Quadrupole radiation from terahertz dipole antennas

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1556OPTICSLETTERS/Vol.25,No.20/October15,2000Quadrupoleradiationfromterahertzdipoleantennas

J.V.RuddPicometrix,Inc.,P.O.Box130243,AnnArbor,Michigan48113

JonL.JohnsonandDanielM.MittlemanDepartmentofElectricalandComputerEngineering,MS-366,RiceUniversity,P.O.Box1892,Houston,Texas77251-1892

ReceivedJune12,2000Wereportwhatistoourknowledgethefirstdetailedinvestigationofthepolarizationstateofradiationfromlens-coupledterahertzdipoleantennas.Theradiationexhibitsaweakbutmeasurablecomponentthatispolarizedorthogonallytotheorientationoftheemitterdipole.Theangularradiationpatternofthiscross-polarizedemissionrevealsthatitisquadrupolar,ratherthandipolar,innature.Onecanunderstandthisresultbytakingintoaccountthephotocurrentflowinginthestriplinesthatfeedthedipoleantenna.AFresnel–Kirchhoffscalardiffractioncalculationisusedforcalculatingthefrequency-dependentangulardis-tributionoftheradiationpattern,providingsatisfactoryagreementwiththemeasurements.©2000OpticalSocietyofAmericaOCIScodes:320.7080,260.5430,260.5150.

Thetechniqueknownasterahertz(THz)time-domainspectroscopyhasbecomeapopulartoolforspectroscopyandimaging,andnumeroussensingandmonitoringapplicationshavebeenproposed.Asare-sultofthegrowingimportanceofthistechnique,muchresearchhasbeendevotedtocharacterizingtheTHzbeamgeneratedfromaphotoconductiveswitchcoupledtoasubstratelens.1Thebeamdivergence,thespatialmode,andthefrequency-dependenttransverseprofilehavebeendiscussedinseveralpublications.2–5Thepolarizationstateoftheradiationhasreceivedlessattention.ForseveralmeasurementsthepolarizationpropertiesoftheTHzradiationhavebeenexploited,6–8andotherauthorshavenotedpolarization-dependentpropertiesofopticalelements.9,10AsTHztime-domainspectroscopybecomesmorewidespread,itisbecomingclearthatthedegreeofpolarizationoftheTHzradiationisanimportantaspectofthesesystems.Inmostcases,theemissionfromTHzantennashasbeendescribedbyuseoftheapproximationofanidealdipolelinear.4Ofcourse,thedipolesusedinrealTHzsystemsarenotideal,sothepolarizationstateoftheradiationisnot,ingeneral,purelylinear.Thecon-ventionalwisdomisthatthetypicalemittergeneratesacross-polarizedcomponentthatisoftheorderofafewpercentaslargeasthecomponentpolarizedalongthedipole.Yet,toourknowledge,therearealmostnoreportsintheliteraturethatcorroboratethisstatement.Caietal.foundthatthecross-polarizedradiationhasanamplituderoughly7%aslargeasthedominantpolarizationcomponent,11althoughtheyprovidednoexplanationfortheoriginofthissmallcomponent.Garetetal.10reportedafrequency-dependentvariationinthelinearpolarizationaxis,whichtheyattributedtosubstrate-lensmisalignment.WeareawareofnothoroughcharacterizationofthepolarizationoftheTHzbeam.Herewereportmeasurementsofthecross-polarizedcomponentofthefieldradiatedfromalens-coupledTHzantenna,similartooneoftheantennasde-scribedinRef.11.Theangulardependenceofboththes-polarizedandthep-polarizedcomponentsoftheE-planeemissionhasbeenmeasured.Forthes-polarizedemission,thelargestpeak-to-peakelectricfieldamplitudeisapproximately7%ofthep-polarizedemission,aswaspreviouslynoted.11However,wehavefoundthatthemaximuminthes-polarizedemissionoccursatanangleofapproximately6±away

fromtheopticalaxis.Theseresultsindicatethatthes-polarizedcomponentisquadrupolar,ratherthandipolar,incharacter.Thesemeasurementsrepresentwhatistoourknowledgethefirstdetailedcharacteri-zationoftheorthogonallypolarizedcomponentoftheradiationfromTHzantennas.ForthesemeasurementswehaveusedaTHztime-domainspectroscopysysteminwhichthefem-tosecondopticalpulsesaredeliveredtotheantennasviasingle-modeopticalfiber.Withoutthisadvantageitwouldhavebeennecessarytorealigntheopticalbeamontothedetectorateachangularpositiontoensurethatthedetectionefficiencywasnotcompro-misedbymisalignment.12Anevenmoreimportant

advantageaffordedbythefibercouplingisthatonemaymountthereceiverantennaontheendofarailthatpivotsaboutthepositionoftheemitter,asinFig.1(a).Thisprocedureguaranteesthatthereceiverantennaisalwaysorientednormallytothepropagationdirectionoftheemittedwave,whichremovestheissueoftheangularsensitivityofthereceiverfromtheanalysis.Asaresult,themeasure-mentsreflecttheangulardistributionoftheemittedradiationratherthanaconvolutionoftheemitterandreceiverantennapatterns.Withtheexceptionofthefibercouplingoftheanten-nas,thesystemisafairlyconventionalTHzspectrome-ter.Thestructureoftheemitterantenna[Fig.1(b)]isequivalenttothestructurelabeledFTMinRef.11withadipolelengthL෇60mm.Wechosethisstruc-tureforourinitialinvestigationbecauseithasbecomequitepopularrecently,asaresultoftheenhanced