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LasertechnologyR.E.SlusherBellLaboratories,LucentTechnologies,MurrayHill,NewJersey07974Lasertechnologyduringthe20thcenturyisreviewedemphasizingthelaser’sevolutionfromsciencetotechnologyandsubsequentcontributionsoflasertechnologytoscience.Asthecenturydrawstoaclose,lasersaremakingstrongcontributionstocommunications,materialsprocessing,datastorage,imagerecording,medicine,anddefense.Examplesfromtheseareasdemonstratethestunningimpactoflaserlightonoursociety.Laseradvancesarehelpingtogeneratenewscienceasillustratedbyseveralexamplesinphysicsandbiology.Free-electronlasersusedformaterialsprocessingandlaseracceleratorsaredescribedasdevelopinglasertechnologiesforthenextcentury.[S0034-6861(99)02802-0]1.INTRODUCTIONLighthasalwaysplayedacentralroleinthestudyofphysics,chemistry,andbiology.Lightiskeytoboththeevolutionoftheuniverseandtotheevolutionoflifeonearth.Thiscenturyanewformoflight,laserlight,hasbeendiscoveredonoursmallplanetandisalreadyfacilitatingaglobalinformationtransformationaswellasprovidingimportantcontributionstomedicine,industrialmaterialprocessing,datastorage,printing,anddefense.Thisreviewwilltracethedevelopmentsinscienceandtechnologythatledtotheinventionofthelaserandgiveafewexamplesofhowlasersarecontributingtobothtechnologicalapplicationsandprogressinbasicscience.Therearemanyotherexcellentsourcesthatcovervariousaspectsofthelasersandlasertechnologyincludingarticlesfromthe25thanniversaryofthelaser(AusubellandLangford,1987)andtextbooks.,Siegman,1986;AgrawalandDutta,1993;andReady,1997).Lightamplificationbystimulatedemissionofradiation(LASER)isachievedbyexcitingtheelectronic,vibrational,rotational,orcooperativemodesofamaterialintoanonequilibriumstatesothatphotonspropagatingthroughthesystemareamplifiedcoherentlybystimulatedemission.Excitationofthisopticalgainmediumcanbeaccomplishedbyusingopticalradiation,electricalcurrentanddischarges,orchemicalreactions.Theamplifyingmediumisplacedinanopticalresonatorstructure,forexamplebetweentwohighreflectivitymirrorsinaFabry-Perotinterferometerconfiguration.Whenthegaininphotonnumberforanopticalmodeofthecavityresonatorexceedsthecavityloss,aswellaslossfromnonradiativeandabsorptionprocesses,thecoherentstateamplitudeofthemodeincreasestoalevelwherethemeanphotonnumberinthemodeislargerthanone.Atpumplevelsabovethisthresholdcondition,thesystemislasingandstimulatedemissiondominatesspontaneousemission.Alaserbeamistypicallycoupledoutoftheresonatorbyapartiallytransmittingmirror.Thewonderfullyusefulpropertiesoflaserradiationincludespatialcoherence,narrowspectralemission,highpower,andwell-definedspatialmodessothatthebeamcanbefocusedtoadiffraction-limitedspotsizeinordertoachieveveryhighintensity.Thehighefficiencyoflaserlightgenerationisimportantinmanyapplicationsthatrequirelowpowerinputandaminimumofheatgeneration.Whenacoherentstatelaserbeamisdetectedusingphoton-countingtechniques,thephotoncountdistributionintimeisPoissonian.Forexample,anaudiooutputfromahighefficiencyphotomultiplierdetectingalaserfieldsoundslikeraininasteadydownpour.Thislasernoisecanbemodifiedinspecialcases,.,byconstantcurrentpumpingofadiodelasertoobtainasqueezednumberstatewherethedetectedphotonssoundmorelikeamachinegunthanrain.Anopticalamplifierisachievedifthegainmediumisnotinaresonantcavity.Opticalamplifierscanachieveveryhighgainandlownoise.InfacttheypresentlyhavenoisefigureswithinafewdBofthe3dBquantumnoiselimitforaphase-insensitivelinearamplifier,.,theyaddlittlemorethanafactoroftwotothenoisepowerofaninputsignal.Opticalparametricamplifiers(OPAs),wheresignalgainisachievedbynonlinearcouplingofapumpfieldwithsignalmodes,canbeconfiguredtoaddlessthan3dBofnoisetoaninputsignal.InanOPAthenoiseaddedtotheinputsignalcanbedominatedbypumpnoiseandthenoisecontributedbyalaserpumpbeamcanbenegligiblysmallcomparedtothelargeamplitudeofthepumpfield.2.HISTORYEinstein(1917)providedthefirstessentialideaforthelaser,stimulatedemission.Whywasn’tthelaserinventedearlierinthecenturyMuchoftheearlyworkonstimulatedemissionconcentratesonsystemsnearequilibrium,andthelaserisahighlynonequilibriumsystem.Inretrospectthelasercouldeasilyhavebeenconceivedanddemonstratedusingagasdischargeduringtheperiodofintensespectroscopicstudiesfrom1925to1940.However,ittookthemicrowavetechnologydevelopedduringWorldWarIItocreatetheatmosphereforthelaserconcept.CharlesTownesandhisgroupatColumbiaconceivedthemaser(microwaveamplificationbystimulatedemissionofradiation)idea,basedontheirbackgroundinmicrowavetechnologyandtheirinterestinhigh-resolutionmicrowavespectroscopy.SimilarmaserideasevolvedinMoscow(BasovandProkhorov,1954)andattheUniversityofMaryland(Weber,1953).ThefirstexperimentallydemonstratedmaseratColumbiaUniversity(Gordonetal.,1954,1955)wasbasedonanammoniamolecularbeam.Bloembergen’sideasforgaininthreelevelsystemsresultedinthefirstpracticalmaseramplifiersintherubysystem.ThesedeviceshavenoisefiguresveryclosetothequantumlimitandwereusedbyPenziasandWilsoninthediscoveryofthecosmicbackgroundradiation.Towneswasconfidentthatthemaserconceptcouldbeextendedtotheopticalregion(Townes,1995).Thelaserideawasborn(SchawlowandTownes,1958)whenhediscussedtheideawithArthurSchawlow,whounderstoodthattheresonatormodesofaFabry-Perotinterferometercouldreducethenumberofmodesinteractingwiththegainmaterialinordertoachievehighgainforanindividualmode.ThefirstlaserwasdemonstratedinaflashlamppumpedrubycrystalbyTedMaimanatHughesResearchLaboratories(Maiman,1960).Shortlyafterthedemonstrationofpulsedcrystallasers,acontinuouswave(CW)He:NegasdischargelaserwasdemonstratedatBellLaboratories(Javanetal.,1961),firstatmmandlaterattherednmwavelengthlasingtransition.AnexcellentarticleonthebirthofthelaserispublishedinaspecialissueofPhysicsToday(Bromberg,1988).Themaserandlaserinitiatedthefieldofquantumelectronicsthatspansthedisciplinesofphysicsandelectricalengineering.Forphysicistswhothoughtprimarilyintermsofphotons,somelaserconceptsweredifficulttounderstandwithoutthecoherentwaveconceptsfamiliarintheelectricalengineeringcommunity.Forexample,thelaserlinewidthcanbemuchnarrowerthanthelimitthatonemightthinktobeimposedbythelasertransitionspontaneouslifetime.CharlesTowneswonabottleofscotchoverthispointfromacolleagueatColumbia.Thelaserandmaseralsobeautifullydemonstratetheinterchangeofideasandimpetusbetweenindustry,government,anduniversityresearch.Initially,duringtheperiodfrom1961to1975therewerefewapplicationsforthelaser.Itwasasolutionlookingforaproblem.Sincethemid-1970stherehasbeenanexplosivegrowthoflasertechnologyforindustrialapplications.Asaresultofthistechnologygrowth,anewgenerationoflasersincludingsemiconductordiodelasers,dyelasers,ultrafastmode-lockedTi:sapphirelasers,opticalparameteroscillators,andparametricamplifiersispresentlyfacilitatingnewresearchbreakthroughsinphysics,chemistry,andbiology.3.LASERSATTHETURNOFTHECENTURYSchawlow’s‘‘law’’statesthateverythinglasesifpumpedhardenough.Indeedthousandsofmaterialshavebeendemonstratedaslasersandopticalamplifiersresultinginalargerangeoflasersizes,wavelengths,pulselengths,andpowers.Laserwavelengthsrangefromthefarinfraredtothex-rayregion.Laserlightpulsesasshortasafewfemtosecondsareavailableforresearchonmaterialsdynamics.Peakpowersinthepetawattrangearenowbeingachievedbyamplificationoffemtosecondpulses.Whenthesepowerlevelsarefocusedintoadiffraction-limitedspot,theintensitiesapproach1023W/cm2.Electronsintheseintensefieldsareacceleratedintotherelativisticrangeduringasingleopticalcycle,andinterestingquantumelectrodynamiceffectscanbestudied.Thephysicsofultrashortlaserpulsesisreviewedisthiscentennialseries(Bloembergen,1999).Arecentexampleofalarge,powerfullaseristhechemicallaserbasedonaniodinetransitionatawavelengthofmmthatisenvisionedasadefensiveweapon(Forden,1997).ItcouldbemountedinaBoeing747aircraftandwouldproduceaveragepowersof3megawatts,equivalentto30acetylenetorches.Newadvancesinhighqualitydielectricmirrorsanddeformablemirrorsallowthisintensebeamtobefocusedreliablyonasmallmissilecarryingbiologicalorchemicalagentsanddestroyitfromdistancesofupto100km.This‘‘starwars’’attackcanbeaccomplishedduringthelaunchphaseofthetargetmissilesothatportionsofthedestroyedmissilewouldfallbackonitslauncher,quiteagooddeterrentfortheseevilweapons.CaptainKirkandthestarshipEnterprisemaybeusingthisoneontheKlingons!Attheoppositeendofthelasersizerangearemicrolaserssosmallthatonlyafewopticalmodesarecontainedinaresonatorwithavolumeinthefemtoliterrange.Theseresonatorscantaketheformofringsordisksonlyafewmicronsindiameterthatusetotalinternalreflectioninsteadofconventionaldielectricstackmirrorsinordertoobtainhighreflectivity.Fabry-PerotcavitiesonlyafractionofamicroninlengthareusedforVCSELs(verticalcavitysurfaceemittinglasers)thatgeneratehighqualityopticalbeamsthatcanbeefficientlycoupledtoopticalfibers(ChoquetteandHou,1997).VCSELsmayfindwidespreadapplicationinopticaldatalinks.4.MATERIALSPROCESSINGANDLITHOGRAPHYHighpowerCO2andNd:YAGlasersareusedforawidevarietyofengraving,cutting,welding,soldering,and3Dprototypingapplications.rf-excited,sealedoffCO2lasersarecommerciallyavailablethathaveoutputpowersinthe10to600Wrangeandhavelifetimesofover10000hours.Lasercuttingapplicationsincludesailclothes,parachutes,textiles,airbags,andlace.Thecuttingisveryquick,accurate,thereisnoedgediscoloration,andacleanfusededgeisobtainedthateliminatesfrayingofthematerial.Complexdesignsareengravedinwood,glass,acrylic,rubberstamps,printingplates,plexiglass,signs,gaskets,andpaper.ThreedimensionalmodelsarequicklymadefromplasticorwoodusingaCAD(computer-aideddesign)computerfile.Fiberlasers(Rossi,1997)arearecentadditiontothematerialsprocessingfield.ThefirstfiberlasersweredemonstratedatBellLaboratoriesusingcrystalfibersinanefforttodeveloplasersforundersealightwavecommunications.Dopedfusedsilicafiberlasersweresoondeveloped.Duringthelate1980sresearchersatPolaroidCorp.andattheUniversityofSouthamptoninventedcladding-pumpedfiberlasers.Theglasssurroundingtheguidingcoreintheselasersservesbothtoguidethelightinthesinglemodecoreandasamultimodeconduitforpumplightwhosepropagationisconfinedtotheinnercladdingbyalow-refractiveindexouterpolymercladding.Typicaloperationschemesatpresentuseamultimode20Wdiodelaserbarthatcouplesefficientlyintothelargediameterinnercladdingregionandisabsorbedbythedopedcoreregionoveritsentirelength(typically50m).Thedopantsinthecoreofthefiberthatprovidethegaincanbeerbiumforthemmwavelengthregionorytterbiumforthemmregion.Highqualitycavitymirrorsaredepositeddirectlyontheendsofthefiber.Thesefiberlasersareextremelyefficient,withoverallefficienciesashighas60%.Thebeamqualityanddeliveryefficiencyisexcellentsincetheoutputisformedasthesinglemodeoutputofthefiber.Theselasersnowhaveoutputpowersinthe10to40Wrangeandlifetimesofnearly5000hours.Currentapplicationsoftheselasersincludeannealingmicromechanicalcomponents,cuttingof25to50mmthickstainlesssteelparts,selectivesolderingandweldingofintricatemechanicalparts,markingplasticandmetalcomponents,andprintingapplications.ExcimerlasersarebeginningtoplayakeyroleinphotolithographyusedtofabricateVLSI(verylargescaleintegratedcircuit)chips.AstheIC(integratedcircuit)designrulesdecreasefrommm(1995)tomm(2002),thewavelengthofthelightsourceusedforphotolithographicpatterningmustcorrespondinglydecreasefrom400nmtobelow200nm.Duringtheearly1990smercuryarcradiationproducedenoughpoweratsufficientlyshortwavelengthsof436nmand365nmforhighproductionratesofICdevicespatternedtommandmmdesignrulesrespectively.Asthecenturyclosesexcimerlasersourceswithaverageoutputpowersinthe200Wrangearereplacingthemercuryarcs.Theexcimerlaserlinewidthsarebroadenoughtopreventspecklepatternformation,yetnarrowenough,lessthan2nmwavelengthwidth,toavoidmajorproblemswithdispersioninopticalimaging.Thekryptonfluoride(KF)excimerlaserradiationat248nmwavelengthsupportsmmdesignrulesandtheArFlasertransitionat193nmwillprobablybeusedbeginningwithmmdesignrules.Atevensmallerdesignrules,downtommby2008,theF2excimerlaserwavelengthat157nmisapossiblecandidate,althoughtherearenophotoresistsdevelopedforthiswavelengthatpresent.Higherharmonicsofsolid-statelasersarealsopossibilitiesashighpowerUVsources.Atevenshorterwavelengthsitisverydifficultforopticalelementsandphotoresiststomeettherequirementsinthelithographicsystems.Electronbeams,x-raysandsynchrotronradiationarestillbeingconsideredforthe70nmdesignrulesanticipatedfor2010andbeyond.5.LASERSINPHYSICSLasertechnologyhasstimulatedarenaissanceinspectroscopiesthroughouttheelectromagneticspectrum.Thenarrowlaserlinewidth,largepowers,shortpulses,andbroadrangeofwavelengthshasallowednewdynamicandspectralstudiesofgases,plasmas,glasses,crystals,andliquids.Forexample,Ramanscatteringstudiesofphonons,magnons,plasmons,rotons,andexcitationsin2Delectrongaseshaveflourishedsincetheinventionofthelaser.Nonlinearlaserspectroscopieshaveresultedingreatincreasesinprecisionmeasurementasdescribedinanarticleinthisvolume(Ha¨nschandWalther1999).Frequency-stabilizeddyelasersanddiodelaserspreciselytunedtoatomictransitionshaveresultedinultracoldatomsandBose-Einsteincondensates,alsodescribedinthisvolume(Wiemanetal.,1999).Atomicstatecontrolandmeasurementsofatomicparitynonconservationhavereachedaprecisionthatallowstestsofthestandardmodelinparticlephysicsaswellascrucialsearchesfornewphysicsbeyondthestandardmodel.Inrecentparitynonconservationexperiments(Woodetal.,1997)Ceatomsarepreparedinspecificelectronicstatesastheypassthroughtworeddiodelaserbeams.Thesepreparedatomsthenenteranopticalcavityresonatorwheretheatomsareexcitedtoahigherenergylevelbyhigh-intensitygreenlightinjectedintothecavityfromafrequency-stabilizeddyelaser.Appliedelectricandmagneticfieldsinthisexcitationregioncanbereversedtocreateamirroredenvironmentfortheatoms.Aftertheatomexitstheexcitationregion,theatomexcitationrateismeasuredbyathirdreddiodelaser.Verysmallchangesinthisexcitationratewithamirroringoftheappliedelectricandmagneticfieldsindicateparitynonconservation.Theaccuracyoftheparitynonconservationmeasurementhasevolvedoverseveraldecadestoalevelof%.Thismeasurementaccuracycorrespondstothefirstdefinitiveisolationofnuclear-spin-dependentatomicparityviolation.