High harmonic generation in 真空装置

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Highharmonicgenerationinagas-filledhollow-corephotoniccrystalfiber

O.H.Heckl·C.R.E.Baer·C.Kränkel·S.V.Marchese·F.Schapper·M.Holler·T.Südmeyer·J.S.Robinson·J.W.G.Tisch·F.Couny·P.Light·F.Benabid·U.Keller

Received:28September2009/Publishedonline:14October2009

©TheAuthor(s)2009.ThisarticleispublishedwithopenaccessatSpringerlink.com

AbstractHighharmonicgeneration(HHG)ofintensein-fraredlaserradiation[1,2]enablescoherentvacuum-UV(VUV)tosoft-X-raysources.Intheusualsetup,energeticfemtosecondlaserpulsesarestronglyfocusedintoagasjet,restrictingtheinteractionlengthtotheRayleighrangeofthefocus.Theaveragephotonfluxislimitedbythelowcon-versionefficiencyandthelowaveragepowerofthecom-plexlaseramplifiersystems[3–6]whichtypicallyoperateatkilohertzrepetitionrates.Thisrepresentsaseverelimitationformanyexperimentsusingtheharmonicradiationinfieldssuchasmetrologyorhigh-resolutionimaging.DrivingHHGwithnovelhigh-powerdiode-pumpedmulti-megahertzlasersystemshasthepotentialtosignificantlyincreasetheaver-agephotonflux.However,thehigheraveragepowercomesattheexpenseoflowerpulseenergiesbecausetherepeti-tionrateisincreasedbymorethanathousandtimes,andefficientHHGisnotpossibleintheusualgeometry.Sofar,twopromisingtechniquesforHHGatlowerpulseen-ergiesweredeveloped:externalbuild-upcavities[7,8]and

O.H.Heckl(󰀝)·C.R.E.Baer·C.Kränkel·S.V.Marchese·F.Schapper·M.Holler·T.Südmeyer·U.Keller

DepartmentofPhysics,InstituteforQuantumElectronics,ETHZurich,8093Zurich,Switzerlande-mail:heckl@phys.ethz.ch

J.S.Robinson

DepartmentofPhysics,UniversityofCaliforniaatBerkeleyandMaterialsSciencesDivision,LawrenceBerkeleyNationalLaboratory,Berkeley,CA94720,USA

J.W.G.Tisch

QuantumOpticsandLaserScienceGroup,BlackettLaboratory,ImperialCollegeLondon,LondonSW72BW,UK

F.Couny·P.Light·F.Benabid

DepartmentofPhysics,UniversityofBath,BathBA27AY,UK

resonantfieldenhancementinnanostructuredtargets[9].Herewepresentathirdtechnique,whichhasadvantagesintermsofeaseofHHGlightextraction,transversebeamqual-ity,andthepossibilitytosubstantiallyincreaseconversionefficiencybyphase-matching[10–14].Theinteractionbe-tweenthelaserpulsesandthegasoccursinaKagome-typeHollow-CorePhotonicCrystalFiber(HC-PCF)[15],whichreducesthedetectionthresholdforHHGtoonly200nJ.Thisnoveltypeoffiberguidesnearlyallofthelightinthehollowcore[16],preventingdamageevenatintensitiesrequiredforHHG.Ourfiberguided30-fspulseswithapulseenergyofmorethan10μJ,whichismorethanfivetimeshigherthanforanyotherphotoniccrystalfiber[17].

PACS42.81.Qb·42.65.Ky·42.55.Xi

370Infreespace,thenonlinearinteractionoflightwithgasesislimitedbydiffraction.Guidinglightingas-filledHC-PCFsenabledseveralmajorbreakthroughsinnonlinearoptics.Combiningasmallmodeareawithalongeffectivein-teractionlengthreducesthethresholdofvariousnonlin-earprocessesbyseveralordersofmagnitudeandincreasestheirefficiencydramatically[15,16,18].Accordingtonu-mericalsimulations,HC-PCFsalsohavealargepoten-tialforhighharmonicgeneration[11–13,17–19].Sofar,wave-guidequasi-phasematchingtechniquesforHHGwereonlyrealizedincapillarieswithdiametersabove100μm[20,21].HC-PCFsoffersubstantiallylowertransmissionlosses,smallermodeareas,andlargerdesignfreedom.How-ever,employingHC-PCFsintheareaofhighfieldscienceischallenging,becauseintensities>1013W/cm2havetobeachievedinthehollowcore.Thisisdifficultwithstan-dardphotonicbandgap(PBG)HC-PCFsbecausethefieldintensityinsurroundingsilicais≈1%ofthemaximumin-tensityinthecore[22].Sofar,thehighestguidedenergyre-portedforfemtosecondpulsepropagationinPBGHC-PCFsis1.8μJusing40-fspulses[17].Inourexperiment,weusedaKagome-typeHC-PCFforwhichthefieldoverlapismorethantentimeslower[16].IncontrasttoPBGHC-PCF,theguidingmechanismisnotbasedonabandgapbutonthein-hibitedcouplingbetweenthecoreandcladdingmodes[16].Thefiberisimmunetocore-modeandsurface-modecou-pling[23],whichrepresentsoneofthemajorsourcesofop-ticaloverlapwiththesilicasurroundingthefibercore.Sincethedamagethresholdscaleswiththeinverseofthisfraction,guidanceofveryhighpulseenergiesandpeakpowerlev-elsbecomesfeasible.Furthermore,Kagome-typePCFshaveadvantagesintermsofdispersionmanagementandanultra-broadguidingbandwidth,whichareparticularlyimportantforapplicationswithultrashortlaserpulses[16].Ascanningelectronmicroscope(SEM)imageofourKagome-typePCFcanbeseeninFig.1.Thefiberwasdesignedforoperationatacentralwavelengthof800nmandhasaneffectivecorediameterof15μm.

Inourexperimentalsetup(Fig.2),weusetwovacuumchamberswhichareconnectedbya15-mmlongHC-PCF.ThedrivinglaserisastandardTi:sapphireamplifiersystemgenerating30-fspulsesat800nmwitharepetitionrateof1kHz.TheentrancefaceoftheHC-PCFissituatedinthepre-chamberfilledwithxenonatavariablepressureofupto30mbar.Theexitfaceofthefiberextendedintotheadjacentspectrometervacuumchamberwhichissealedofffromthepre-chamberexceptforthegasflowthroughtheHC-PCFandsomeparasiticleakagethroughthefibermount.Thevacuumchambercontainsatunablegratingandanelectronmultipliertube(EMT)detector.Thisiridium-coatedgratinghas1200lines/mm,resultinginaspectralresolutionofap-proximately4nmforourgeometry.TheEMTissensitiveinthespectralregionfrom30nmto150nmwithanestimatedpeakefficiencyof16%atawavelengtharound70nm.

O.H.Heckletal.

Fig.1SEMimageoftheKagome-typeHC-PCF.Theeffectivemodediameter(correspondingtothehollowcentercore)isabout15μm

Fig.2Experimentalsetup:Thelaserpulsesarefocusedontotheen-trancefaceofthehollowcorephotoniccrystalfiber(HC-PCF)infreespace.Thefiberismountedinthemiddleofthereddisc,whichislo-catedinachamberbackfilledwithanadjustablexenonpressure.Thexenoninletisdisplayedinyellow.Thefiberexitfaceisinavacuumchamberwhichcontainsthespectrometergrating(framedinblue)andthedetector(green)

Asafirststep,weinvestigatedthetransmissionproper-tiesanddamagethresholdofourKagomeHC-PCFinvac-uum(noxenonfillinginthepre-chamber).Kagome-typefiberstypicallyoperateathighcouplingefficienciesandwithtransmissionlossesof≈0.5dB/m[16].However,inourexperiment,theinputbeamwasellipticalandnotwellmode-matchedtotheroundfibermode,whichresultedinacouplingefficiencyofonly30%to40%.Forapulse-energyof30μJincidentonthefiber,wetransmitted10.5μJpulses.Thespectrumattheoutputwasidenticaltotheinputspec-

Highharmonicgenerationinagas-filledhollow-corephotoniccrystalfiber371Fig.3MeasuredspectrumofthehighharmonicsgeneratedinXewith4.2μJ,30fslaserpulsesofaTi:sapphire.ThegraphiscorrectedforspectralresponseoftheEMT

trum,showingthatthelowfieldoverlapofthecoremodewiththesilicadidnotintroducesignificantselfphasemod-ulation.Thedurationoftheoutputwasnotmeasured,butisexpectedtobethesameastheinputpulsedurationduetothelowdispersionofthefiber.Wedidnotobserveanydamageforthesepulseenergiesdespitepeakintensitiesof3×1014W/cm2andthelargeamountofun-launchedpower.Whenwefloodedthepre-chamberwithxenongas,weobservedthegenerationofhighharmonics.Figure3showsthemeasuredspectrumforpulseswith4.2μJenergyintheHC-PCFandaxenonpressureofP0=27mbarinthepre-chamber.Wedidnotincreasethepressureabove30mbarbecausetheparasiticleakagethroughthefibermountde-gradedthevacuuminthemainchambertotheEMToper-ationlimit(<1.3×10−4mbar).The7thto13thharmoniccanbeclearlyidentified.The3rdand5thharmonicorderscannotbeobservedsincethedetectorisinsensitivetowave-lengthsabove140nm.Weobtainaconversionefficiencyofη≈2×10−9(whichisdefinedasη=harmonicen-ergy/launchedlaserenergy),byintegratingoverthehar-monicorders7–13andtakingintoaccountthegratingef-ficiency(calculatedwiththeprogramREFLEC[24]).Fig-ure4showsthesignalintensityofthe7thharmonicversusthelaunchedpulseenergyELonmonicsignalscalesasEp

alogarithmicplot.Thehar-Lwherep=3.6.Thisisaweakerdependencethanexpectedfromlowestorderperturbationtheory(p∼7)indicatingthattheatomicresponsecannotbeaccuratelydescribedwithintheweak-fieldlimitforourconditions[25].WeobservedaHHGdetectionthresholdaslowas200nJinsidetheHC-PCF.Thetransversebeampro-fileoftheharmonicswasmeasuredbygraduallyinsertingaknifeedgeintothehighharmonicbeam(locatedinvac-uumchamberbetweenfiberoutputandgrating,seeFig.2).InFig.5,weshowthemeasurementofthebeamprofilefor

Fig.4Measuredsignalintensityforthe7thharmonicasafunctionofthelaserpulseenergyincidentonthefiber.Theerrorbarsdisplaythestandarderrorofthemeasurement.The7thHHGdetectionthresholdwasmeasuredtobe200nJinsidetheHC-PCF

Fig.5Aknifeedgemeasurementofthe7thharmonic70mmfromthefiberendrevealsanexcellentGaussianbeamprofilewhichcanbewellapproximatedbyaGaussianfit

the7thharmonicforalaunchedpulse-energyof4μJandaxenonpressureof30mbar.ThebeamprofilecanbewellapproximatedbyaGaussianfit.

OurresultsshowthatHHGinaHC-PCFisadvanta-geousintermsofthreshold,lightextractionandbeamqual-ity.Extractingtheharmonicradiationinenhancementcavi-tiesissubstantiallymorechallenging[7,8]becausethein-teractionofthelaserpulseswiththegastargetoccursin-sideahigh-finessecavityforwhichthelossesusuallyarekeptwellbelow1%.IncontrasttoHHGbasedonresonantfieldenhancementinnanostructuredtargets[9],thereisnobeamprofiledistortionimprintedontotheharmonicradia-

372tion,insteadaroundbeamwithGaussiantransverseprofileisachieved(Fig.5).

ThecurrentconversionefficiencyissimilartoHHGbasedonnanostructuredtargets,butsubstantiallylowerthanforrecenthighpowerenhancementcavitysystems[26].Thisisduetotheimperfectphasematchingbetweenfun-damentalandharmonicradiationinthesefirstproof-of-principleexperiments.HHGquasi-phasematchingtech-niquesarealreadyestablishedforinteractionsincapillaries[21]andfreespace[27],forwhichtheyincreasedtheeffi-ciencybymorethan2ordersofmagnitude.HC-PCFsallowustoaddressalargerparameterspaceforphasematching,duethesubstantialdesignflexibilityoftheirdispersionandmodearea.Thisisinstarkcontrasttostandardcapillariesforwhichthedispersionandhencephasematchingcondi-tionsaredirectlyconstrainedbythemodearea.Forexam-ple,theabilitytotailortherefractiveindexofthefiberatthefundamentalwavelengthmayallowforadjustmentofphase-matchingpressurewithoutcompromisingthemodeareare-quiredforlowenergyHHG.Inconjunctionwiththemuchincreasedinteractionlengthaffordedbythehightransmis-sionoftheHC-PCF,efficientgenerationofhighharmonicsinregimesthatarenotlimitedbytheabsorptionlengthofthenonlinearmediummaybepossible.Additionally,onecanemployquasi-phasematchingtechniquesforHHGal-readydemonstratedincapillaries[21],forexample,bymod-ulatingthemodearea(e.g.,bythefiberdrawingmethodorheattreatmentwithaCO2laser[28]).Onemayalsoutilizecounter-propagatingbeams[14]asdiscussedinrecentsim-ulationsofHHGinaPCF[11],whichshowthatharmonicswithphotonenergiesofseveralhundredsofelectron-voltscanbeproducedbyfemtosecondpulseswithmicrojouleen-ergies.

Inconclusion,wehavedemonstratedthegenerationofhighharmonicsinagasfilledHC-PCFforthefirsttime.Thefibertransmittedlaserpulseswithpulseenergiesofmorethan10μJwithoutdamage,whichisseveraltimeshigherthanpreviouslyreportedforthepropagationoffemtosecondpulsesinaHC-PCF.Usingxenonasthenonlinearmedium,weobservedthegenerationofthe7thto13thharmonicofthefundamentalwavelengthofaTi:sapphirelasersystemoperatingaround800nm.Thiscorrespondstothegenera-tionoflightintheVUV/XUV-spectralregionfrom50nmto130nm.TheobservedHHGdetectionthresholdisbelow200nJ.Thisiswellwithinreachofnovelmulti-megahertzdiode-pumpedsolid-statelasersandfiberamplifiers,whichachievesubstantiallyhigherpowerlevelsandrepetitionratesthanthecurrentlydominantkilohertzTi:sapphiream-plifiersystems.Diode-pumpedSESAM-mode-lockedfem-tosecondthindisklaserscurrentlygeneratecloseto100Wdirectlyfromtheoscillatorwithoutanyfurtheramplifica-tion[4,29,30].Chirped-pulsefemtosecondfiberamplifiersystemsachieveupto325Waveragepower[5,6].There-sultinghighaveragepower,compactcoherentXUV/VUV

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sourcewillhaveahighimpactfornumerousapplicationsinfieldsasdiverseasmedicine,biology,chemistry,physicsandmaterialsscience.Evenatalowconversionefficiencyofonly10−8,anaverageHHGphotonfluxof>1μWwouldbeachieved,whichissufficientformanyapplica-tions.Betterconversionefficiencieswithimprovedphasematchingwillreducethepowerrequirementandcomplex-ityofthesystemfurther.Operatingatmulti-megahertzin-steadofkilohertzrepetitionratesincreasesthesignal-to-noiseratioandreducesthetimerequiredformanymeasure-ments,as,forexample,inXUVmetrology,highresolutionXUVimaging,andmicroscopy[31].Increasingtherepe-titionrateisespeciallyimportantinexperiments,forwhichthetotalnumberofsimultaneousionizationprocessesislim-itedbyspacechargeeffects(i.e.,electro-magneticforcesbe-tweenthegeneratedchargedparticleswithintheinteractionvolume).Importantexamplesaresurfacescienceandcon-densedmatterstudiesusingangle-resolvedphotoemissionspectroscopy(ARPES)[32].

AcknowledgementTheauthorswouldliketothankPhilipSt.J.RussellforfruitfulinitialdiscussionsonHHGinaHC-PCFandhelpingtosetupthecollaborationwithBathUniversity.

OpenAccessThisarticleisdistributedunderthetermsoftheCre-ativeCommonsAttributionNoncommercialLicensewhichpermitsanynoncommercialuse,distribution,andreproductioninanymedium,providedtheoriginalauthor(s)andsourcearecredited.

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