用低温生长砷化镓检测高达20赫兹

Detectionofupto20THzwithalow-temperature-grownGaAsphotoconductiveantennagatedwith15fslightpulses

ShunsukeKono,a)MasahikoTani,PingGu,andKiyomiSakai

KansaiAdvancedResearchCenter,CommunicationsResearchLaboratory,MPT,588-2Iwaoka,Nishi-ku,Kobe,651-2492Japan

͑Received27June2000;acceptedforpublication16October2000͒

Wereportontheultrabroadbandcoherentdetectionofradiationinwavelengthsspanningfromfartomidinfraredwithalow-temperature-grownGaAsphotoconductivedipoleantennagatedwith15fslightpulses.Thedetectedspectralfrequencyexceeds20THz.©2000AmericanInstituteofPhysics.͓S0003-6951͑00͒05451-6͔

Thegenerationanddetectionofterahertz͑THz͒radia-tionusingultrashortopticalpulseshasbeenintensivelystud-iedduringthelastdecade.Thepulsewidthofcommerciallyavailablemode-lockedTi:sapphirelasersisapproaching10fs.Withsuchultrashortpulses,awiderdetectionbandwidthisexpectedtobepossible.CoherentdetectionofTHzradia-tionbasedonaphotoconductive͑PC͒antenna,however,wasreportedtobelowerthan7THz.1,2ThislimitationhasbeenexplainedtobearesultofthefinitecarrierlifetimeandtheRCtimeconstantofthePCantenna.Therefore,interestinultrafastdetectionoftheradiationhasrecentlyshiftedtothefree-spaceelectro-optic͑EO͒samplingtechniquebecauseEOcrystalsareassumedtohaveaninstantaneousnonlinearresponse,andmostofthemaretransparentinthefartomid-infraredradiationregime.Byexploitingtheseadvantages,ultrabroadbanddetectionofTHzradiationbasedonEOsam-plinghasbeenreported.3,4Toobtainhigh-frequencyre-sponseusingEOsampling,theEOcrystalsshouldbethinenoughtoreducethegroupvelocitymismatchbetweenthenear-infraredprobebeamandtheTHzradiation.

However,evenwithaPCantennafabricatedonslowcarrierlifetimesemiconductors,suchassemi-insulating͑SI͒GaAsorSIInP,thedetectionofrelativelybroadband͑ϳ3THz͒THzradiationhasbeenreported.5,6Thisdetectionwithaslowcarrierlifetimewaspossiblyduetothefast-risingedgeofthecarrierinjectedbytheultrashortopticalpulses.Thissuggeststhatthedetectionbandwidthisnotstronglyrestrictedbythecarrierlifetimeandispossiblyextendedbyusingshorterlaserpulses.Thus,itisworthwhiletoinvesti-gatethehigh-frequencylimitofaPCantennagatedwithultrashortopticalpulseswhosewidthiscloseto10fs.

Inthisletter,wereportontheultrabroadbanddetectionofelectromagneticradiation,extendinguptothemidinfraredregime,withaPCantennafabricatedonalow-temperature-grownGaAs͑LTGaAs͒substrateandgatedwith15fslaserpulses.Thedetectedradiationfrequencyexceeded20THz.ThisisthehighestfrequencyobservedbyPCantennasre-portedsofar.

The12fslightpulsesweredeliveredfromamode-lockedTi:sapphirelaser͑FemtolasersProduktions,Femto-sourcePRO͒atacenterwavelengthof800nmandwithaspectralwidthof90nm͑fullwidthathalfmaximum͒.The

a͒

Electronicmail:kono@crl.go.jp

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averageoutputpowerofthelaserwas320mW.Thelaserbeamwasdividedintopumpandprobebeamsbya1-mm-thickglass-platebeamsplitter.Theaveragepowerofthepumpbeamwasabout110mW.Thepumpbeamwasfo-cusedontoaSIInP͑100͒waferbyasilver-coatedoff-axisparabolicmirrorwithanincidentangleof45°.TheTHzradiationfromtheemitterwascollectedatthereflectionangleoftheincidentpumpbeambyapairofoff-axispara-bolicmirrorsandthenfocusedontothePCdetector.Asili-conaplanatichemisphericallenswhosediameterwas12mmwasattachedtothePCantennatofocustheTHzradiation.

ThePCantennawasa30-␮m-longdipoleantennawitha5␮mgapatthecenter,fabricatedonanLTGaAswafer.The1.5-␮m-thickLTGaAslayerwasgrownat250°ContheGaAssubstratewhosethicknesswas0.4mm.ThecarrierlifetimeoftheLTGaAswasestimatedtobeabout1.4psbyatransientphotoreflectancemeasurement.Theprobebeamwasfocusedontothephotoconductivegapwithareflection-typeobjectivelenstoavoidbroadeningoftheopticalpulsesduetothedispersionandcoloraberrationtypicallycausedbyanordinaryglasslens.ThetimingbetweentheTHzpulsesandtheprobepulseswasscannedbythetimedelaylineinthepathoftheprobebeamwithacornerreflectoronahigh-precisionmotorizedtranslationstage.

Thelaserpulseswerenegativelychirpedattheexitofthelaserduetomultiplereflectionsbetweenthepairofchirpedmirrors.Evenafterthechirpofthelaserpulseswerepositivelycompensatedbytheinsertionofthebeamsplittersandneutraldensityfilters,thepulsewidthonthesemicon-ductoremitterandPCantennawasapproximately15fs.ThephotocurrentsignalfromthePCantennawaspreamplifiedwithalow-noisecurrentamplifierandthendetectedwithalock-inamplifierreferencedtoanopticalchopper͑2kHz͒inthepumpbeampath.

Figure1͑a͒showsthewaveformoftheTHzradiationfromtheSIInP͑100͒emitterforasinglescan.Thetimeconstantofthelock-inamplifierwas0.1s,andthescanningtimeforthe3pstimewindowwasabout3minwitha1␮mstepresolutioninthedelay-linetranslationstage.ThereisasingleTHzpulsecenteredaround0.5ps,whichisattributedtotheradiationduetothecurrentmodulatedfromthesur-facefieldoftheInP.Veryfastoscillationsaresuperimposedonthispulse.Theshortestoscillationperiodisabout45fsandtheperiodsoftheoscillationsarenotconstant.Wedid

©2000AmericanInstituteofPhysics

0003-6951/2000/77(25)/4104/3/$17.00

Appl.Phys.Lett.,Vol.77,No.25,18December2000FIG.1.͑a͒Beginningofthetime-resolvedwaveformofTHzradiationfromtheInP͑100͒surface.͑b͒Theentiretime-resolvedTHzwaveform.ThiswaveformwasusedforthecalculationoftheFouriertransformedspectrashowninFig.2.

notclearlyobservetheexpected95fsoscillationduetothecoherentoscillationofthelongitudinaloptical͑LO͒phononoftheInPemitter,asshowninFig.3͑a͒ofRef.4.Thus,thesefastoscillationsmaybeattributedtotheradiationgen-eratedbytheopticalrectificationeffectintheemitter.TheringsafterthepeaksignalaremainlyattributedtothebeatsbetweenthecoherentoscillationoftheLOphononintheInPemitterandthefastoscillationduetotheopticalrectificationeffectbecausetheperiodsoftheringsarenotconstant.Thesebeatsare,atthesametime,affectedbythedispersionandabsorptionintheGaAssubstrateofthedetectorandabsorptionbywatervaporintheambientair.

Figure2showstheFouriertransformedspectrumoftheTHzradiationwaveformshowninFig.1͑b͒.Thespectrumextendsbeyond20THz.ThisbandwidthisalmostthreetimeswiderthanthewidestbandwidtheverachievedbyaPCantenna.Theabsorptionbandfrom7to9THzisdueto

FIG.2.FouriertransformedspectraofTHzradiationwaveforminFig.1͑b͒.ThedashedcurverepresentstheRCresponsefunctionofthereceiverantennawithanRCtimeconstantof0.2ps.

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FIG.3.CalculatedtimederivativeoftheTHz-radiationwaveformoftheTHzradiationshowninFig.1͑a͒.

thephononresonanceinthe0.4-mm-thickGaAssubstrateofthePCdetector͑Reststrahlenband͒.Thepeakat10.5THzcorrespondstotheLOphononfrequencyofInP.ThefastoscillationsduetotheopticalrectificationeffectextendthetailofthespectrumhigherthanthisLO-phononresonance.Manyoftheabsorptionlinesobservedinthespectrumareduetothewatervaporintheambientair.Thestrongabsorp-tionlinesat6.08,11.24,and11.83THzcorrespond7tothewatervaporabsorptionlinesintheliteraturewithinthespectralresolutionof36.6GHz.Theabsorptioncenteredat15.5THzisduetothephononoscillationintheSihemi-sphericallens.8Theabsorptionbandcenteredatabout18.2THzismainlyattributedtothetwo-phononabsorptionduetothecombinationofatransverseopticalandatransverseacousticphononintheSihemisphericallens.9

ItisrathersurprisingthatthebandwidthofthePCan-tennaexceeded20THz.Weneedtoexplaintheoriginofthisultrabroadbandresponse.IthasbeenreportedthataPCan-tennawithaverylongcarrierlifetime͑ϳ100ps͒wasabletodetectTHzradiationwithalmostthesamedetectionband-widthofaPCantennawithasubpicosecondcarrierlifetime.5,6Thefasttemporalresponseofaslowphotocon-ductiveantennacanbeexplainedbythefastriseofitspho-tocurrentonexcitationbytheultrashortlaserpulse.IftheresponsefunctionofthePCantennawithaslowcarrierlife-timeisastepfunction,thePCantennaworksasasamplingdetectorinanintegrationmode.Thisappeartobethecaseinthepresentexperiment:thecarrierlifetimeoftheLTGaAs͑ϳ1ps͒wasmuchlongerthanthegatingpulsewidth.Figure3presentsthecalculatedtimederivativeofthewaveformshowninFig.1͑a͒.Thistime-differentiatedwaveformissimilartoFig.3ofRef.3.ThissupportsthetheorythatthePCantennawasworkinginanintegrationmode.Thephysi-caloriginofthefastphotocurrentwithintheinitialϳ100fsstillneedstobeinvestigated.However,itmaybeexplainedbytheballistictransportofthephotoexcitedelectronsinthebiasedelectricfield,whichwasexperimentallyobservedbyHuetal.inthesametimerange.10

Inadditiontothecarrierlifetime,theRCconstantoftheantennaisanimportantparameterindeterminingthefre-quencyresponse.Inapreviousreport,weestimatedanRCtimeconstantforthesametypeofantennatobeapproxi-mately0.2ps.11ThisfiniteRCtimeconstantrestrictsthebandwidthofthePCantennaandreducesitsresponsivityathigherfrequencies.Ourobservationofradiationatunexpect-

4106Appl.Phys.Lett.,Vol.77,No.25,18December2000edlyhighfrequencieswithaPCantennacanbeexplainedbytherelativelyslowdecayofthefrequencyresponsefortheRCtimeconstant.ThefrequencyresponseofthePCantennawasapproximatedbytheequationofthedifferentialcircuit

G(␻)ϭXC/ͱR2ϩX2C,whereXandCistheCϭ1/␻C,RistheresistanceofthePCantenna,capacitanceformedbetweentheantennaelectrodes.Forexample,fora0.2psRCtimeconstant,thecutofffrequencywascalculatedtobeabout0.8THz.Theresponsivityoftheantennaat5and10THz,re-spectively,decreasesto25%and13%ofthatatthecutofffrequency.ThecircuitresponsefunctioninthefrequencydomaincalculatedwithanRCtimeconstantof0.2psisindicatedbythedashedcurveinFig.2.TheRCresponsefunctionreproducesthespectralprofileforthefrequencyrangefrom2to11THz,exceptfortheReststrahlenabsorp-tionband.Weneedtoincludeotherfactors,suchastheemitterbandwidthandabsorptionorreflectionloss,forbetterreproductionoftheoverallspectralprofile.

Inconclusion,wedemonstratedthataPCantennagatedwith15fslaserpulseswascapableofultrabroadbanddetec-tionupto20THz,comparabletothatreportedwithEOsamplingusingathinEOcrystal.IntheEOsamplingtech-nique,thestrongabsorptionanddispersionatthephononresonancefrequenciesofEOcrystalsisadisadvantage.We

Konoetal.alsoobservedstrongphononabsorptionintheGaAssub-stratebutthisproblemcanbeavoidedbyfabricatingtheLTGaAsonasuitablesubstratematerial,suchashighresistivitysilicon.

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