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Spie proceedings final_prof_eb_lpw

  1. 1. 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 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 being“pulled” by industrial applications in broadband wireless and fiber-optic communications. Two examples are the
  2. 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. It’s 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. 3. Active Region, 9x9 Micron Desired 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. 4. (a) Interdigital + Electrode Interdigital Gap Electrodes Contacts Ultrafast ~1 m Photoconductor 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. Photoelectrostatics As 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): 2 1 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. 5. 240  modified Booker’s Impedance [Ohms] Real resistance 150 9-micron 9-micron gaps arms 0 0 Imaginary -150 Active Bias 0.2 0.6 1.0 1.4 1.8 2.2 2.6 3.0 Region Lead 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 (<< 1 ps) inherent to ultrafast photoconductors, the gain averaged over theactive volume in Fig. 2(b) is typically ~0.1 in optimized structures, whether made from LTG-GaAs or ErAs:GaAs. II.B. THz Electromagnetics As the ultrafast PC devices have such high differential resistance, a simple way to increase the THz power is toincrease the driving-point resistance of the THz load circuit. At microwave frequencies this would be a relativelystraightforward application of a transformer circuit of some sort. But at THz frequencies, and with the decade or morebandwidth these devices offer, transformation is not so easy. So, to date, the common strategy has been to embed PCdevices in the driving gap of planar antennas, or nearby, to minimize coupling losses caused by the parasitic impedancesthat invariably affect THz integrated circuits. Some of the first useful PC-coupled antennas came from the class oftraveling wave antennas, such as the tapered dipole (that is, bow-tie) [7] and tapered slot (called Vivaldi if the taper is
  6. 6. exponential) [8]. Comparable bandwidth, but superior THz efficiency and beam patterns, were then demonstrated fromlog periodic [7] or log-spiral [8] designs. If designed with special symmetry properties such as equiangularity or log-periodicity, such antennas have a frequency independent radiation pattern and a nearly real and frequency-independentradiation resistance RA=Re{ZA}. If the antenna is also self-complementary in form, then its impedance can approach themodified form of Booker’s relation RA ≈0[2(eff)1/2], applicable to an air-dielectric interface, where 0 is thecharacteristic impedance of free space and eff is the effective dielectric constant [9]. The high THz permittivity (r =12.8) of GaAs yields a Booker resistance RA ≈more than 5x lower than 0 and undesirable for the standpoint ofTHz generation efficiency. On the other hand, a large r makes most of the radiation propagate into the substrate side ofthe interface, which then makes simple spherical-lens coupling quite effective on the backside of the substrate [11].Remarkably, some 30 years after its first demonstration at THz frequencies, spherical-lens coupling is still the preferredway of coupling radiation from ultrafast PC devices to free space. Another drawback of the common self-complementary antennas is physical size, which generally must extendover at least one free-space wavelength in diameter for good wideband performance. A simpler but less exploredantenna structure is the square spiral shown in Fig. 3(a). While lacking the equiangular-or log-periodic symmetryproperties, it is still self-complementary and thus offers the possibility of large bandwidth [10]. Fig. 3(a) shows abaseline square-spiral design for THz frequencies, and Fig. 3(b) shows its radiation impedance computed using acommercial Method-of-Moments code. The real part of the computed resistance above 200 GHz varies between about100  at the valleys and 240  at the peaks. This is in good agreement with the frequency-dependent variationsobserved experimentally. Somewhat surprising, but beneficial, is the large deviation of the resistance from the 72-Booker’s value. As expected from the Kramers–Kronig relations, the imaginary part is always significant, starting outmostly inductive between ~200 and 500 GHz, and becoming capacitive at higher frequencies. The real part stays abovethe modified Booker formula until 1.9 THz, and then falls below it at all higher frequencies. With its high average driving point resistance below 1.0 THz, the square spiral has produced the highest powerlevels we have ever achieved from photomixers and photoconductive switches. This includes a photomixer cw power ofover 10 W around 100 GHz [5], and a PC switch average power of over 1 mW spread over the range from ~0.1 to 1.0THz. The latter result is discussed in more detail later. II.C. Laser Coupling and Thermal Management As in all optoelectronic devices, external laser coupling is an important factor for photonic-to-THz conversionefficiency and laser stability too since even back-reflection from a photomixer, for example, can create diode-laserinstabilities if not isolated to a very high degree. In addition all semiconductors are imperfect absorbers with absorptioncoefficients typically in the range between 5,000 and 10,000 cm-1 (depending on the proximity of the drive wavelengthto the band-gap wavelength). Thus a significant amount of laser power is absorbed ~1 micron or deeper in the activelayer where according to Fig. 2(b) the electrostatic collection of photocarriers is much worse than at the top.
  7. 7. Silicon nitride film h Ultrafast 0.31 m Layer AlxGa1-xAs 1.09 m Heat Gold Spreader electrode AlAs/ 10 AlGaAs repeat Dielectric units Mirror Semi-insulating GaAs substrate Silicon Lens Dielectric Lens THz Output Beam Fig. 4. Improved THz photoconductive-switch and photomixer device structure in which the ultrafast (subpicosecond-lifetime) photoconductive layer is separated from the GaAs substrate by an AlGaAs heat-spreading layer and an AlAs/GaAs dielectric-mirror stack. Good top-side laser coupling entails some simple optical procedures. The first applies to interdigital-electrodePC devices such as that shown in Fig. 1 whereby the polarization of the incident laser beam is oriented perpendicular tothe electrodes to minimize reflection by grating effects. Of course there is still specular reflection from the electrodemetal that increases with the metal fill-fraction, but this can generally be kept at 10% or less. The second procedure isjust an antireflection coating as shown in the cross-sectional view of Fig. 4. At the laser wavelengths typically used forGaAs (~780 nm) or In0.53Ga0.47As (~1550 nm), it relatively easy to deposit a /4-wave-thick film of silicon nitride,silicon dioxide, or some ternary alloy that can reduce the air-semiconductor reflection to well below 10%. If properlydeposited, such films can also act as surface passivants and protective coatings for both GaAs and InGaAs. Improving the laser coupling within the active layer is more difficult, but made feasible by the molecular-beamepitaxy growth technique commonly used to grow ultrafast PC materials. As shown in Fig. 4, one can grow a dielectricmirror between the active layer and the substrate that reflects laser radiation not absorbed on the first pass through theactive layer. This takes advantage of the availability of aluminum and its ternary alloys AlGaAs and InAlAs in the MBEprocess, and the fact that the optical refractive index of the Al-bearing compounds is significantly lower than GaAs orInGaAs. About 10 alternating layers of GaAs and Al0.9Ga0.1As, for example, creates a dielectric mirror having areflectivity of ~90%. It is also important to judiciously locate the mirror with respect to the top air-semiconductorinterface to create a constructive interference. By so doing, one can create a “resonant optical cavity (ROC)” in whichfor a given incident laser power, far more photoelectron hole pairs are created per unit volume than in a single-passdevice [11]. This then allows one to make the active layer much thinner than 1 micron, which according to Fig. 2(b)
  8. 8. allows more of the photoelectron generation to be near the surface where dc fields are stronger and electrostaticcollection is more efficient. Such an ROC structure was, in fact, used in the most powerful photomixer that we haveever tested [5]. Like practically all solid-state THz sources, PC devices are ultimately limited in output power and performanceby thermally-related failure. The two primary sources of heat are the optical power absorption and the Joule heatingfrom photocurrent flowing in the bias field. A secondary source prevalent in the narrow-band-gap THz PC materials likeInGaAs, is Joule heating from dark current. The “junction temperature” TJ (at the top air-semiconductor interface at thecenter of the active area) can then be estimated from elementary thermal analysis as TJ = T0 + PQ·RTH where T0 is theambient temperature, PQ is the total power dissipation by heat, and RTH is the device thermal resistance. Being a planardevice and assuming a round heating area of radius REQ, we can re-write this as TJ = T0 + PQ/[(2)1/2REQ], where  isthe bulk thermal conductivity [12]. For a typical GaAs photomixer, for example,  ≈ 0.45 W/cm-K and REQ ≈ 5 m, sothat RTH ≈ 964oC/W ! If we then estimate the maximum rise above ambient as 120oC (a rule-of-thumb for some GaAsdevices, corresponding to a maximum junction temperature of 150oC), the maximum total power dissipation of PQ = 124mW. Indeed, this is close to what is observed experimentally in GaAs devices, where the combined laser power isgenerally limited to 80 mW or less, and the photocurrent is typically about 1.0 mA at a maximum bias voltage of 30 V,for a total PQ of 110 mW. To extend the lifetime of critical devices such as those packaged into sophisticatedinstruments, the total laser power must be backed off about 2x below this. SHG specs: 49 MHz PRF Center  782 nm = Pave = 20 mW EDFA specs: Si Hyperhemisphere Bias 49 MHz PRF Pulsewidth < 200 fs Supply Center  = 1572 nm Pave = 100 mW Pulsewidth < 100 fs Power SHG Meter Unit SMPM Fiber Microscope Mode-Locked Thermopile Photoconductive Objective EDFA Head Switch (a) 3.5 3 Current [microamp] Ave Power [mW] Ave Power [mW] 2.5 2 1.5 1 0.5 0 0 50 100 150 Bias Voltage [V] (b)Fig. 5. (a). Experimental set-up for testing high-power photoconductive switches. (b) Dark I-V curve and THzaverage power vs bias voltage.
  9. 9. III. HIGH-POWER PHOTOCONDUCTIVE SWITCH The photoconductive (sometimes called “Auston”) switch is the oldest and simplest of the ultrafastphotoconductive devices, but not as well characterized as photomixers in terms of THz power. The reason is simple:from their introduction in the early 1990s, photomixers were contrasted against indigenous devices such as Schottky-diode multiplier chains, because of their potential application as local oscillators in THz superheterodyne receivers. Toqualify for this application, it was important that the photomixers minimally supply a power level adequate for drivingcryogenic superconductive mixers, for example, which means roughly 1 microwatt cw. Not having such conventionalapplications allowed the PC switch to evolve successfully as the transmit and receive element in time-domains systemswithout a good understanding of its absolute power capacity. Given this situation, the author embarked on characterizing the average power of PC switches with the samelevel of scrutiny normally applied to photomixers, and with similar metrological methods. To make the comparison asobjective as possible, several design factors were kept constant, including the ultrafast material (ErAs:GaAs), theantenna design (square spiral), and the pump wavelength (780 nm). The PC switch was embedded in the three-turn, self-complementary, square spiral antenna shown of Fig. 3(a). The active area is the 9 x 9 micron driving gap at the center ofthe antenna. The experimental set-up used to characterize the PC switch is shown in Fig. 5(a). The switch was drivenby an erbium-doped fiber mode-locked-laser with a PPLN doubler to produce ~780-nm pulses [13]. Initially, a Golaycell was used to measure the power, but was quickly driven to saturation. So it was replaced with a small thermopile(sensitive to the mW-level) which started recording at ~0.1 mW. The results for THz average power Pave vs PC switchbias voltage are plotted in Fig. 5(b) along with the dark current-voltage characteristics. As in typical PC switches andphotomixers, Pave rises monotonically with bias voltage and approaches a maximum value of 1.6 mW. Higher biasvoltages were not attempted because of the onset of impact ionization seen in Fig. 2. To the best of our knowledge, thisis the highest Pave ever reported for a THz PC switch and exceeds by almost ten times the initial report from a devicehaving similar design [14]. The discrepancy is attributed to saturation of small-signal free-space-coupled THz detectors(Golay cell in Ref. [16]) typically used to measure power. Thermopiles are well-known for large dynamic range and theability to measure pulses having high peak power Ppeak. In the present case, the maximum Ppeak can be estimated fromthe 150-V bias data using Ppeak ~ Pave/(frep · tp) where frep = 49 MHz is the laser repetition frequency and tp ~ 1 ps is theapproximate THz pulse width into free space. The result is Ppeak = 33 W - an impressive number for the THz regionwhere powerful sources, pulsed or cw, are lacking.
  10. 10. Transimpedance Wavemeter Lock-In Amp Fixed DFB Laser Isolator  780 nm Beam Receive Combiner + Photomixer Tunable Focusing Transmit DFB Laser Isolator Lens Photomixer >780 nm Silicon Nanofluidic Cell (Top View) Lens Nanofluidic SiO2 Chip Channel Limiting Aperture. Chip Holder SiO2 Wall THz Path THz circular polarization Off-Axis ParaboloidsFig. 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 APPLICATIONS IV.A. Frequency Domain Arguably 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. 11. Magnitude Power [Arb Units] 80 dB Phase Sensitive 60 dB 40 dB Noise Floor Frequency [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. 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-µL) 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. 13.   100 101 si-RNA signatures Transmitted Power [Arb Units] 100 si-RNA Transmission Sample Background 10-1 10-1 10-2 Noise Floor 10-3 (1) (3) (2) 10-4 10-2 500 600 700 800 900 1000 1100 1200 700 800 900 1000 1100 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. 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 (<< 1 ns) impulse response and wide video bandwidth (> 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 (NET) usedwidely in radiometric imaging, but here tailored to water detection – the noise-equivalent change in water concentration
  15. 15. (a) “Halo” Feature (b) (c) Fig. 10. (a) Visible photograph of branded burn made on in-vitro porcine skin. (b) THz image of same burn. (c) THz image of same burn through five layers of gauze. In the THz images, the spatial resolution is 2 mm, and the image size and acquisition time were 1 KPixel and 5 min, respectively.(NEWC). Performing calibrated evaporation experiments, we have determined NEWC  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 <1 min over 1 KPixel and 1 sq. inch. V. ACKNOWLEDGEMENTS This work was sponsored by the U.S. Army Research Office, U.S. Army TATRC, and the National ScienceFoundation. Special thanks goes to Dr. Dwight Woolard for supporting THz research consistently for over a decade.
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