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The Truth About Terahertz

The Truth About Terahertz

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spectrum.ieee.orgMonth 2012 • IEEE Spectrum • NA 37


meaning its photons are not energeticenough to knock electrons off atoms andmolecules in human tissue, which couldtrigger harmful chemical reactions. <strong>The</strong>waves also stimulate molecular and electronicmotions in many materials—reflectingoff some, propagating through others,and being absorbed by the rest. <strong>The</strong>se featureshave been exploited in laboratorydemonstrations to identify explosives,reveal hidden weapons, check for defectsin tiles on the space shuttle, and screen forskin cancer and tooth decay.But the goal of turning such laboratoryphenomena into real-world applicationshas proved elusive. Legions ofresearchers have struggled with thatchallenge for decades.<strong>The</strong> past 10 years have seen the mostintense work to tame and harness thepower of the terahertz regime. I firstbecame aware of the extent of theseefforts in 2007, when I cochaired a U.S.government panel that reviewed compactterahertz sources. <strong>The</strong> review’schief goal was to determine the state ofthe technology. We heard from about30 R&D teams, and by the end we hada good idea of where things stood. Whatthe review failed to do, though, was givea clear picture of the many challenges ofexploiting the terahertz regime. What Ireally wanted were answers to questionslike, What exactly are terahertz frequenciesbest suited for? And how demandingare they to produce, control, apply, andotherwise manipulate?So I launched my own investigation.I studied the key issues in developingthree of the applications thathave been widely discussed in defense,security, and law-enforcement circles:communication and radar, identificationof harmful substances from a distance,and through-wall imaging. I also lookedat the 20 or so compact terahertz sourcescovered in the 2007 review, to see if theyshared any performance challenges,despite their different designs and features.I recently updated my findings,although much of what I concluded thenstill holds true now.My efforts aren’t meant to discouragethe pursuit of this potentially valuabletechnology—far from it. But thereare some unavoidable truths that anyoneworking with this technology inevitablyhas to confront. Here’s what I found.Although terahertz technologyhas been much in the news lately,the phenomenon isn’t really new. It justwent by different names in the past—near milli meter, submillimeter, extremefar infrared. Since at least the 1950s,researchers have sought to tap its appealingcharacteristics. Use of this spectralband by early molecular spectroscopists,for example, laid the foundationfor its application to ground-based radiotelescopes, such as the Atacama LargeMillimeter/ submillimeter Array, inChile. Over the years a few other nicheuses have emerged, most notably spacebasedremote sensing. In the 1970s, spacescientists began using far -infrared andsubmillimeter-wave spectrometers forinvestigating the chemical compositionsof the interstellar medium and planetaryatmospheres. One of my favorite statistics,which comes from astronomer DavidLeisawitz at NASA Goddard Space FlightCenter, is that 98 percent of the photonsreleased since the big bang reside in thesubmillimeter and far-infrared bands, afact that observatories like the HerschelSpace Observatory are designed to takeadvantage of. Indeed, it’s safe to say thatthe current state of terahertz technologyrests in good measure on advances inradio astronomy and space science.But orbiting terahertz instrumentshave a big advantage over their terrestrialcounterparts: <strong>The</strong>y’re in space!Specifically, they operate in a near -vacuum and don’t have to contend witha dense atmosphere, which absorbs,refracts, and scatters terahertz signals.Nor do they have to operate in inclementweather. <strong>The</strong>re is no simple way toget around the basic physics of the situation.You can operate at higher altitudes,where it’s less dense and there’sATMOSPHERICEFFECTS:Terrestrial signalssent at terahertzfrequenciescan experienceextremeatmosphericabsorption, dueprimarily to watervapor and oxygen.For horizontaltransmission atsea level andnormal humidity,as shown here, thesignal attenuationclearly peaksbetween 1 and10 terahertz.Atmospheric absorption (decibels/kilometer)1 000 000100 00010 0001 000100101less moisture, but many of the envisionedterahertz applications are foruse on the ground. You can boost thesignal’s strength in hopes that enoughradiation will get through at the receivingend, but at some point, that’s just notpractical, as we’ll see.Obviously, atmospheric attenuationposes a problem for using terahertz frequenciesfor long-range communicationand radar. But how big a problem? Toanswer that question, I compared differentscenarios for horizontal transmissionat sea level—good weather, bad weather,a range of distances (from 1 meter up to6 kilometers), and specific frequenciesbetween 35 gigahertz and 3 terahertz—to determine how much the signalstrength degrades as conditions vary. Forshort-range operation—that is, for signalstraveling 10 meters or less—the effects ofthe atmosphere and bad weather don’treally come into play.Try to send anything farther thanthat and you hit what I call the “terahertzwall”: No matter how much youboost the signal, essentially nothinggets through. A 1-watt signal with a frequencyof 1 THz, for instance, will dwindleto nothing after traveling just 1 km.Well, not quite nothing: It retains about10 -30 percent of its original strength. Soeven if you were to increase the signal’spower to the ridiculously high level of,say, a petawatt, and then somehow manageto propagate it without ionizing theatmosphere in the process, it would bereduced to mere femtowatts by the timeit reached its destination. Needless tosay, there are no terahertz sources capableof producing anything approaching0.1Source: M.J. Rosker& H. B. Wallace,June 20070.010.01 0.1 1.0 10Frequency, terahertzillustrations: George retseck38 NA • iEEE Spectrum • september 2012 spectrum.ieee.org


Transmitted power (decibel-milliwatts)1007550250-25-50-75-1006 kma petawatt; the closest is a free- electronlaser, which has an output in the low tensof megawatts and isn’t exactly a fielddeployabledevice. (For comparison, theoutput power of today’s compact sourcesspans the 1-microwatt to 1-W range—more on that later.) And that’s underordinary atmospheric conditions. Rainand fog will deteriorate the signal evenmore. Attenuation that extreme all butrules out using the terahertz region forlong-range ground-based communicationand radar.Another potentially invaluable andmuch hyped use for terahertz waves isidentifying hazardous materials fromafar. In their gaseous phase, many naturaland man-made molecules, includingammonia, carbon monoxide, hydrogensulfide, and methanol, absorb photonswhen stimulated at terahertz frequencies,and those absorption bands canserve as chemical fingerprints. Even so,outside the carefully calibrated conditionsof the laboratory or the sparse environmentof space, complications arise.Let’s say you’re a hazmat worker andyou’ve received a report about a possiblesarin gas attack. Obviously, you’llwant to keep your distance, so you pullout your trusty portable T-wave spectrometer,which works something likethe tricorder in “Star Trek.” It sends adirected beam of terahertz radiation intothe cloud; the gas absorbs the radiationwith a characteristic spectral frequencysignature. Unlike with communicationsor radar, which would probably use anarrowband signal, your spectrometersends out a broadband signal, fromabout 300 GHz to 3 THz. Of course, to100metersFrequency, terahertz1 km10 meters1 meterGood weatherRain and fog0.01 0.1 1.0 10TERAHERTZWALL: <strong>The</strong> powerneeded to senddata at terahertzfrequencies wouldbe impracticallyhigh in manycases. For a lineof-sightterrestrialcommunicationlink using fixed-gainantennas, shownhere, transmittingat distances of lessthan 100 metersis the only wayto avoid the“terahertz wall.”ensure that the signal returns to yourspectrometer, it will need to reflect offsomething beyond the gas cloud, likea building, a container, or even sometrees. But as in the case above, the atmospherediminishes the signal’s strengthas it travels to the cloud and then backto your detector. <strong>The</strong> atmosphere alsowashes out the spectral features of thecloud because of an effect known as pressurebroadening. Even at a distance ofjust 10 meters, such effects would makeit difficult, if not impossible, to get anaccurate reading. Yet another wrinkleis that the chemical signatures of somematerials—table sugar and some plasticexplosives, for instance—are so remarkablynondescript as to make distinguishingone from another impossible.By now, you won’t be surprised tohear that through-wall imaging, anothermuch-discussed application of terahertzradiation, also faces major hurdles. <strong>The</strong>idea is simple enough: Aim terahertzradiation at a wall of some sort, withan object on the other side. <strong>Terahertz</strong>waves can penetrate some—but not all—materials that are opaque in visible light.So depending on what the wall is made ofand how thick it is, some waves will getthrough, reflect off the object, and thenmake their way back through the wallto the source, where they can reveal animage of the hidden object.Realizing that simple idea is anothermatter. First, let’s assume that the objectitself doesn’t scatter, absorb, or otherwisedegrade the signal. Even so, thequality of the image you get will dependlargely on what your wall is made of. Ifthe wall is made of metal or some othergood conductive material, you won’t getany image at all. If the wall containsany of the common insulating or constructionmaterials, you might still getserious attenuation, depending on thematerial and its thickness as well as thefrequency you are using. For example, a1-THz signal passing through a quarter -inch-thick piece of plywood would have0.0015 percent of the power of a 94-GHzsignal making the same journey. Andif the material is damp, the loss is evenhigher. (Such factors affect not justimaging through barriers but also terahertzwireless networks, which wouldrequire at the least a direct line of sightbetween the source and the receiver.) Soyour childhood dream of owning a pairof “X-ray specs” probably isn’t going tohappen any time soon.It’s true that some researchers havesuccessfully demonstrated through-wallimaging. In these demonstrations, theradiation sources emitted impulses ofradiation across a wide range of frequencies,including terahertz. Given what weknow about attenuation at the higher frequencies,though, some scientists who’vestudied the results believe it’s highlylikely that the imaging occurred not inthe terahertz region but rather at thelower frequencies. And if that’s the case,then why not just use millimeter-waveimagers to begin with?Before leaving the subject of imaging,let me add one last thought on terahertzfor medical imaging. Some of themore creative potential uses I’ve heardinclude brain imaging, tumor detection,and full-body scanning that would yieldmuch more detailed pictures than anyexisting technology and yet be completelysafe. But the reality once againfalls short of the dream. Frank De Lucia,a physicist at Ohio State University, inColumbus, has pointed out that a terahertzsignal will decrease in powerto 0.0000002 percent of its originalstrength after traveling just 1 mm insaline solution, which is a good approximationfor body tissue. For now at least,terahertz medical devices will be usefulonly for surface imaging of things likeskin cancer and tooth decay and laboratorytests on thin tissue samples.So those are some of the basicchallenges of exploiting the terahertzregime. <strong>The</strong> physics is indeed daunting,but that hasn’t prevented developersfrom continuing to pursue lots of differentterahertz devices for those and otherspectrum.ieee.orgseptember 2012 • IEEE Spectrum • NA 39


applications. So the next thing I lookedat was the performance of systems capableof generating radiation at terahertzfrequencies. I decided to focus on thesesources—and not detectors, receivers,control devices, and so on—becausewhile those other components are certainlycritical, people in the field prettymuch agree that what’s held up progressis the lack of appropriate sources.<strong>The</strong>re’s a very good reason for theshortage of compact terahertz sources:<strong>The</strong>y’re really hard to build! For manyapplications, the source has to be powerfulenough to overcome extreme signalattenuation, efficient enough toavoid having to wheel around your ownpower generator, and small enough tobe deployed in the field without havingto be toted around on a flatbed truck.(For some applications, the source’s spectralpurity, tunability, or bandwidth ismore important, so a lower power isacceptable.) <strong>The</strong> successful space-basedinstruments mentioned earlier merelydetect terahertz radiation that celestialbodies and events naturally emit;although some of those instruments usea low-power source for improved sensitivity,they don’t as yet attempt to transmitat terahertz frequencies.<strong>The</strong> government review in 2007loosely defined a compact terahertzsource as having an average output powerin the 1-mW to 1-W range, operating in the300-GHz to 3-THz frequency band, andbeing more or less “portable.” (We choseaverage power rather than peak powerbecause, ultimately, it’s the average powerthat counts in nearly all of the envisionedapplications.) In addition, we asked thatthe sources have a conversionRelative signal power (referenced to 1 milliwatt)post yourco mmentsonline at http://spectrum.ieee.org/terahertz091210.10.010.0010.00010.000010.000001Referencesignature0 1 2 3Frequency, terahertzety of optoelectronic RF generators).Vacuum devices and lasers exhibitedthe highest average power at the lowerand upper frequencies, respectively.Solid-state devices came next, followedby photonic devices. To be fair, callinga gyrotron a compact source is quite astretch, and while photonic sources canproduce high peak power, ranging fromhundreds of watts to kilowatts, they alsorequire high optical-drive power.Despite their considerable design differencesand some variations in performance,these three classes of terahertztechnology have similar challenges. Onesignificant issue is their uniformly lowconversion efficiency, which is typicallymuch less than 1 percent. So to get a 1-Wsignal, you might need to start with kilowattsof input power, or greater. Othereveryday electronic and optical devicesare, by comparison, far moreefficient. <strong>The</strong> RF power amplifierin a typical 2-GHz smartphone,for example, operatesat around 50 percent eff i-ciency. A commercial reddiode laser can convert electricalpower to light with anefficiency of more than 30 percent.That low efficiency combined withthe devices’ small size leads to anotherproblem: extremely high power densities(the amount of power the devices musthandle per unit area) and current densities(the amount of current they musthandle per unit area). For the vacuumand solid-state devices, the power densitieswere in the range of several megawattsper square centimeter. Supposeyou want to use a conventional vacuumtraveling-wave tube, or TWT, that’s beenReflection10 meters100 metersTHE UNDETECTED:When attemptingto identify unknownsubstances at adistance, nearlyall of the terahertzsignal will be lostor distorted by theatmosphere. Here,the gray line is afictitious signaturefor a sample beingprobed in reflectionmode. At distancesof 10 meters and100 meters [blueand red lines], thesample’s distinctspectral featuresare washed away.efficiency of at least 1 percent—for every 100 W of input power,the source would produce asignal of 1 W or more. Eventhat modest goal, it turns out,is quite challenging.<strong>The</strong> 2007 review includedabout 20 terahertz sources. I don’t haveroom here to describe how each of thesedevices works, but in general they fallinto three broad categories: vacuum(including backward-wave oscillators,klystrons, grating-vacuum devices,traveling -wave tubes, and gyrotrons),solid state (including harmonic frequencymultipliers, transistors, andmonolithic microwave integrated circuits),and laser and photonic (includingquantum cascade lasers, opticallypumped molecular lasers, and a variscaledup to operate at 1 THz. Such anapparatus would require you to focusan electron beam with a power densityof multiple megawatts per square centimeterthrough an evacuated circuithaving an inner diameter of 40 µm—about half the diameter of a human hair.(<strong>The</strong> solar radiation at the surface of thesun, by contrast, has a power density ofonly about 6 kilowatts per square centimeter.)A terahertz transistor, with itsnanometer features, operates at similarlyhigh power density levels. And all of theelectrical and photonic devices examined,even the quantum cascade laser,require high current densities, rangingfrom kiloamperes per square centimeterto multimega-amperes per square centimeter.Incidentally, the upper portion ofthat current density range is typical ofwhat you’d see in the pulsed-power electricalgenerators used for nuclear effectstesting, among other things.Compact electrical and opticaldevices can handle conditions like that,but you’re asking for trouble—if thedevice isn’t adequately cooled, the internalpower dissipation minimized, andthe correct materials used, it can quicklymelt or vaporize or otherwise breakdown. And of course, eventually youreach an upper limit, beyond which yousimply can’t push the power density andcurrent density any higher.As a device physicist, I was naturallyinterested in the relationship betweenthe sources’ output power and theirfrequency, what’s known as powerfrequencyscaling. When you plot thedevice’s average power along the y-axisand the frequency along the x, you wantto see the flattest possible curve. Such40 NA • iEEE Spectrum • september 2012 spectrum.ieee.orgillustrations: George retseck


f latness means that as the frequencyincreases, the output power remainssteady or at least does not plummet. Intypical radio-frequency devices, suchas transistors, solid-state diodes, andmicrowave vacuum tubes, the powertends to fall as the inverse of the frequencysquared. In other words, if youdouble the frequency, the output powerdrops by a factor of four.Most of the electrical terahertzsources we reviewed in 2007, however,had much steeper power-frequencycurves that basically fell off into theabyss as they were pushed into the terahertzrange. In general, the power scaledas the inverse of the frequency to thefourth, or worse, which meant that asthe frequency doubled, the output powerdropped by a factor of 16. So a device thatcould generate several watts at 100 GHzwas capable of only a few hundred microwattsas it went to 1 THz. Lasers, too, felloff in power in the terahertz region fasterthan you would expect.Given what I mentioned earlier aboutextreme signal attenuation in the terahertzregion and the sources’ low conversionefficiencies, this precipitous drop-offin power represents yet another high hurdleto commercializing the technology.Average power, watts10 0001 00010010spectrum.ieee.org10.10.010.0010.00010.000010.0000010.0000001Pλ = constantPf 2 = constantExtended-interaction klystronMonolithic microwaveintegrated circuitDifference-frequencygeneratorMicrovacuumtraveling-wave tubeOptical rectificationFine, you say, but can’t all of theseproblems be attributed to the fact that thesources are still technologically immature?Put another way, shouldn’t weexpect device performance to improve?Certainly, the technology is getting better.In the several years between my initialanalysis and this article, here aresome of the highlights in the device technologiesI reviewed:• <strong>The</strong> average power of microfabricatedvacuum devices rose twoorders of magnitude, from about 10 µWto over a milliwatt at 650 GHz, andresearchers are now working on multibeamand sheet-beam devices capableof higher power than comparablelow-voltage single round-beam units.• <strong>The</strong> average power of submillimetermonolithic microwave integratedcircuits and transistors climbed by afactor of five to eight, to the 100 mWlevel at 200 GHz and 1 mW at 650 GHz.• T h e o p e r a t i n g f r e q u e n c yrange for milliwatt-class cryogenicallycooled quantum cascade laserswas extended down to 1.8 THz in2012, compared to 2.89 THz in 2007.With an eye toward use outsidethe laboratory, researchers have beenenhancing their sources in other ways,GyrotronPhotomixerGrating-vacuumdevicesOptically pumpedmolecular lasersVacuumSolid statePhotonicsLasersBackward-waveoscillatorQuantumcascade laserMultipliers0.01 0.1 1 10Frequency, terahertzSource sampler: Compact terahertz sources exhibit low power and conversionefficiencies of much less than 1 percent. And in nearly every case, as the frequency risesinto the terahertz range, the source’s output power plummets. Here, the Pf 2 = constantline is the power-frequency slope you’d expect to see in a more mature RF device,while the Pλ = constant line is the expected slope for some commercial lasers.too, including improved packaging forphotonic devices and lasers and highertemperatureoperation for quantumcascade lasers. Given the amount ofeffort and interest in the field, therewill certainly be more advances andimprovements to come. (For more onthe current state of technology, I suggestconsulting the IEEE Transactionson <strong>Terahertz</strong> Science and Technology andsimilar journals.)That said, my main points still hold:While terahertz molecular spectroscopyhas continuing scientific uses in radioastronomy and space remote sensing,some of the well- publicized mainstreamproposals for terahertz technology continueto strain credulity. In addition,despite recent progress in cracking theterahertz nut, it is still exceedingly difficultto efficiently produce a useful level ofpower from a compact terahertz device.I strongly feel that any application toutedas using terahertz radiation should bethoroughly validated and vetted againstalternative approaches. Does it reallyuse terahertz frequencies, or is someother portion of the electromagneticspectrum involved? Is the applicationreally practical, or does it require suchrarefied conditions that it may neverfunction reliably in the real world? Arethere competing technologies that workjust as well or better?<strong>The</strong>re is still a great deal that wedon’t know about working at terahertzfrequencies. I do think we should keepvigorously pursuing the basic scienceand technology. For starters, we need todevelop accurate and robust computationalmodels for analyzing device designand operation at terahertz frequencies.Such models will be key to futureadvances in the field. We also need a betterunderstanding of material propertiesat terahertz frequencies, as well as generalterahertz phenomenology.Ultimately, we may need to apply outof-the-boxthinking to create designsand approaches that marry new devicephysics with unconventional techniques.In other areas of electronics, we’ve overcomeenormous challenges and beatimprobable odds, and countless pastpredictions have been subsequentlyshattered by continued technological evolution.Of course, as with any emergingpursuit, Darwinian selection will have itssay on the ultimate survivors. oNOTE: <strong>The</strong> views presented in this articleare solely those of the author.september 2012 • IEEE Spectrum • NA 41

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