Terahertz Sensors, Conscripting 4-1-12

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from photonics.com: 04/01/2012http://www.photonics.com/Article.aspx?AID=50538

Conscripting Terahertz SensorsLynn Savage, Features Editor, [email protected]

Imaging and spectroscopy in the terahertz frequency range eventually will give troops a welcome advantage in the field.

Terahertz waves are short enough to provide resolution of less than 1 mm, yet long enough to penetrate mostnonmetallic substances, such as the materials used to make clothing, rucksacks and tarps. As such, they are usefulto security agents and military personnel alike for revealing concealed weapons, chemical explosives and biologicalagents. Besides security applications such as airport screeners, higher-resolution terahertz sensors could provide

enhanced identification of battlefield targets, better missile guidance and other combat advantages.

Soldiers, marines and fighter pilots are increasingly trained to use not only the visible wavelengths that their eyescan process, but infrared wavelengths as well. To some in the military-aligned industries, sensors that readterahertz frequencies could help augment what the US Army calls its Future Combat System by supporting a widerange of microdevices that scan multiple frequencies at all times.

The US Army Research Office is a main driver of all terahertz-related defense technologies, according to DwightWoolard of the US Army Research Laboratory in Research Triangle Park, N.C. However, he noted, because theterahertz spectrum is very broad with extreme diversity across the regime, the ARO places emphasis on selectscientific and technology projects that are high risk and high reward.

As far as researchers in the field are concerned, the terahertz range runs from 300 GHz to as high as 10 THz. The

spectral territory beyond that is largely unexplored until you get closer to microwave frequencies.

Terahertz technology looks at the collective motion of molecules stacked in a group, said M. Hassan Arbab of theUniversity of Washington in Seattle. The spectral lines come from the vibrational modes of large molecules.

Spectral information acquisition has thus far relied upon only a couple of basic technologies, namely antenna-coupled semiconductors, cryogenically cooled bolometers and uncooled microbolometers paired with quantum-cascade lasers (QCLs).

Antenna-coupled sensors operate in the subterahertz (0.1 to 1 THz) or low-terahertz range (


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RDX (cyclotrimethylenetrinitramine, or cyclonite) are composed of a mash of volatilechemicals and binders.

Capturing reflectance from such mixtures requires a certain amount of surfaceroughness in the target material. Plastic explosives such as C-4, for example, havefairly rough particles, which have the same approximate size as the terahertz wavesthemselves. However, particles over a certain size tend to overly scatter terahertzwaves, affecting overall absorption. Therefore, despite being mostly acceptabletargets of terahertz radiation, some targets will have particles that naturally dampenthe ability of a scanner to successfully identify them.

Measured absorption characteristics of ametamaterial structure and THz-QCLemission spectrum. Courtesy of GamaniKarunasiri, Naval Postgraduate School.

Oddly, alpha-lactose, a form of thecommon sugar found in milk, makes ahandy (and safe) stand-in for someexplosives. Particles made of thematerials, when excited, exhibit

resonances at 540 GHz, 1.2 THz and 1.38THz. Pellets made chiefly of lactoseparticles are available in a multitude of sizes representing various surface roughness grades (akin to sand-paperratings), and tests using these have shown that terahertz waves become less reliable as the particle size and thusoverall surface roughness increases.

In 2010, researchers at the University of Washington devised a mathematical transform method to address theissue of surface roughness. Typical time-domain spectrographic analysis involves acquiring multiple pulsedreflection measurements from several spots on the target surface, then calculating the Fourier transform of eachmetric. At terahertz frequencies, however, target irregularities scatter so much energy that any signal gets buried inan avalanche of noise.

Lactose pellets stand in for more volatile chemicals when testing the

effects of surface roughness on terahertz sensing. a 5 400 grit (23-mparticle diameter); b 5 150 grit (92 m); c 5 P80 grit (201 m). (d) showsthe normalized reflection spectral amplitudes of these samples.Reprinted with permission of Applied Physics Letters.

Instead, the Washington group Arbab and his colleagues Antao Chen,Eric I. Thorsos and Dale P. Winebrenner tweaked a wavelet transformmethod, not uncommon in terahertz investigations. Dubbed themaximal overlap discrete wavelet transform, the groups techniquepermits a better spectral signal-to-noise ratio from targets, even givenvery rough surface equivalents and only a few disjoint terahertzmeasurements of the target particles. The researchers report, however, that more work is needed to nail down the

minimum number of measurements needed for certainty.

Their development of the wavelet transform was reported in Proceedings of SPIE, Vol. 7601, and in AppliedPhysics Letters, Vol. 97, 181903.

Illumination

Since terahertz radiation is highly absorbed by the atmosphere, long-range standoff imaging requires relativelyhigh-power terahertz illuminators, depending on the distance, said Karunasiri of the NPS. The best laser sourcesavailable to date are either terahertz QCLs or free-electron lasers (FELs), which require [a] large infrastructure tooperate.

FELs offer high power and a wide range of tunability within the terahertz frequency range, which is important not

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only for imaging, but also for spectroscopy, which permits better identification of chemical and biological threats.More compact FELs that are under development should lead to compact high-power sources for field applications,Karunasiri said.

Graphene individual sheets of carbon atoms also is gaining attention as a possible terahertz source. Theoptoelectronic properties of graphene make the material an interesting subject for a number of research groups,including that of Alexander W. Holleitner at the Technical University of Munich in Garching, Germany.

When pumped by a QCL laser, two or more layers of graphene freely

suspended on bimetal supports emit terahertz radiation. This could lead tocompact emitters necessary for portable operation. Courtesy of Alexander W.Holleitner, Technical University of Munich.

Generally, my group explores the time-resolved optoelectronic dynamics in avariety of nanoscale materials, including nanowires, carbon nanotubes, metalnanoparticles, molecules and corresponding hybrid structures, Holleitner said.The dynamics comprise ultrafast dielectric displacement currents, the driftcurrents of photogenerated charge carriers and recombination processes.Furthermore, the tunable bandgap in bilayer graphene is in the terahertzrange, he said.

Holleitner and his colleagues reported in the April 12, 2011, issue of Nature Communicationsthat they tested thephotoelectric response of graphene. They placed bilayered graphene into a circuit that also included a pair oftitanium and gold striplines.

Using a mode-locked Ti:sapphire laser with a pulse width of ~160 fs and a repetition rate of about 76 MHz, theinvestigators pumped the graphene. Terahertz radiation was emitted from a charge-carrier plasma that wasgenerated by the optically pumped graphene. Increasing the laser power, Holleitner said, increased the frequencyof the terahertz emission.

The striplines are made out of t itanium and gold, but in principle, any metal will do it, Holleitner said. The co-planarstriplines fulfilled two purposes for the researchers. First, they acted as a near-field antenna for the terahertzradiation; and second, they transduced the signal to an ultrafast low-temperature-grown GaAs photodetector placedto the side of one of the metal striplines. The stripline circuit is up to five orders of magnitude more sensitive inpicking up the terahertz signal than are far-field detection mechanisms, Holleitner said.

Attenuation due to atmospheric effects is a problem. Attenuation can be hundreds of decibels per kilometer. Atshorter distances say, hundreds of meters it is less of an issue. Large bands in the 300-GHz to 2-THz and 6- to10-THz ranges are particularly less prone to atmospheric effects. More powerful lasers, such as free-electrondevices, can increase the amount of safe distance, but are so large as to preclude field use.

Diode lasers are more compact, of course, but are generally very low in power. This is a trade-off that researchersare attempting to avoid with new technologies.

For example, one alternative to addressing the attenuation problem has been proposed: Xi-Cheng Zhang of theUniversity of Rochester in New York and his colleagues have developed a method to generate terahertz wavesclose to a target, then analyze and communicate the spectroscopic findings to personnel a safe distance away via800-nm waves.

The future warrior

The most important terahertz application, Karunasiri said, is standoff detection of concealed weapons andidentification of explosives. At the NPS, work on future terahertz sensors includes the development of thermaldetectors based on microbolometers and on bimaterial pixels.

Bimaterial detectors, which combine heat-sensitive substances such as SiO2 and aluminum, rely on the deflectionresulting from differences in the thermal expansion of the two materials, Karunasiri said. Besides focal plane arrayscomprising SiO2 and aluminum (sans substrate), his group has developed nanoscale films made of chromium ornickel that provide up to 50 percent terahertz absorption within the 1- to 10-THz range. The film thickness andconductivity play an important role in achieving the 50% absorption for a given metal. The group has alsodeveloped metamaterial-based narrowband (~1 THz) devices that absorb nearly 100 percent at a targeted terahertz


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frequency in the 1- to 10-THz range.

A schematic (below) and micrograph (above) show the design andfabrication of a metamaterial structure tuned to a quantum cascadelaser (QCL) frequency. Courtesy of Gamani Karunasiri, NavalPostgraduate School.

Our goal is to develop highly sensitive terahertz focal plane arrays for

real-time terahertz imaging, he said.

Everything is 10 to 20 years down the road, Arbab said. But one day,robots will roll down the road, emitting laser beams toward suspectedtargets and identifying dangerous contents within innocuous-lookingpackages, he suggests. Currently, terahertz technology can be used tolook for the spectral signatures of C-4, TNT, RDX and more. A spectraldatabase of most/all of the known explosives is practically there, hesaid.

Its now a matter of making the technology catch up.

The Main Hurdles to Terahertz Technology

#1. Terahertz imaging and terahertz spectroscopy use different partsof the spectrum and require differenttechnologies. Imaging at 300 GHz (as with airport scanners) is not harmful to people, but you dont get vitalspectrographic information from it.

#2. Water vapor, even in very dry deserts, brings about major atmospheric attenuation; for example, in a room with20 percent humidity, a terahertz signal ends at 1 m.

#3. More powerful lasers are needed to make both terahertz spectroscopy and imaging possible from greaterdistances between source and target, and powerful QCLs and FELs are too large and expensive to hand out totroops in the field.

Applying Terahertz Waves to Burn Triage

Terahertz radiation, it turns out, isnt only good for determining the threat level of a would-be terrorist or combatant.

The problem with military/security applications is that youre always playing hide-and-seek, said M. Hassan Arbabof the University of Washington in Seattle. Its important, but it occurred to us that there are targets that dont hide.

More than a million people are treated for burns in the US every year; battle wounds and burns also must beevaluated to determine which tissue is damaged, how badly it is damaged and which is still healthy. Third-degreeburns are the most problematic because they require excision of the injured tissue. Its not as easy as making aquick visual inspection; current techniques for evaluating third-degree burns are only about 60 percent successful,and that is under optimal conditions.

Terahertz radiation, which is nonionizing and thus will not further harm skin cells, is highly sensitive to the watercontent of the dermal layers of the skin. When heat burns the skin, fluids build up between the skin cells, protectingwhats left of the dermis. Arbab and his colleagues reported in the Aug. 11, 2011, issue of Biomedical Opticsthatterahertz signals increase when the fluids called interstitial edema are present. They found, for example, thatthird-degree burns have about 30 percent higher reflectivity in the terahertz range than does normal skin.

The variations in terahertz reflectivity readings permit rapid diagnosis of not only the current state of a burn, but alsoof its future. Terahertz radiation helps tell which partial-thickness burns will progress to full-thickness, Arbab said.


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Terahertz Sensors, Conscripting 4-1-12