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QuantumCascadeLasersandApplications
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DaylightSolutions
QuantumCascadeLasers(QCLs)werefirstdemonstratedin1994bytheBellLabsTeamheadedbyJérmeFaistandFedericoCappasso.[1]TheopticalphysicsofQCLsdifferfromthatofothersemiconductorlasersinthatthelasingtransitionoccursbetweenstateswithinagivenquantumwell.(Incontrast,thelasingtransitionofadiodelaseroccursbetweentheconductionbandandvalenceband.)ThewelldepthsdependonthethicknessoflayerscreatedduringthefabricationprocessandhencethewavelengthoftheQCLcanbe"engineered."Bycarefuldesignofthequantumwells,lasingfrom2.75μm[2,3]to161μm[4](1.9THz)hasbeenobserved.Thelongerwavelengthdevicesstillrequirecryogeniccoo领,butroomtemperatureoperationispossibletoatleast16μm[5].Commercialavailabilityhasconcentratedinthemid-infrared(3.5-13μm).
ThegainprofileofaQCLcanbequitebroad(500cm-1inselectcases).ByprovidingwavelengthfeedbackeitherthroughtheuseofDistributedFeedBackorbyconstructinganexternalcavity(ECqcL),thelinewidthoftheemissioncanbepassivelynarrowedtoaslittleas0.00002cm-1(500kHz),butapracticallimitiscloserto5-50MHz.Further,inbothdevicearchitectures,theemissionwavelengthcanbetuned(throughtemperatureorgratingrotationrespectively)althoughtheDFBislimitedtoonlyacoupleofwavenumberswhereastheECqcLcanprovidehundredsofwavenumbers.Thusnarrow-band,widelytunablemid-infraredlightisobtainedinasingle-stage,semiconductordevice.
InthespectralregionservedbyQCLs,manyspecieshavestrongfundamentalabsorptionsandsoaccesstothemid-infraredfacilitatestheirdetectionandidentification.Detectionsinthepartspertrillionrange[6]and/ordiscriminationbetweensimilarspeciesarepossible.Figure1isarepresentationofthemid-infraredportionofthespectrumwithanumberofspeciesplacedwheretheirstrongabsorptionsoccur.Itcanbeseenthatthemid-IRisrichininformationforthosewishingtoprobe,detect,image,orquantifytheseandmanyotherspeciesincludingexplosives,nerveagents,andtoxins.
Figure1:GraphicalRepresentationoftheLocationofStrongAbsorptionsofMoleculesofInterest
AkeyapplicationforQCLsisstand-offexplosivesdetection.Inthisdevelopingfieldresearchershavesettheambitiousgoalofdetectinganddiscriminatingnanogramquantitiesofvariousexplosivesatdistancesupto50mwitheye-safelasers.Thereareanumberoftacticsbeingemployed,[7,8]oneapproachbeingThermalImaging.[9,10]Whenacompoundabsorbsinfraredlight,itre-emitsmostoftheabsorbedlightisotropicallyasheatwhichcanbeimagedbyinfraredcameras.Sinceeachanalytehasauniqueabsorptionspectrum,eachwillheatselectivelyastheIRsourceistunedthroughtheseabsorptionsandmaybeidentifiedunambiguouslybyanalysisofthemulti-spectralorhyperspectraldatacubeproduced.
WhileQCLsserveastheenginesfornewtechniquesinspectroscopyinthemid-IR,theyalsocanprovideroweratnewperformancelevels.Powersexceeding5Whavebeendemonstratedfromsingleroom-temperaturedevices.[11]CombiningperformancesuchasthiswithruggedizedpackaginghasenabledanewgenerationofInfraredCountermeasure(IRCM)devices.High-power,solid-statelasersthatoperateinmid-infrared"atmosphericwindows"canbeusedbypointer-trackerstodisabletheheatseekingmechanismemployedonsurface-to-airmissiles,thussafeguardingsoldiersinbattlefieldsituations.Multiple"socket"QCL-basedlasersystemshavebeenmilitarilyhardenedandhavecompletedhelicopterflighttesting.
QuantumCascadeLasersarearelativelynewtechnologyforaccessingthemid-infraredouttoTerahertzwavelengths.Theyhavemovedfromlaboratorycuriositythroughtechnologyacceptanceandintotechnologyrelianceasrobustsystemsarecommerciallydeliveredinproductionquantitiesforanumberofapplications.FormoreinformationonECqcLtechnology,pleasevisittheDaylightSolutionswebsite.
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