and thatphotons absorbed deeper in the structure will have a progressively weaker effect. As the bias voltage is increased toenhance the THz generation from the deeper-absorbed photocarriers, surface breakdown tends to occur from thecombination of dc leakage current and photocurrent under the high bias voltage and high laser drive power. Because ofthe short electron-hole recombination time (780 nm Silicon Nanofluidic Cell (Top View)Lens Nanofluidic SiO2 ChipChannel Limiting Aperture.Chip HolderSiO2WallTHz Path THz circular polarization Off-AxisParaboloidsFig. 6. Improved THz photoconductive-switch and photomixer device structure in which the ultrafast (subpicosecond-lifetime) photoconductive layer is composed of ErAs:In0.53Ga0.47As and grown on a semi-insulating indium phosphide(InP) substrate and separated from the top-side electrodes and THz circuit (antenna or transmission line) by ablocking layer of In0.52Al0.48As, which has a direct bandgap significantly larger than the ultrafast material.IV. SYSTEM APPLICATIONSIV.A. Frequency DomainArguably the most successful application of photomixers to date is high-resolution THz spectroscopy based onthe fully coherent photomixing transceiver [15,16]. It consists of two THz photomixers, each driven by the same pair ofsingle-frequency, single-mode, temperature-tunable distributed feedback (DFB) lasers. One photomixer acts as thetransmitter, and the other as the receiver. The temperature variable provides a continuously tunable coherent tone frombelow 100 GHz to 1.5 THz or higher with instantaneous linewidth of ~100 MHz or better [17]. A block diagram of oneconfiguration of the transceiver is shown in Fig. 6. The radiation from the transmit photomixer is coupled from theantenna to free space through a high-resistivity silicon hyperhemispherical lens. The THz beam is then collimated usingan aspherical optic, usually an off-axis paraboloid. The reciprocal process occurs between free space and the receivephotomixer. The sample under test can be mounted either in the collimated beam half-way between the two photomixerswhere the beam is collimated, or as shown in Fig. 6, close to the transmit photomixer where the beam is quite small (~3mm diameter) and more intense. 11. MagnitudePower [Arb Units]80 dBPhase Sensitive60 dB 40 dBNoise FloorFrequency [GHz]Fig. 7. Transfer function of coherent photomixing transceiver along with background noise floor. The phase-sensitivecurve plotted in gray is the in-phase (I) output of the receive photomixer (Ref. [21]). Because the lasers driving receive and transmit photomixers are mutually coherent, the THz beam into thereceive photomixer is mixed down in frequency by homodyne conversion. A simple amplitude modulation on thetransmit photomixer then allows for dc offset and straightforward synchronous detection with all the benefits oftraditional homodyne transceivers. As in any coherent system, the output of the transceiver maintains phase information.Fig. 7 shows the in-phase (I2) response (gray curve), the power response ([I2 + Q2] (black curve), and the noise floorobtained as the power response with the THz beam blocked but all other settings kept the same. The ratio of the powerresponse to the noise floor is the signal-to-noise (SNR) ratio, which is ~80 dB at 200 GHz, 60 dB at 1.0 THz, and 40 dBat 1.8 THz. These are excellent SNR values for a room-temperature system with such wide tuning bandwidth and highresolution, and can be attributed largely to the sensitivity advantage of coherent processing over incoherent (or direct)techniques [18]. Furthermore, the photomixing transceiver has no moving parts, runs at room temperature, and requiresno high voltages or large magnetic fields. The power response associated with Figs. 7 also exemplifies the complicated baseline that typically occurs incoherent THz spectroscopy. Visible at 556, 752 and at several frequencies above 1.1 THz are absorption lines from theambient water vapor in the ~1-foot path between the transmitter and receiver. However, away from these are otherundulations associated with variations in the intrinsic system transfer function. Fortunately, these undulations are not sodeep or plentiful as to preclude high-resolution spectroscopy. 12. A unique aspect of this instrument already utilized but not widely appreciated is the combination of spatial,coherence, temporal coherence, and wide tunability. The vertical orientation in Fig. 6 allows one to orient smallsamples, such as nanofluidic chips, horizontally. This facilitates the initial wetting and subsequent filling of thechannels. It also allows for locating the chip immediately below the transmit photomixer-coupling lens (a siliconhyperhemisphere) where the spot diameter is small (~3 mm diameter), as determined by the photomixer spiral antennaand the thickness of the lens. Assuming an average power of 1.0 W and instantaneous linewidth of 20 MHz, the THzbeam at this point has a spatial intensity of ~1.4x10-5 W/cm2, and the power spectral intensity is 0.7x10-3 W/cm2-GHz.We have found the latter quantity to be a good performance metric for THz sources in wideband spectroscopy.THz transmission experiments were carried out with the coherent photomixing transceiver customized for high-resolution measurement of weak absorption signatures, and a nanofluidic chip designed for biomolecular spectroscopy.By capillary action, the RNA-bearing solution filled the silica nanofluidic channels, which were 800 nm wide by 1000nm deep, on a pitch of ~1200 nm. The raw experimental results are plotted in Fig. 8(a) in the frequency range 800 to1200 THz - a band having two strong water vapor lines at 1098 and 1164 GHz, and a relatively weak line around 990GHz. The top curve is the background signal PB through the nanofluidic chip containing buffer solution only, themiddle curve is the sample signal PS with RNA suspended in the buffer, and the bottom curve is the noise floor PNobtained by blocking the THz path with a metal plate. In a typical experiment, the sequence of THz spectra acquisitionconsisted of first mounting the chip within an auto-aligning rail that enables precise and repeatable positioning of thenanofluidic chip sample in and out of the beam path of the THz spectrometer, followed by the acquisition of backgroundspectra of the chip in the absence of any fluids in the channels. Following this dry-chip background measurement, asecond wet-chip background was collected by placing a ~100-L drop of buffer in the nanofluidic chip fluidreservoirs, and measuring the background spectra of the buffer-filled channels. Finally, si-RNA drops (~100-L) wereadded to the reservoirs, allowed to disperse, and the THz spectrum was measured. The measurement was repeated onthe same sample six times, and good reproducibility was obtained. The three curves in Fig. 8(a) are used to compute thenormalized and noise-referenced transmission function T vs plotted in Fig. 8(b) based on T() = [PS() PN()] /[PB() PN()]. The transmission shows three prominent resonant signatures centered at 916, 962, and 1034 GHz, labeled (1),(2), and (3), respectively in Fig 8(b). There is also a broad and weaker signature (perhaps a multiplet) between 830 and875 GHz, and a narrow but weaker one centered at 1075 GHz. The feature around 1100 GHz is questionable since it ismixed with a very strong water vapor line, evident from the background transmission in Fig. 8(a). These features are ingood qualitative agreement with our previous experimental results obtained by similar methodology but using silicananochannels fabricated on high-resistivity silicon substrates rather than fused quartz [19]. The previous results yieldedprominent resonances centered at 1034, 950, 875, and 1084 GHz, all having comparable spectral widths as presentedhere but with weaker resonant absorbance and a lower signal-to-noise ratio. 13. 100 101 si-RNA signatures Transmitted Power [Arb Units] 100si-RNA TransmissionSample Background 10-110-1 10-2Noise Floor 10-3(1)(3) (2) 10-4 10-2500 600700800 900 1000 1100 1200 700 800 900 10001100 Frequency [GHz](a) Frequency [GHz](a)(b) Fig. 8. (a) Raw experimental data for the nanofluidic chip with pre-wet (glycerol-EDTA buffer), the nanofluidic chip with si-RNA solution filling the channels, and the spectrometer noise floor. (b) Normalized transmission spectra computed from the three raw data spectra in (a). The prominent attenuation signatures are labeled (1), (2), and (3).IV.B. Time Domain One of the most promising applications of THz today is in the field of biomedical imaging, particularlyburns and other lesions of human skin tissue. Most of these applications rest on the acute sensitivity of THz radiation towater concentration. Water has long been a bane of the THz radiation region in both the vapor and liquid states. Watervapor greatly attenuates the propagation through the terrestrial atmosphere, particularly between ~0.5 and 3.0 THz wherea large set of strong molecular rotational transitions occur. Liquid water is even worse because its attenuation occursover a broad continuum with absorption coefficient well exceeding 100 cm-1 above 0.5 THz [20,21,22,23]. In bothcases, the attenuation is absorptive and associated with the high built-in dipole moment (1.85 Debye) and relatively highmobility of the H2O molecule. From the Fresnel equations, we know that strong absorption can affect the reflection tooif the associated imaginary part of the refractive index is comparable to the real part. This is exactly what happens withwater in the THz region. Furthermore, human tissue is generally a composite of water and some biomolecular material(e.g., protein or polysaccharide, such as collagen). The biomaterials are not as polar as water, so they have little impacton the reflection. Hence the composite reflection is a strong function of the water concentration, which is the basis forour sensing technique. 14. (a) (b)Fig. 9. (a) Block diagram of THz impulse radar configured in reflection mode. (b) Power spectra obtained from thePC switch alone (dashed black line), and the portion collected by a WR-1.5 zero-bias Schottky diode (solid red line). Traditional THz time-domain imaging systems would work for this application, and has been addressed widelyin other review articles; however, this is not what we have focused on with the high-power PC switches. Instead wehave focused on a simpler and more portable type of sensor inspired by traditional radar design, specifically impulseradar. A strong motivation for our design is affordability. At the cost of traditional time-domain systems, THzbiomedical imaging would likely only be done in medical research clinics. With a simpler impulse radar design, it mightbe possible to reach a much broader medical arena, such as the urgent-care or general practitioner.Our sensor is the THz impulse radar design presented in Fig. 9(a). The transmitter is a high-efficiencyphotoconductive (PC) switch driven by a low-cost, 780-nm fiber mode-locked laser (MLL) having a pulse width of 230fs and pulse repetition frequency (PRF) = 20 MHz. The radiation from the PC switch consists of a train of pulses, eachhaving ~1 ps width. In the frequency domain, the power spectrum is broadly spread over a comb starting with =PRF and every harmonic thereof, and extending beyond 1.0 THz. While being a poor spectral match to molecular lines,it is a good match to the inherently broadband reflection of liquid water. In other words, a large fraction of the THzradiation from the PC switch contributes to the instantaneous reflected power from the sample. The reflected beam iscollected and rectified by a WR-1.5 waveguide-mounted (cutoff frequency 400 GHz), zero-bias Schottky barrier diodehaving fast ( 10 GHz). The received power spectrum is plottedin Fig. 9(b), showing a bandpass behavior centered at ~500 GHz. The resulting spatial resolution is far better than canbe achieved from typical ( ~ 3 mm) mm-wave imaging systems [25]. The diode output is then gated at the PRF with adelay-controlled reference pulse, and the baseband DC component is time-averaged to achieve a good signal-to-noiseratio (SNR). On specular surfaces such as smooth skin, the SNR reaches levels of 30 dB or higher with ~ 16 msintegration time.The best sensing metric for our radar is a variant of the noise-equivalent temperature difference (NET) usedwidely in radiometric imaging, but here tailored to water detection the noise-equivalent change in water concentration 15. (a)HaloFeature (b) (c)Fig. 10. (a) Visible photograph of branded burn made on in-vitro porcine skin. (b) THz image of same burn. (c) THzimage of same burn through five layers of gauze. In the THz images, the spatial resolution is 2 mm, and the image sizeand acquisition time were 1 KPixel and 5 min, respectively.(NEWC). Performing calibrated evaporation experiments, we have determined NEWC 0.054%. The bestdemonstration to date of this acute sensitivity was made by 2D imaging of in-vitro, physiological porcine skinconsidered by medical researchers as a good simulant for human skin. Fig. 10(a) shows the visible photograph and 10(b)the THz image of a branded burn with no obscuration. Fig. 10(c) shows the same burn through five layers of cottongauze. Fig. 10(b) displays interesting features not seen in the visible image of the burn, such as the "halo" that maydemark the spatial extent of tissue damage. Fig. 10(c) also supports the consensus that THz radiation can detect andimage through fabric materials that are opaque to infrared and visible radiation. The THz impulse radar imager iscurrently in rapid engineering evolution, and our near-term performance specifications are 2D image acquisition with ~1mm spatial resolution in