THz Review

1. REVIEW ARTICLE
26 nature materials | VOL 1 | SEPTEMBER 2002 | www.nature.com/naturematerials
BRADLEY FERGUSON1,2
AND
XI-CHENG ZHANG*1
1
CenterforTerahertzResearch,RensselaerPolytechnicInstitute,
1108th
StreetTroy,NewYork,12180-3590,USA
2
CentreforBiomedicalEngineeringandDepartmentofElectrical
andElectronicEngineering,TheUniversityofAdelaide,South
Australia5005,Australia
*e-mail:zhangxc@rpi.edu
Recentyearshaveseenaplethoraof significantadvances
inmaterialsdiagnosticsbyterahertzsystems,ashigher-
powersourcesandmoresensitivedetectorsopenupa
rangeof potentialuses.Applicationsincluding
semiconductorandhigh-temperaturesuperconductor
characterization,tomographicimaging,label-free
geneticanalysis,cellularlevelimagingandchemicaland
biologicalsensinghavethrustterahertzresearchfrom
relativeobscurityintothelimelight.
Theterahertz(THz)regionof theelectromagnetic
spectrumhasproventobeoneof themostelusive.
Terahertzradiationislooselydefinedbythefrequency
rangeof 0.1to10 THz(1012
cyclespersecond).
Beingsituatedbetweeninfraredlightandmicrowave
radiation(seeFig. 1),THzradiationisresistanttothe
techniquescommonlyemployedinthesewell-
establishedneighbouringbands.Highatmospheric
absorptionconstrainedearlyinterestandfundingfor
THzscience.Historically,themajoruseof THz
spectroscopyhasbeenbychemistsandastronomersin
thespectralcharacterizationof therotationaland
vibrationalresonancesandthermal-emissionlinesof
simplemolecules.Thepast20yearshaveseena
revolutioninTHzsystems,asadvancedmaterials
researchprovidednewandhigher-powersources,and
thepotentialof THzforadvancedphysicsresearchand
commercialapplicationswasdemonstrated.Terahertz
technologyisanextremelyattractiveresearchfield,with
interestfromsectorsasdiverseasthesemiconductor,
medical,manufacturing,spaceanddefenceindustries.
Severalrecentmajortechnicaladvanceshavegreatly
extendedthepotentialandprofileof THzsystems.
Theseadvancesincludethedevelopmentof aquantum
cascadeTHzlaser1
,thedemonstrationof THzdetection
of singlebase-pairdifferencesinfemtomolar
concentrationsof DNA2
andtheinvestigationof the
evolutionof multiparticlechargeinteractionswithTHz
spectroscopy3
.Thisarticleprovidesanoverviewof these
andmanyotherimportantrecentdevelopments.
THZ SPECTROSCOPY SYSTEMS
Terahertzspectroscopyallowsamaterial’sfar-infrared
opticalpropertiestobedeterminedasafunctionof
frequency.Thisinformationcanyieldinsightinto
materialcharacteristicsforawiderangeof applications.
ManydifferentmethodsexistforperformingTHz
spectroscopy.Fouriertransformspectroscopy(FTS)is
perhapsthemostcommontechniqueusedforstudying
molecularresonances.Ithastheadvantageof an
extremelywidebandwidth,enablingmaterial
characterizationfromTHzfrequenciestowellintothe
infrared.InFTSthesampleisilluminatedwitha
broadbandthermalsourcesuchasanarclamporaSiC
globar.Thesampleisplacedinanopticalinterferometer
systemandthepathlengthof oneof theinterferometer
armsisscanned.Adirectdetectorsuchasahelium-
cooledbolometerisusedtodetecttheinterference
signal.TheFouriertransformof thesignalthenyields
thepowerspectraldensityof thesample.
Onedisadvantageof FTSisitslimitedspectral
Materials for terahertz science and
technology
Terahertz spectroscopy systems use far-infrared radiation to extract molecular spectral information in an
otherwise inaccessible portion of the electromagnetic spectrum. Materials research is an essential
component of modern terahertz systems: novel, higher-power terahertz sources rely heavily on new
materials such as quantum cascade structures. At the same time, terahertz spectroscopy and imaging
provide a powerful tool for the characterization of a broad range of materials, including semiconductors
and biomolecules.
103 106 109 1012 1015 1018 1021 1024
100
Microwaves Visible
kilo mega giga tera peta exa zetta yotta
X-ray γ-ray
THz
Electronics Photonics
Hz
Radar ???
Frequency (Hz)
Example
industries:
Radio
communications
MF, HF, VHF, UHF, SHF, EHF
Optical
communications
Medical
imaging
Astrophysics
Figure 1Theelectromagnetic
spectrum.Thedevelopmentof
efficientemittersanddetectors
withineachofthespectral
regimeshasresultedinthebirth
ofnumerousindustries.
Thesearchforpotential
applicationsofTHzradiationis
steadilyintensifyingasmaterials
researchprovidesimproved
sourcesanddetectors.
© 2002 Nature Publishing Group

2. REVIEW ARTICLE
nature materials | VOL 1 | SEPTEMBER 2002 | www.nature.com/naturematerials 27
resolution.Spectralmeasurementshavingamuch
higherresolutionmaybemadeusinganarrowband
systemwithatunableTHzsourceordetector.Inthese
systems,thesourceordetectoristunedacrossthe
desiredbandwidthandthesample’sspectralresponseis
measureddirectly.BothFTSandnarrowband
spectroscopyarealsowidelyusedinpassivesystemsfor
monitoringthermal-emissionlinesof molecules,
particularlyinastronomyapplications.
Athird,morerecenttechniqueistermedTHztime-
domainspectroscopy(THz-TDS).THz-TDSusesshort
pulsesof broadbandTHzradiation,whicharetypically
generatedusingultrafastlaserpulses.Thistechnique
grewfromworkinthe1980satAT&TBellLabsandthe
IBMT.J.WatsonResearchCenter4,5
.Althoughthe
spectralresolutionof THz-TDSismuchcoarserthan
narrowbandtechniques,anditsspectralrange
significantlylessthanthatof FTS,ithasanumberof
advantagesthathavegivenrisetosomeimportantrecent
applications.ThetransmittedTHzelectricfieldis
measuredcoherently,whichprovidesbothhigh
sensitivityandtime-resolvedphaseinformation.Itis
alsoamenabletoimplementationwithinanimaging
systemtoyieldrichspectroscopicimages.ATHz-TDS
systemisdescribedinFig. 2.TypicalTHz-TDSsystems
haveafrequencybandwidthbetween2and5 THz,a
spectralresolutionof 50 GHz,anacquisitiontimeunder
oneminuteandadynamicrangeof 1 × 105
inelectric
field.Signal-processingalgorithmsmaybeusedto
improvethesignal-to-noiseratioof themeasured
signalsbyalmost30%(ref.6).
THZ SOURCES
Thelackofahigh-power,low-cost,portableroom-
temperatureTHzsourceisthemostsignificantlimitation
ofmodernTHzsystems.However,thereisavastarrayof
potentialsourceseachwithrelativeadvantages,and
advancesinhigh-speedelectronics,laserandmaterials
researchcontinuetoprovidenewcandidates.Sources
maybebroadlyclassifiedaseitherincoherentthermal
sources,broadbandpulsed(‘T-ray’)techniquesor
narrowbandcontinuous-wavemethods.
BROADBAND THZ SOURCES
MostbroadbandpulsedTHzsourcesarebasedonthe
excitationof differentmaterialswithultrashortlaser
pulses.Anumberof differentmechanismshavebeen
exploitedtogenerateTHzradiation,including
photocarrieraccelerationinphotoconductingantennas,
second-ordernon-lineareffectsinelectro-opticcrystals,
plasmaoscillations7
andelectronicnon-linear
transmissionlines8
.Currently,conversionefficienciesin
allof thesesourcesareverylow,andconsequently,
averageTHzbeampowerstendtobeinthenano-to
microwattrange,whereastheaveragepowerof the
femtosecondopticalsourceisintheregionof 1 W.
Photoconductionandopticalrectificationaretwo
of themostcommonapproachesforgenerating
broadbandpulsedTHzbeams.Thephotoconductive
approachuseshigh-speedphotoconductorsas
transientcurrentsourcesforradiatingantennas9
.
Typicalphotoconductorsincludehigh-resistivity
GaAs,InPandradiation-damagedsiliconwafers.
Metallicelectrodesareusedtobiasthe
photoconductivegapandformanantenna.
ThephysicalmechanismforTHzbeamgeneration
inphotoconductiveantennasbeginswithanultrafast
laserpulse(withaphotonenergylargerthanthe
bandgapof thematerialhν ≥ Eg
),whichcreates
electron–holepairsinthephotoconductor.Thefree
carriersthenaccelerateinthestaticbiasfieldtoforma
transientphotocurrent,andthefast,time-varying
currentradiateselectromagneticwaves.Several
materialparametersaffecttheintensityandthe
bandwidthof theresultantTHzradiation.
ForefficientTHzradiation,itisdesirabletohaverapid
photocurrentriseanddecaytimes.Thussemiconductors
withsmalleffectiveelectronmassessuchasInAsand
InPareattractive.Themaximumdriftvelocityisalso
animportantmaterialparameter,butitisgenerally
limitedbytheintrabandscatteringrateorby
intervalleyscatteringindirectsemiconductorssuchas
GaAs10–12
.Becausetheradiatingenergymainlycomes
fromstoredsurfaceenergyintheformof thestaticbias
field,theTHzradiationenergyscalesupwiththebias
andopticalfluency13
.Thebreakdownfieldof the
materialisanotherimportantparameterbecausethis
determinesthemaximumbiasthatmaybeapplied.
Photoconductiveemittersarecapableof relativelylarge
averageTHzpowersinexcessof 40 µW(ref.14)and
bandwidthsashighas4 THz(ref.15).
Opticalrectificationisanalternativemechanismfor
pulsedTHzgeneration,andisbasedontheinverse
processof theelectro-opticeffect16
.Again,femtosecond
laserpulsesarerequired,butincontrastto
photoconductingelementswheretheopticalbeam
functionsasatrigger,theenergyof theTHzradiationin
opticalrectificationcomesdirectlyfromtheexciting
laserpulse.Theconversionefficiencyinoptical
rectificationdependsprimarilyonthematerial’snon-
linearcoefficientandthephase-matchingconditions.
Thistechniquewasfirstdemonstratedfor
generatingfar-infraredradiationusingLiNbO3
(ref.17).MuchresearchhasfocusedonoptimizingTHz
generationthroughinvestigatingtheelectro-optic
propertiesof differentmaterials,includingtraditional
semiconductorssuchasGaAsandZnTe,andorganic
crystalssuchastheionicsalt4-dimethylamino-N-
methylstilbazoliumtosylate(DAST)amongmany
others18–20
.Becauseopticalrectificationrelieson
couplingof theincidentopticalpowertoTHz
frequenciesatrelativelylowefficiency,itusually
providesloweroutputpowersthanphotoconductive
Sample
THz
detector
Beam
Delay stage
THz
emitter
Femtosecond
pulses
Pump beam
Probe beam
Parabolic
Figure 2IllustrationofaTHz-
TDSpumpprobesystem.
Theultrafastlaserbeamissplit
intopumpandprobebeams.
Thepumpbeamisincidenton
theTHzemittertogenerateTHz
pulses,andtheTHzpulsesare
collimatedandfocusedonthe
targetusingparabolicmirrors.
Aftertransmissionthroughthe
target,theTHzpulseis
collimatedandre-focusedon
theTHzdetector.Theoptical
probebeamisusedtogatethe
detectorandmeasurethe
instantaneousTHzelectricfield.
Adelaystageisusedtooffset
thepumpandprobebeamsand
allowtheTHztemporalprofileto
beiterativelysampled.
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3. REVIEW ARTICLE
28 nature materials | VOL 1 | SEPTEMBER 2002 | www.nature.com/naturematerials
antennas,butithastheadvantageof providingvery
highbandwidths,upto50 THz(ref.21).Phase-
matchedopticalrectificationinGaSeallows
ultrabroadbandTHzpulsestobegeneratedwitha
tunablecentrewavelength.Tuninguptoafrequencyof
41 THzisaccomplishedbytiltingthecrystalaboutthe
horizontalaxisperpendiculartothepumpbeamto
modifythephase-matchingconditions22,23
.
NARROWBAND THZ SOURCES
NarrowbandTHzsourcesarecrucialforhigh-resolution
spectroscopyapplications.Theyalsohavebroad
potentialapplicationsintelecommunications,andare
particularlyattractiveforextremelyhighbandwidth
intersatellitelinks.Forthesereasonstherehasbeen
significantresearchinterestinthedevelopmentof
narrowbandsourcesoverthepastcentury24
.Amultitude
of techniquesareunderdevelopment,including
upconversionof electronicradio-frequencysources,
downconversionof opticalsources,lasersandbackward-
wavetubes.Severalcomprehensivereviewsof thisfield
areavailable25
.
Thetechniquemostusedforgeneratinglow-power
(<100 µW)continuous-waveTHzradiationisthrough
upconversionof lower-frequencymicrowaveoscillators,
suchasvoltage-controlledoscillatorsanddielectric-
resonatoroscillators.Upconversionistypicallyachieved
usingachainof planarGaAsSchottky-diodemultipliers.
Usingthesemethods,frequenciesashighas2.7 THz
havebeendemonstrated26
.Researchalsocontinuesto
increasethefrequencyof GunnandIMPATTdiodesto
thelowerreachesof theterahertzregionusingalternate
semiconductingstructuresandimprovedfabrication
techniques27
.GaslasersareanothercommonTHz
source.Inthesesources,acarbondioxidelaserpumpsa
low-pressuregascavity,whichlasesatthegasmolecule’s
emission-linefrequencies.Thesesourcesarenot
continuouslytunable,andtypicallyrequirelargecavities
andkilowattpowersupplies,howevertheycanprovide
highoutputpowersupto30 mW.Methanoland
hydrogencyanidelasersarethemostpopular,and
theyareincommonuseforspectroscopyand
heterodynereceivers.
Extremelyhigh-powerTHzemissionshaverecently
beendemonstratedusingfree-electronlaserswith
energy-recoveringlinearaccelerators28
.Free-electron
lasersuseabeamofhigh-velocitybunchesofelectrons
propagatinginavacuumthroughastrong,spatially
varyingmagneticfield.Themagneticfieldcausesthe
electronbunchestooscillateandemitphotons.
Mirrorsareusedtoconfinethephotonstotheelectron
beamline,whichformsthegainmediumforthelaser.
Suchsystemsimposeprohibitivecostandsize
constraintsandtypicallyrequireadedicatedfacility.
However,theymaygeneratecontinuousorpulsedwaves,
andprovideanaveragebrightnessofmorethansix
ordersofmagnitudehigherthantypical
photoconductiveantennaemitters.Free-electronlasers
havesignificantpotentialinapplicationswhere
improvedsignal-to-noiseratioisessential,orinthe
investigationofnon-linearTHzspectroscopy.Bench-top
variationsonthesametheme,termedbackward-wave
tubesorcarcinotrons,arealsocapableofproviding
milliwattoutputpowersatTHzfrequencies,andare
commerciallyavailable.
Anumberof opticaltechniqueshavealsobeen
pursuedforgeneratingnarrowbandTHzradiation.
Originaleffortsbeganinthe1970susingnon-linear
photomixingof twolasersources,butstruggledwith
lowconversionefficiencies29
.Inthistechnique,two
continuous-wavelaserswithslightlydifferingcentre
frequenciesarecombinedinamaterialexhibitinga
highsecond-orderopticalnon-linearitysuchasDAST.
Thetwolaserfrequenciesmutuallyinterfereinthe
2
Injector
Active region
1
Injection barrier
Energy
Distance
104.9 mm
Figure 3Simplifiedconduction
bandstructureoftheTHz
quantumcascadelaser
demonstratedbyKohleretal.
(afterref.1).Electronsare
injectedthroughthe4.3-nm
AlGaAsinjectionbarrierintothe
level2energystateoftheactive
region.Theactivetransitionfrom
level2tolevel1resultsinthe
emissionof4.4-THzphotons.
Theelectronsthenescapetothe
subsequentinjectorband.Atotal
of104repeatsofthebasic7well
structurewasusedinthelaser.
Eachquantumwellconsistsofa
thinlayer(10–20 nm)ofGaAs
betweentwopotentialbarriersof
AlGaAs(0.6–4.3 nmthick).
Thelayerthicknessesandthe
appliedelectricfieldwere
tunedtoprovidetherequired
tunnellingcharacteristics.
-4
-2
0
2
4
3
2
1
0
Time (ps)
a b
Electro-optic
signal
(nA)
Transmission
0.1
1
10
60
50
40
30
20
10
0
THz
Figure 4AbroadbandTHzpulse
withafrequencyspectrum
extendingintotheinfrared.a,
TemporalwaveformofaTHz
pulsegeneratedusingoptical
rectificationina27-µmZnTe
emitter,andmeasuredusing
freespaceelectro-opticsampling
ina30-µmZnTesensor.b,
FrequencyspectrumoftheTHz
field.Theabsorptionresonance
at5.3THzisduetophonon
modesintheZnTecrystals.
© 2002 Nature Publishing Group

4. REVIEW ARTICLE
nature materials | VOL 1 | SEPTEMBER 2002 | www.nature.com/naturematerials 29
material,resultinginoutputoscillationsatthesumand
differenceof thelaserfrequencies.Suchsystemscanbe
designedsuchthatthedifferencetermisintheTHz
range.Tunablecontinuous-waveTHzradiationhas
beendemonstratedbymixingtwofrequency-offset
lasersinGaAsgrownatlowtemperature30
andby
mixingtwofrequencymodesfromasinglemultimode
laser.Furthertechniquesuseopticalparametric
generatorsandoscillatorswhereaQ-switchNd:YAG
(neodymium:yttrium-aluminium-garnet)laserpump
beamgeneratesasecondidlerbeaminanon-linear
crystal,andthepumpandidlersignalbeattoemitTHz
radiation31,32
.Opticaltechniquesprovidebroadly
tunableTHzradiationandarerelativelycompact
owingtotheavailabilityof solid-statelasersources.
Outputpowersinexcessof 100 mW(pulsed)have
beendemonstrated33
.Opticaldownconversionisarich
areaformaterialsresearchbecausemolecularbeam
epitaxyandothermaterialsadvancesallowsthe
generationof engineeredmaterialswithimproved
photomixingproperties34
.
Semiconductorlasersareafurthertechniquewith
extremepromisefornarrowbandTHzgeneration.
Thefirstsuchlaserwasdemonstratedover20yearsago
inlightlydopedp-typegermaniumasaresultof hole
populationinversioninducedbycrossedelectricand
magneticfields35
.Theselasersaretunablebyadjusting
themagneticfieldorexternalstress.Terahertzlasingin
germaniumhasalsobeendemonstratedbyapplyinga
stronguniaxialstresstothecrystaltoinducethehole
populationinversion36
.Suchlasershavemanyinherent
limitationsincludinglowefficiency,lowoutputpower
andtheneedforcryogeniccoolingtomaintainlasing
conditions.Recently,semiconductordeposition
techniqueshaveadvancedtoalevelwherethe
constructionof multiplequantum-wellsemiconductor
structuresforlaseremissionisfeasible.Quantum
cascadelaserswerefirstdemonstratedin1994basedona
seriesof coupledquantumwellsconstructedusing
molecularbeamepitaxy37
.Aquantumcascadelaser
consistsof coupledquantumwells(nanometre-thick
layersof GaAssandwichedbetweenpotentialbarriersof
AlGaAs).Thequantumcascadeconsistsof arepeating
structureinwhicheachrepeatunitismadeupof an
injectorandanactiveregion.Intheactiveregiona
populationinversionexistsandelectrontransitiontoa
lowerenergyleveloccurs,emittingphotonsataspecific
wavelength.Theelectronsthentunnelbetween
quantumwellsandtheinjectorregioncouplesthemto
thehigherenergylevelintheactiveregionof the
subsequentrepeatunit.
Quantumcascadelasershavebeendemonstrated
withintheinfraredspectrum,butuntilveryrecently
severalsignificantproblemshadpreventedTHz
quantumcascadelasersfrombeingrealized.
Theprincipalproblemsarecausedbythelong
wavelengthof THzradiation.Thisresultsinalarge
opticalmode,whichresultsinpoorcouplingbetween
thesmallgainmediumandtheopticalfield,andinhigh
opticallossesowingtofreeelectronsinthematerial
(theselossesscaleasthesquareof thewavelength).
Kohleretal.1
addressedtheseandotherproblemsin
theirrecentinnovativedesignof aTHzquantum
cascadelaseroperatingat4.4 THz.Thelaserconsisted
of 104repetitionsof thebasicunit(showninFig. 3)and
atotalof over700quantumwells.Thissystem
demonstratedpulsedoperationatatemperatureof
10 K;however,optimizedfabricationpromisestolead
tocontinuous-waveoperationatliquidnitrogen
temperatures(of theorderof 70 K).
THZ DETECTORS
Thedetectionof THz-frequencysignalsisanotherarea
of importantactiveresearch.Thelowoutputpowerof
THzsourcescoupledwiththerelativelyhighlevelsof
thermalbackgroundradiationinthisspectralrangehas
necessitatedhighlysensitivedetectionmethods.
Forbroadbanddetection,directdetectorsbasedon
thermalabsorptionarecommonlyused.Mostof these
requirecoolingtoreducethermalbackground.The
mostcommonsystemsarehelium-cooledsilicon,
germaniumandInSbbolometers.Pyroelectricinfrared
detectorsmayalsobeusedatTHzfrequencies.
Superconductorresearchhasyieldedextremely
sensitivebolometersbasedonthechangeof stateof a
superconductorsuchasniobium.Interferometric
techniquesmaybeusedtoextractspectralinformation
usingdirectdetectors.AsinglephotondetectorforTHz
photonshasrecentlybeendemonstrated38
.
Thisdetectorusesasingle-electrontransistorconsisting
of aquantumdotinahighmagneticfield,tooffer
unparalleledsensitivity.Althoughdetectionspeedsare
currentlylimitedto1 ms,high-speeddesignsare
proposedandthishasthepotentialtorevolutionizethe
fieldof THzdetection.
a b c
40
40
30
30
20
20
10
10
0
0
mm
mm
Refractive
index
1.5
1.4
1.3
1.2
1.1
1
Figure 5T-raycomputed
tomographyimageoftwoplastic
cylinders(afterref.57).
a,Anopticalimageofthetest
structure.b,Thetargetwas
imagedusingthetomography
system,andtherefractiveindex
ofeachcross-sectionalslice
wasreconstructed.Thecentral
sliceisshown.Thegreyscale
intensityindicatestherefractive
indexofthetwodifferenttypes
ofplastic.c,Thecross-sectional
slicesarecombinedtoforma3D
image.Asurface-rendered
imageisshown.
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5. REVIEW ARTICLE
30 nature materials | VOL 1 | SEPTEMBER 2002 | www.nature.com/naturematerials
Inapplicationsrequiringveryhighspectral
resolutionofthesensor,heterodynesensorsarepreferred.
Inthesesystems,alocaloscillatorsourceattheTHz
frequencyofinterestismixedwiththereceivedsignal.
Thedownshiftedsignalisthenamplifiedandmeasured.
Atroomtemperaturesemiconductorstructuresmaybe
used.AplanarSchottky-diodemixerhasbeenoperated
successfullyat2.5 THzforsensingapplicationsinspace39
.
Cryogeniccoolingisusedforhighersensitivity
inheterodynesuperconductordetectors.Several
superconductorstructureshavebeenusedforover
20years.Themostwidelyusedisthesuperconductor-
insulator-superconductortunneljunctionmixer40
.
High-temperaturesuperconductorssuchasYBCO
(yttrium–barium–copperoxide)aretoutedfortheir
potentialforhigherbandwidthoperation.Anumberof
generalreviewsofnarrowbandTHzreceiversare
available41
.Alternativenarrowbanddetectors,suchas
electronicresonantdetectors,basedonthefundamental
frequencyofplasmawavesinfield-effecttransistorshave
beendemonstratedupto600 GHz42
.
ForpulsedTHzdetectioninTHz-TDSsystems,
coherentdetectorsarerequired.Thetwomostcommon
methodsarebasedonphotoconductivesamplingand
free-spaceelectro-opticsampling,bothof whichagain
relyonultrafastlasersources.Fundamentally,the
electro-opticeffectisacouplingbetweenalow-
frequencyelectricfield(THzpulse)andalaserbeam
(opticalpulse)inthesensorcrystal.Simpletensor
analysisindicatesthatusinga<110>-oriented
zincblendecrystalasasensorprovidesthehighest
sensitivity.TheTHzelectricfieldmodulatesthe
birefringenceof thesensorcrystal;thisinturnmodulates
thepolarizationellipticityof theopticalprobebeam
passingthroughthecrystal.Theellipticitymodulation
of theopticalbeamcanthenbeanalysedtoprovide
informationonboththeamplitudeandphaseof the
appliedelectricfield43,44
.
Theuseof anextremelyshortlaserpulse(<15 fs)
andathinsensorcrystal(<30 µm)allowelectro-optic
detectionof signalsintothemid-infraredrange.
Figure 4ashowsatypicalmid-infraredpulsedTHz
waveform.TheFouriertransformof theTHzpulseis
showninFig. 4b;thehighestfrequencyresponse
reachesover30 THz.Theresonantabsorptionat
5.3 THziscausedbythephononmodesof theZnTe
crystals.Whenthinsensorsareused,extremelyhigh
detectionbandwidths,inexcessof 100 THz,have
beendemonstrated45
.
Photoconductiveantennasarewidelyusedfor
pulsedTHzdetection.Anidenticalstructuretothe
photoconductiveantennaemittermaybeused.
Ratherthanapplyingabiasvoltagetotheelectrodesof
theantenna,acurrentamplifierandmeterareusedto
measureatransientcurrent.Ultrahighbandwidth
detectionhasbeendemonstratedusing
photoconductiveantennadetectorswithdetectable
frequenciesinexcessof 60 THz(ref.46).
THZ APPLICATIONS
Oneof theprimarymotivationsforthedevelopmentof
THzsourcesandspectroscopysystemsisthepotentialto
extractmaterialcharacteristicsthatareunavailablewhen
usingotherfrequencybands.Astronomyandspace
researchhasbeenoneof thestrongestdriversforTHz
researchbecauseof thevastamountof information
availableconcerningthepresenceof abundant
moleculessuchasoxygen,waterandcarbonmonoxide
instellardustclouds,cometsandplanets47
.Inrecent
years,THzspectroscopysystemshavebeenappliedtoa
hugevarietyof materialsbothtoaidthebasic
understandingof thematerialproperties,andto
demonstratepotentialapplicationsinsensingand
diagnostics.Wereviewseveralof themorerecently
demonstratedapplications.
MATERIAL CHARACTERIZATION
Oneof themajorapplicationsof THzspectroscopy
systemsisinmaterialcharacterization,particularlyof
lightweightmoleculesandsemiconductors.Terahertz
spectroscopyhasbeenusedtodeterminethecarrier
concentrationandmobilityof dopedsemiconductors
suchasGaAsandsiliconwafers48–50
.TheDrudemodel
maythenbeusedtolinkthefrequency-dependent
dielectricresponsetothematerialfree-carrierdynamic
properties,includingtheplasmaangularfrequencyand
thedampingrate.Animportantfocusisonthe
measurementof thedielectricconstantof thinfilms51
.
High-temperaturesuperconductor
characterizationisanotherimportantapplicationof
THzspectroscopy.Severalsuperconductingthinfilms
havebeenanalysedtodeterminematerialparameters
includingthemagneticpenetrationdepthandthe
superconductingenergygap.THz-TDShasrecently
beenusedtostudyMgB2
,thematerialnewlydiscovered
tobesuperconducting.Thismaterialexhibitsan
extremelyhightransitiontemperatureof 39 Kandis
currentlynotwellunderstood.THz-TDSwasusedto
determinethesuperconducting-gapenergythreshold
of approximately5 meV.Thiscorrespondstoonlyhalf
thevaluepredictedbycurrenttheoryandpointstothe
existenceof complexmaterialinteractions52
.
Experiments with optical-pump THz-probe
systems can reveal additional information about
1.6 mm
Figure 6THzimageofanonion
cellmembrane(afterref.60).
ATHzimagingsystembasedon
opticalrectificationandfree-
spaceelectro-opticsamplingin
ZnTecrystals30 µmthickwas
usedwithabandwidthupto40
THz.Thisallowedaspatial
resolutionoflessthan50 µmto
beachieved.Thecellular
structureofthetissuemembrane
isclearlyvisible.
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6. REVIEW ARTICLE
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materials.In these experiments,the material is excited
using an ultrafast optical pulse,and a THz pulse is
used to probe the dynamic far-infrared optical
properties of the excited material.Leitenstorfer et al.
used an optical-pump THz-probe system to identify
the time evolution of charge–charge interactions in an
electron–hole plasma excited in GaAs using ultrafast
optical pulses.This study added experimental
evidence to quantum-kinetic theoretical predictions
regarding charge build-up or dressed quasi-particles3
.
THZ IMAGING AND TOMOGRAPHY
PulsedTHz-waveimaging,or‘T-rayimaging’,wasfirst
demonstratedbyHuandNuss53
in1995,andsincethen
hasbeenusedforimagingawidevarietyof targets
includingsemiconductors54
,canceroustissue55
and
flames56
.Theattractionof THzimagingislargelydueto
theavailabilityof phase-sensitivespectroscopicimages,
whichholdsthepotentialformaterialidentificationor
‘functionalimaging’.
THzsystemsareidealforimagingdrydielectric
substancesincludingpaper,plasticsandceramics.
Thesematerialsarerelativelynon-absorbinginthis
frequencyrange,yetdifferentmaterialsmaybeeasily
discriminatedonthebasisof theirrefractiveindex,
whichisextractedfromtheTHzphaseinformation.
Manysuchmaterialsareopaqueatopticalfrequencies,
andprovideverylowcontrastforX-rays.THzimaging
systemsmaythereforefindimportantnicheapplications
insecurityscreeningandmanufacturingqualitycontrol.
Animportantgoalinthiscontextisthedevelopmentof
three-dimensional(3D)tomographicT-rayimaging
systems57
.Figure 5illustratesareconstructedcross
sectionand3D-renderedimageof twoplasticcylinders
withdifferingrefractiveindex.Thesystemisbasedon
thesameprinciplesasX-raycomputedtomography,but
providesawealthof informationaboutthematerial’s
frequency-dependentopticalpropertiesthrough
broadband,phase-sensitiveTHzdetection58
.
InterestinusingTHzimagingtostudycellular
structureisalsoincreasing.Afundamentallimitationin
thiscontextistheresolutionof currentsystems.
TheRayleighcriterionlimitsthefar-fieldresolutionof
animagingsystemtotheorderof thewavelength
(0.3 mmat1 THz).Forthisreason,researchersare
relyingonimaginginthenear-fieldtoachieveimproved
spatialresolution.Usingnear-fieldtechniques,similarto
thoseusedinnear-fieldopticalmicroscopy,resolutions
of 7 µmhavebeendemonstratedusingradiationwitha
centrewavelengthof 600 µm(ref.59).Analternative
methodtoimprovetheresolutionistousehigher-
frequencyTHzpulses.Figure 6showsaTHzimageof a
membraneof onioncells.Theresolutionof
approximately50 µmisachievedbyusingvery
broadbandTHzpulsesextendingintothemid-infrared.
Thecontrastintheimageisattributedprimarilyto
differencesinthewatercontentof thecellsandthe
intercellularregions60
.
BIOMATERIAL THZ APPLICATIONS
THzsystemshavebroadapplicabilityinabiomedical
context.Activefieldsof researchrangefromcancer
detection61
togeneticanalysis.Biomedicalapplications
of THzspectroscopyarefacilitatedbythefactthatthe
collectivevibrationalmodesof manyproteinsandDNA
moleculesarepredictedtooccurintheTHzrange.THz
spectroscopyhasalsobeenheraldedforitspotential
abilitytoinferinformationonabiomolecule’s
conformationalstate.Thecomplexrefractiveindexof
pressedpelletsconsistingof DNAandother
biomoleculeshasbeendeterminedandshows
absorptionconsistentwithalargedensityof low-
frequencyinfrared-activemodes62,63
.
DNAanalysersareusedtoidentifypolynucleotide
basesequencesforavarietyof geneticapplications.
Productionof genechipsisanincreasinglypopular
techniquewhereunknownDNAmoleculesarebound
tofluorescentlylabelledpolynucleotideswithknown
basesequences.Fluorescentlabellingcanaffect
diagnosticaccuracyandincreasesthecostand
preparationtimeof genechips.Asaresult,several‘label-
free’methodsarebeingresearched.THzimaging
appearstohavepromiseinthiscontext.THz
spectroscopyhasshownthecapabilityof differentiating
THz
Biotin-avidin
Pure avidin
Delay time (ps)
THz
electric
field
(au)
1.0
0.5
0.0
0 10 20 30 40 50 60
–0.5
–1.0
a
b
c
Figure 7Abiotin-avidinT-ray
biosensor(afterref.65).
a,Thetestslideconsistingofa
biotinthin-film,halfcoatedwith
anavidinsolution.b,Illustration
ofthemeasurementprocedure
wheretheslideismountedona
galvanometricshakerand
translatedbackandforthinthe
THzbeamtoallowthedifferential
signaltobemeasured.c,TheTHz
pulsemeasuredafter
transmissionthroughthebiotin-
avidinsensorwith(dashedline)
andwithout(solidline)exposure
to0.3 ng cm–2
ofavidin
moleculeschemicallyboundto
agarosebeadsinsolution.
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7. REVIEW ARTICLE
32 nature materials | VOL 1 | SEPTEMBER 2002 | www.nature.com/naturematerials
betweensingle-anddouble-strandedDNAowingto
associatedchangesinrefractiveindex64
.Recentlythe
samegrouphasdemonstratedaTHzsensingsystem
capableof detectingDNAmutationsof asinglebase
pairwithfemtomolesensitivity2
.
Afurtherbiomedicalapplicationof THzsystemsis
theT-raybiosensor65
.Asimplebiosensorhasbeen
demonstratedfordetectingtheglycoproteinavidinafter
bindingwithvitaminH(biotin).Afilmof biotinis
depositedonasolidsubstrate(Fig. 7a),andhalf of the
biosensorslideisexposedtotheenvironmentorsolution
of interest.Avidinhasaverystrongaffinityforbiotinand
bindstoanybiotin-containingmolecules.Themodified
far-infraredopticalpropertiesof theboundbiotinfilm
canthenbedetectedusingthetechniqueof differential
THz-TDS66
.Theslideismountedonagalvanometric
shakerandtheTHzbeamisalternatelyfocusedthrough
theavidinandcontrolportionof theslide(Fig. 7b).
Acomparisonof thesignalmeasuredusingaslide
treatedwith0.3 ng cm–2
avidinsolutionandaplain
biotinsliderevealedadetectabledifference(Fig. 7c).
Thisdetectablelimitissignificantlyenhancedby
chemicallybindingtheavidinmoleculestoagarose
beadstoprovideincreasedcontrastof refractiveindex67
.
Suchtechniquesmayfindbroadapplicationsintracegas
sensingandproteomics.
OUTLOOK
THzspectroscopysystemshaveundergonedramatic
changesoverthepastdecade.Improvedsourceand
detectorperformancecontinuetoexpandapplication
areasandfacilitatethetransitionof THzsystemsfrom
thelaboratorytocommercialindustry.Biomedical
imagingandgeneticdiagnosticsaretwoof themost
obviouspotentialapplicationsof thistechnology,but
equallypromisingistheabilitytoinvestigatematerial
characteristics,probedistantgalaxiesandstudy
quantuminteractions.THzradiationhasfurther
potentialforrevolutionarynewuses,suchasinthe
manipulationof boundatoms,whichholdspotentialfor
futurequantumcomputers68
.Severalkeyresearchareas
promisesignificantcontinuingadvancesinTHz
technology.Centralamongthesearecurrentefforts
towardshigher-powerTHzsources—whichwillgiverise
tonon-linearTHzspectroscopy,withthepotentialto
extractadditionalmaterialcharacteristics—higher-
sensitivityreceiversandimprovedunderstandingof the
interactionbetweenTHzradiationandmaterialssuchas
quantumstructuresandbiomaterials.
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Acknowledgements
This work was supported in part by the US Army Research Office, the National
Science Foundation, the Australian Research Council and the Cooperative Research
Centre for Sensor, Signal and Information Processing. The authors thank D.
Abbott, D. Gray,A. Menikh, S. P. Mickan and the Australian-American Fulbright
Commission.
Correspondence and requests for materials should be addressed to X.C.Z.
© 2002 Nature Publishing Group