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Recognizing Outstanding Ph.D. Research
Marcus Seidel
A New Generation of High-Power, Waveform Controlled, Few-Cycle Light Sources SpringerTheses RecognizingOutstandingPh.D.Research
AimsandScope Theseries “SpringerTheses” bringstogetheraselectionoftheverybestPh.D. thesesfromaroundtheworldandacrossthephysicalsciences.Nominatedand endorsedbytworecognizedspecialists,eachpublishedvolumehasbeenselected foritsscienti ficexcellenceandthehighimpactofitscontentsforthepertinent field ofresearch.Forgreateraccessibilitytonon-specialists,thepublishedversions includeanextendedintroduction,aswellasaforewordbythestudent’ssupervisor explainingthespecialrelevanceoftheworkforthe field.Asawhole,theserieswill provideavaluableresourcebothfornewcomerstotheresearch fieldsdescribed, andforotherscientistsseekingdetailedbackgroundinformationonspecial questions.Finally,itprovidesanaccrediteddocumentationofthevaluable contributionsmadebytoday’syoungergenerationofscientists.
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DoctoralThesisacceptedby theMaxPlanckInstituteofQuantumOptics, Garching,Germany
Author Dr.MarcusSeidel InstitutdeScienceetd’
Ingé
nierie
Supramoléculaires
Strasbourg,France
Supervisor
Prof.Dr.FerencKrausz MaxPlanckInstituteofQuantumOptics
Garching,Germany
ISSN2190-5053ISSN2190-5061(electronic)
SpringerTheses
ISBN978-3-030-10790-1ISBN978-3-030-10791-8(eBook) https://doi.org/10.1007/978-3-030-10791-8
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Supervisor ’sForeword Exploitingtheuniquepropertiesofcoherentlaserlighthasbecomeessentialinour modernsociety.High-speedInternetisenabledbyopticallyencodeddatapackages. Lasermaterialprocessingis,forinstance,indispensableintheautomotiveindustry orindisplaymanufacturing.Medicalapplicationssuchaseyesurgeries,cancer diagnosis,andtreatmenthavebeenestablished.Yet,thedevelopmentoflaser technology,promisingnewexcitingapplications,isongoingwithnoendinsight.
Oneoftheuniquepropertiesofcoherentlightisitscompressibilitytoextreme timescalesoffemto-(10 15 s)orevenattoseconds(10 18 s),resultingintwo outstandingproperties:First,ultrashortlaserpulsesarecapableoftracingall motionsofelementarymatteroutsidethenucleusinrealtime.Second,laserpulses canreachpowerlevelsoflarge-scalepowerplants(partiallyevenordersofmagnitudemore)foranultrashorttimeandthusareabletoprovidethehighestdegreeof controlovermolecules,atoms,andelectrons.
Althoughfemtosecondpulsesweredemonstratedinthe1970sforthe firsttime, theirwideapplicabilitywasenabledbytheprogressinthedevelopmentof solid-statelasers.Inparticular,thedemonstrationoftheKerr-lensmode-locked Ti:sapphireoscillatorin1990setamilestoneforultrafastlaserdevelopment. WhereasTi:sapphirelasershaveenabledfullcontrolovertheopticalwaveform, lightpulsegenerationwithdurationsofaboutasinglecarrierwavecycleandoctave spanningbandwidthsaswellasattosecondpulseformationviahighharmonic generation,itremainselusivetotriggerthesespectacularphenomenaatleasta milliontimespersecond.
Here,the “newgenerationofhigh-power,waveformcontrolledfew-cyclelight sources” comesintoplay.Peakandaveragepower-scalablediode-pumpedsolidstatelasers,e.g.thin-disklasers,cannowadaysreadilydeliveruptokilowattsof opticalaveragepower.However,theavailablegainmediaareonlycapableof generatingpulseswithatbestabout100fsofduration,beingfarawayfromthe few-cyclelimitwherefulllight fieldcontrolbecomesimportantandstrong field effectsareattainable.
Thisdissertationdemonstrateswaysofcombiningthebestofbothworlds. MarcusSeidelachievedfew-cyclepulsegenerationandwaveformcontrol,propertiesoftheTi:sapphiretechnology,fromahigh-powerKerr-lensmode-locked thin-disklaseroscillator.
Thework firstbrieflylooksbacktomorethan50yearsofultrashortpulselaser generation.Bydoingso,methodsandgoalsofthetechnicaldevelopmentare highlighted,settingtheroadmapfor5yearsofPh.D.work.Atthesametime,an introductionfornewcomerstothe field,whichpointsoutgeneralideasofthe researchsubject,isprovided.Intheexperimentalchapters,threemajorachievementsaredescribed:First,theimplementationofanamplifi cation-freemulti-watt few-cyclepulsesourcethatcanbecarrier-envelope-phasestabilizedyieldingfull controlovertheopticalwaveformisreported.Second,power-scalablemethodsare demonstratedwhichpresenttheprospectofwaveformcontrolledfew-cyclesources withhundredsofwattsaveragepower.Third,frequencyconversiontothe mid-infraredspectralregionisaccomplishedatunprecedentedhighaveragepowers. Thispresentsanexcellentexampleforapplicationsofthenewpowerfulultrashortlaserpulsesources.Experimentswithlowyield,forinstancefrequency conversiontotheultravioletormid-infrared,willstronglybenefitfromthesignificantlyincreasedpowerlevelsoftheultrafastnear-infrareddrivingsources.This inturnpromisesnovelapplicationsinbothfundamentalandappliedsciences.For instance,myresearchgroupisexploringwaystoemploythisadvancedlaser technologyinearlycancerdiagnosis.
Inthisrespect,MarcusSeidel’sPh.D.thesispresentsaverygoodreferencefor laserscientistsworkingontheforefrontofultrafastopticsaswellasgraduate studentswhoarenewtothe field.Additionally,thereportedresultswillhavean impactonmulti-disciplinaryresearchtopicswhereshortpulselasersserveas essentialinstruments.
Garching,Germany December2018
Abstract Withtheadventofpeakandaveragepower-scalablefemtosecondlasers,inparticularmode-lockedthin-diskoscillators,theneedforequallyscalablepulse compression,carrier-envelope-phasestabilization,andfrequencyconversion schemesarose.Thesetechniqueshavebeenroutinelyappliedtoloweraverage powerultrafastlasers,forinstance,thewidelyusedTi:sapphirebasedones.But theyhadtobereinventedforcutting-edgefemtosecondsourceswith100W-level averageand10MW-levelpeakpowers.
Thisdissertationpresentshowpulsesemittedfroma45-W-averagepower mode-lockedthin-diskoscillatorwerecompressedforthe firsttimetoadurationof onlyafewopticalcycles.Pulsesasshortas7.7fswereattainedfromtwosequential spectralbroadeningandchirpedmirrorpulsecompressionstages.
Thesamelightsourcewasalsothe firstthin-diskoscillator,andsimultaneously the firstoscillatorwithanaveragepowerofmorethan10W,whichwas carrier-envelope-phasestabilized.Twostabilizationmethodsarepresented:The firstoneutilizedintracavitylossmodulationbymeansofanacousto-opticmodulator.Thisresultedin125mradin-loopand270mradout-of-loopresidualphase noise.Thesecondoneemployedpumpdiodecurrentmodulationbymeansofan auxiliarypowersupply.Thisapproachyieldeda390mradresidualin-loopphase noise.
Whereasthepresentedcarrier-envelope-phasestabilizationschemesare power-scalable,thescalabilityoftheinitialfew-cyclepulsegenerationapproachis restrictedbythedamagethresholdofsolid-corephotoniccrystal fiber.Thethesis reportsindetailonthelimitationsofthese fiberswithrespecttomaximally transmittablepeakpowersandattainablespectralbroadeningfactors.Moreover,an alternativeapproachutilizinghollow-coreKagomé-typephotoniccrystal fibersis demonstrated.Adouble-stagebroadeningandcompressionsetupyieldedpulse durationsofonly9.1fs,butalsoshowedasignificantintensitynoiseincreasein comparisonwiththethin-diskoscillatoroutput.
Therefore,spectralbroadeninginbulkcrystalswasstudied.Byexploitingthe opticalKerreffect,spectrawithFouriertransformlimitsof15fswereachieved, openingtheperspectiveforallsolid-statespectralbroadeninginbulkmaterial.
SimulationresultsforasequenceofthinKerrmediapredictagoodpowerefficiencyofthemethod.Furthermore,anexperimentalrealizationofpulsecompressionfrom190fspulseswith90Waveragepowerto30fspulseswith70Waverage powerinself-defocusingBBOcrystalsisreported.Thepresentedcomprehensive studyonspectralbroadeningandpulsecompressiontechniquespavesthewayto few-cyclepulsegenerationathundredsofMWpeakpowerandhundredsofWatts averagepower.
Eventually,thedissertationaddressestheissueoftransferringbroadband, powerfulspectratoawavelengthregionwithahugevarietyofcharacteristic molecularabsorptions themid-infrared.Frequencydown-conversionviaoptical parametricampli ficationresultedinradiationwithupto5Wat4.1 lmand1.3Wat 8.5 lm,correspondingtoanorder-of-magnitudeaveragepowerincreasefor compactfemtosecondlightsourcesoperatingatwavelengthslongerthan5 lm.
Inadditiontothepowermeasurements,bothwavelengthtunabilityandsupercontinuumgenerationbymeansofcascadedquadraticnonlinearitiesarereported, resultinginoverallspectralcoveragefrom1.6to11 lmwithpowerspectral densitiesexceeding1 lW/cm 1 overtheentirerange.
Thepulsecompressionandcarrier-envelope-phasestabilizationschemes demonstratedinthisdissertationwillserveasfundamentaltechniquesforthefurtherdevelopmentofanewgenerationofwaveform-controlledfew-cyclepulse laserswhicharecapableoftriggeringextremenonlineareffectsatunprecedented averagepowersandrepetitionrates.Themulti-octavespanning,mid-infrared femtosecondsourceoffersexcitingopportunitiesformolecular fingerprinting,in particularbymeansoffrequencyup-conversionand field-sensitivetechniquesas wellasfrequencycombspectroscopy.
Preface Theyear2018wasspecialinthehistoryoffemtosecondlasertechnology.On December10,theNobelPrizeinPhysicswasawardedtoGérardMourouandhis formerPh.D.studentDonnaStrickland “fortheirmethodofgenerating high-intensity,ultrashortopticalpulses.” Intheirjointpaperfrom1985,thescientistsproposedanddemonstratedtheconceptofchirpedpulseampli fication.By meansofthisstillverycommonopticalampli ficationmethod,Stricklandand Mourousuccessfullytackledanever-presentdilemmainfemtosecondlasertechnology:Owingtothecompressionoflightenergytoultrashorttimescales, extremelyhigh instantaneous opticalpowerscanreadilybeachievedandnonlinear effectsinanymaterialaretriggered.Ontheonehand,thesenonlinearitiesare essentialforfemtosecondpulse applications.Ontheotherhand,theyaredetrimentalforfemtosecondpulse generation astheyinducedistortionsofthelaser beamorevenmaterialdamage.Today,thelasersemployingStrickland ’sand Mourou’sinventionhavefoundalargevarietyofapplicationsinfundamental scienceandindustry.Thisdemonstratesthesignificanceoffemtosecondlaser developmentwhichisstillasdemandedasinthe1980s,andtheolddilemmaisstill challenged yetonasigni ficantlyhigherlevel.
Thetitleofthisdissertation: “Anewgenerationofhigh-power,waveform controlled,few-cyclelightsources”,alreadyindicatesthatscalingupopticalpower hasnotlostitstopicality.WhereasStricklandandMourouachievedabreakthrough inincreasinginstantaneouspower,thecurrentlydevelopedfemtosecondlaser technologyaimsforcombining bothhighinstantaneousandhighaveragepower. Thisposesnewchallengesonretaininguniqueultrashortlightpulsepropertiesat anyopticalpowerlevel.Suchpropertiesarethecompressionoflightenergydown toafewelectric fieldcyclesandtheprecisecontrolofthelight fieldwaveform.Ina nutshell,thiswasthesubjectoftheresearchIconductedbetweenAugust2012and June2017attheLaboratoryforAttosecondPhysicsinthegroupofProf.Ferenc Krausz.
The firstversionofthisthesiswassubmittedinFebruary2018.Onlyminor changesofthetexthavebeenmadesincethen.Consequently,somenoveldevelopmentsmightbemissinginSect. 1.2 (History)orChap. 5 (Outlook).Thelarge
majorityofthepreludingandconcludingchaptersare,however,asrelevantasthey wereatthebeginningof2018.Chapters 2–5 mainlydescribetheconducted experimentsandsimulations.Ashortoutlineofthechapterisgivenbelow:
Chapter 1 firstlyspecifi estheresearchareaanddefinestheresearchobjectivesof thisthesis.Secondly,itintroducesthemainmethodsfromahistoricalpointofview, outlinestheshortcomingsofcurrentlyprevailinglasertechnologies,andexplains possibleapplicationsforanewgenerationofmode-lockedfemtosecondoscillators. Thirdly,someimportantpropertiesofthethin-disktechnologyareintroduced.In Sect.1.4ofChap. 1,fundamentalphysicalconceptsareexplainedinanillustrative manner.
Chapter 2 demonstratesaproof-of-principleexperimenttowardthin-disk-based high-powerwaveform-controlledfew-cyclepulselightsources.Firstly,themainly usedKerr-lensmode-lockedthin-diskoscillatorischaracterized.Secondly,pulse compressionwithasolid-core fiberandabulkmaterialbroadeningstageis reported.Thirdly,carrier-envelope-phasestabilizationbymeansofanintracavity acousto-opticmodulatorisdemonstrated.Allresultsaresummarizedattheend. ThesubjectofChap. 3 isthepowerscalabilityoftheresultspresentedinChap. 2 InSect. 3.1 solid-coreandhollow-corephotoniccrystal fi berspectralbroadeningis investigated.Section 3.2 focusesonpulsecompressiontechniquesthatrelyonbulk crystals,eitherexhibitingonlytheopticalKerreffectoradditionallyquadratic opticalnonlinearities.Section 3.3 presentsanalternative,power-scalableapproach oncarrier-envelope-phasestabilization.Eventually,Sect. 3.4 givesacomprehensive overviewaboutallstudiedtechniques.
Chapter 4 focusesonfrequencydown-conversionofthenear-infraredradiation emittedbytheultrafastoscillatortothemid-infrared.InSect. 4.1,twopowerful opticalparametricampli fiersaredemonstratedandacomparisontoanalternative down-conversionapproachviaintrapulsedifferencefrequencygenerationispresented.ThesubjectofSect. 4.2 issupercontinuumgenerationinthemid-infraredby meansoflargecascadedquadraticnonlinearities.Inthe fi nalsection,thechapteris summarized.
Chapter 5 firstlyproposeswhatcanbedonenextinordertoapplythe achievementsofthisthesis.Threeapplicationsarediscussed.Secondly,themain resultsofthedissertationarehighlightedoncemoreinconcludingstatements.
Chapter 1 isparticularlyusefulfornewcomersto fieldofultrashortpulselaser development.Theintroductioniseasytoreadasthefundamentalsarepresentedin apictorialwayandwithonlyalimitednumberofequations.However,almost200 referencestotextbooksandjournalpublicationsoffertodigmuchdeeperintothe field.Theextended,alsosomewhatincomplete,historicalintroductionmaybealso interestingformasterorPh.D.studentswhohavebeenworkinginthe fieldfora shorterperiodoftime.Theyaswellasresearcherswhohavebeenworkingwith ultrafastlasersformanyyearswillbenefitmoststronglyfromtheresearchchapters owingtomanytechnicaldetailswhicharepresentedandduetoextensive
Preface
discussionsandcomparisonsofallresearchresults.Thethesisisparticularlyuseful forreaderswhoareinterestedinthemethodsofspectralbroadeningandpulse compression,carrier-envelope-phasestabilizationaswellasdifferencefrequency generationandopticalparametricampli fication.
Strasbourg,FranceDr.MarcusSeidel
Acknowledgements Firstly,IwouldliketothanktheLaboratoryofAttosecondPhysicsteamatMPQ fortheirsupportduringthe fiveyearsofPh.D.work.Iespeciallywanttothankmy advisorFerencKrausz,theheadoftheteam,whohascreatedauniqueresearch environmentIwasstronglybenefittingfrom.
Secondly,IwouldliketothankOlegPronin, “thefatherofKerr-lens mode-lockedthin-diskoscillators”,whohasnotonlydevelopedabeautifultool thatIcouldworkwith,buthasalsoadvisedmeindailydiscussionsandgavemethe freedomtotrymyownideas.IwouldalsoliketothankhimandFerencKrauszfor recommendingmefortheStudienstiftungdesDeutschenVolkes,theTingyeLi InnovationPrizeandmycurrentpositioninStrasbourg.
Moreover,Iwanttothanktheother(former)membersofthe “AGPronin” , namelyJinweiZhang,KaFaiMak,KilianFritsch,MarkusPötzelberger,Sebastian Gröbmeyer,NathalieNagl,FelixSchulze,JohannesStierle,TobiasRumpf,and XiaoXiao.AspecialthankstoJonathanBronswhohasbeenaroundsincethestart ofmyPh.D.work.Iappreciatethediscussionswithhimandthesolidengineering hedid(itwasalmostimpossibletoloosenascrewthathetightenedwithoutspecial tools).
ManythanksalsototheIoachimPupeza’sgroupmembers,forregularexchange ofknowledgeandequipment,andVladimirPervak’steam,forprovidingourbread andbutter,themulti-layermirrors.Forveryhelpfuldiscussions,IwouldinparticularliketothankNikolaiLilienfein,SimonHolzberger,FlorianHabel,and MichaelTrubetskov.
Ialsohadthepleasuretoworkwithmanyscienti ficcollaborators.First,Iwould liketothankThomasUdem,foradvisingonourCEPstabilizationexperimentsand forservingasasecondreviewerofthisdissertation.Aspecialthanksgoesto GunnarArisholmfordevelopingandprovidingthebeautifulSISYFOStoolaswell asforbeingextremelyresponsiveandhelpfulwithrespecttoextendinghiscode andinterpretingourresults.ManythanksalsotoAlexanderHartungforhiscontributiontomaxoutthecapabilitiesofsolid-corePCF.Iwouldalsoliketothank PeterSchunemannforprovidingtheZGPcrystals,DanielSánchezofJensBiegert’s
groupforhisintroductiontoSISYFOS,JohnTraversforsupervisingtheKagomé fiberprojectandJensBethgefromAPEforthepulsemeasurementdiscussionsand theSPIDERtrial.
Iwouldliketoacknowledgethe financialandimmaterialsupportoftheStudienstiftungdesdeutschenVolkes.Iespeciallywanttothankmy “Vertrauensdozent” HannoSchäferformentoring.Iwouldalsoliketothankmy “personalmentor” ChristianSeidelwhosuggestedtoapplyattheIMPRSforadvancedphotonscience. AlltheorganizeroftheMax-Planckresearchschool,Iwouldliketothankaswell. Ireallyenjoyedbeingpartofthegraduateschoolandstillenjoymeetingmany amiablepeopleofthe2012selectionround.
MyPh.D.workwouldnothavebeensuccessfulwithoutmyformeradvisorsand teachersattheUniversityofRostockandatCREOL.Therefore,Iwanttothank StefanLochbrunnerforbeingextremelysupportiveofpavingmyacademicpath firsttotheUSAandlatertoMPQ.Furthermore,IthankEricVanStrylandand DavidHaganforteachingmenonlinearopticswhichhasbecomemostessentialfor mythesis.
Schließlichmöchteichnocheinigenicht-wissenschaftlicheDankworteinmeiner Mutterspracheabfassen:EinganzherzlicherDankfürdiepermanenteUnterstützung aufmeinerLaufbahn(verbundenmitallenUmzügenunddenweitenEntfernungen) gebührtmeinerFamilie,insbesonderemeinenElternBirgitundReinhardsowie meinemBruderDaniel.AuchDoreenmöchteichdanken,diebesondersinder SchlussphasemeinerPromotionoftaufmeineGesellschaftgeduldigverzichtethat undimletztenSommertrotzeinessehrlangenArbeitswegsmitnachStraßburg gezogenist.VielenDankauchdenMünchnerFreundenrundumIMPRSsowie Feierabend/Hauskreis.ZumSchlussbleibtmirnochzuschreiben:GottseiDankist’s gutgelaufenundGottseiDankist’s(endlich)geschafft.
2ProofofConcept:Few-CyclePulseGeneration andCarrier-Envelope-PhaseStabilization
2.1AnUltrafastWorkhorse:TheKerr-LensMode-Locked
2.2EnteringtheFew-CyclePulseRegimewithMode-Locked
2.2.1Solid-CoreFiber-BasedPulseCompression
3PowerScalableConcepts
3.1Fiber-BasedPulseCompression
3.1.1LimitationsofSolid-CoreFiber
3.1.2Kagomé-TypeHollow-CorePhotonicCrystalFibers
3.2AllSolid-StateSpectralBroadeninginBulkMaterial
3.2.1CompressionbyMeansoftheOpticalKerrEffect ofDielectrics ..................................
3.2.2EfficientPulseCompressioninSelf-defocusing BulkMedia
3.3Power-ScalingCarrier-Envelope-PhaseStabilization
4FromtheNear-totheMid-Infrared
4.1OpticalParametricAmpli fiersforFrequency Down-Conversion
4.1.1FrequencyDown-ConversionwithPeriodically PoledLithiumNiobate
4.1.2FrequencyDown-ConversionwithLGS
4.1.3ComparisontoDown-ConversionviaDifference
4.2SupercontinuumGenerationintheMid-Infrared
5.1Near-FutureApplications
5.1.1TheAttosecondOscillator
5.1.2High-Speed,High-RateOpticalSwitching
5.1.3Field-ResolvedOpticalSpectroscopyinthe Mid-infrared ..................................
5.2HaveUltrafastThin-DiskOscillatorsMatured?
Acronyms ACAlternatingcurrent
ANDiAll-normaldispersion
AOIAngleofincidence
AOMAcousto-optic-modulator
APDAvalanchephotodiode
ARAnti-reflection
BBOBetabariumborate, b-BaB2O4
CCDChargecoupleddevice
CEPCarrier-envelopephase
COLTRIMSColdtargetrecoilionmomentumspectroscopy
CPAChirpedpulseamplification
CPSBChirpedpulsespectralbroadening
CSPCadmiumsiliconphosphide,CdSiP2
CWContinuouswave
DCDirectcurrent
DFGDifferencefrequencygeneration
DPDDigitalphasedetector
EOMElectro-opticmodulator
EOSElectro-opticsampling
FOMFigureofmerit
FROGFrequency-resolvedopticalgating
FSFusedsilica
FTIRFouriertransforminfraredspectrometer
FTLFouriertransformlimit
FWHMFullwidthathalfmaximum
GDDGroupdelaydispersion
GVDGroupvelocitydispersion
HC-PCFHollow-corephotoniccrystal fi ber
HHGHighharmonicgeneration
HRHighreflection(dielectriccoating)
INIntensitynoise
IPNIntegratedphasenoise
KLMKerr-lensmode-locked
LGSLithiumgalliumsulfi de,LiGaS2
LMALargemodearea
MFDMode-fi elddiameter
mid-IRMid-infrared
near-IRNearinfrared
OCOutputcoupler
OPAOpticalparametricamplifier
OPCPAOpticalparametricchirpedpulseampli fication
OPOOpticalparametricoscillator
OSAOpticalspectrumanalyzer
OWBOpticalwave-breaking
PBGPhotonicbandgap
PCFPhotoniccrystal fiber
PEEMPhotoemissionelectronmicroscopy
PLFPhaselead filter
PLLPhase-lockedloop
PMPolarizationmaintaining
PPLNPeriodicallypoledMgO-dopedlithiumniobate
PSDPowerspectraldensity
RFRadio-frequency
RINRelativeintensitynoise
RMSRootmeansquare
SCGSupercontinuumgeneration
SESAMSemiconductorsaturableabsorbermirror
SHSecondharmonic
SHGSecondharmonicgeneration
SNRSignal-to-noiseratio
SPMSelf-phasemodulation
TDThin-disk
Ti:sapphTitaniumdopedsapphire
TODThird-orderdispersion
UVUltraviolet
VUVVacuumultraviolet
WLCWhite-lightcontinuum
X-FROGCross-correlationfrequency-resolvedopticalgating
XUVExtremeultraviolet
YAGYttrium-aluminumgranate,Y3Al5O12
Yb:YAGYtterbium-dopedyttrium-aluminumgranate
ZGPZincgermaniumphosphide,ZnGeP 2 xviii
Chapter1 Introduction 1.1WhatDoes“Generation”Referto? Therelativelyyounghistoryofthelaserbeganin1960,whenT.H.Maimandemonstrated Light Amplificationby Stimulated Emissionof R adiationforthefirsttime [1].HisachievementwasbasedonthetheoreticalworkofV.A.Fabrikantandthe NobelprizelaureatesA.Einstein,A.M.Prohorov,N.G.Basov,A.L.Schawlowand Ch.H.Townes[2–5].Duringthepast58years,theresearchanddevelopmentfieldhas proliferatedinmanyrespects.Today,lasertechnologyhasbecomeamulti-billion dollarbusinesswithacontinuousgrowthrateontheorderof5%duringthepast years[6].Moreover,theinventionledtonumerousNobelprizeawardedinventions inphysicsaswellasinchemistry[7].
Theworkpresentedinthisdissertationisfocusedonthedevelopmentofultrashort pulsesolid-statethin-disk(TD)laseroscillators.Althoughsolid-statelasershave onlyasmallmarketshareincomparisontodiodelasers,thetechnologyisfairly interestingforthebiggestlasermarketsegment,materialprocessing[6].Itskey features,whichareaveragepower,intensityandlaserwavelength[8],arein-fact addressedinthiswork.However,themaingoalofthethesisistopresentanovellight sourcewhichenablescutting-edgeapplicationsinfield-sensitivenonlinearopticsas wellasfrequencycombspectroscopy,researchfieldswithtremendouspotentialto beemployedinlifesciencesorsignalprocessing,forinstance.Thistargetrequires theconsiderationofadditionalkeyelements,whicharethegenerationofpulseswith adurationontheorderofanopticalcycleandthefullphase-controlofthelightfield, i.e.waveform-controlenabledbycarrier-envelopephase(CEP)stabilization.These featuresarecloselyrelatedtotheemergenceoffemtosecondtitaniumdopedsapphire (Ti:sapph)oscillatorswhichhavebeenprevailinginultrafastopticslaboratoriessince theiradventin1990/91[9, 10],i.e.formorethan25years.Thisisahugeerainthe younghistoryofthelaser.Nevertheless,theunprecedentedpower-scalabilityofTD oscillatorsanditsmostrecentdevelopments,whichareatleastpartlypresentedinthis dissertation,promisetograduallyreplacetheestablishedbulksolid-statetechnology withinthenextdecades.
©SpringerNatureSwitzerlandAG2019
M.Seidel, ANewGenerationofHigh-Power,WaveformControlled, Few-CycleLightSources,SpringerTheses, https://doi.org/10.1007/978-3-030-10791-8_1
The“newgenerationofhigh-power,waveformcontrolled,few-cyclelightsources” whichisdiscussedinthisthesismustbedistinguishedfromtherecentlyproclaimed “third-generationfemtosecondtechnology”[11, 12].Althoughbothdevelopments aimforenormouslyincreasingbothpeakandaveragepower,thelightsourcespresentedhereareratherconsideredasthefrontendoftheamplifiersystemsdescribed inRefs.[11, 12].Thedistinctadvantagesofusingtheoscillatorsasstandalonesystemsarehigher(MHz)pulserepetitionratesandhencedataacquisitionratesaswell asthevastlyreducedcomplexityofthesystems[13].
Remark:Opticalcycle Thedurationofanopticalcycleiscalculatedbythecarrier(orcentral)wavelengthoftheopticalpulse(λc )dividedbythespeedoflight(c0 ≈ 299.8nm/fs). Thecentralwavelengthoftheutilizedlaseroscillatorisabout1030nmwhich correspondstoa3.4fs=0.0000000000000034scycleduration.
Remark:Mode-locked fiber lasers
Intheearly1990satremendousdevelopmentofultrafastfiberlasersstarted inparalleltotheproliferationofsolid-statebulkoscillatorsas,forinstance, reviewedinRef.[25].Thisdevelopmentisstillrapidlyprogressing,andthus shouldbementionedwithinthecontextofthissection.Bymeansofincreasing mode-fieldareas,chirpedpulseamplification(CPA)andeventuallycoherent combinationofmultiplebeams,fslaserswithkWofaveragepowerandGWof peakpowerhavebeendemonstrated[26–28].Moreover,single-cyclepulses havebeensynthesized[29]andtightlockingofcarrier-envelope-offsetfrequencywasachieved[30].AlthoughthousandsofkWaverageandTWpeak powerhavebeenenvisioned[31],thescalabilityofthetechnologyreliesso-far oncoherentcombiningwherefirstly,powerscalesonlylinearwiththenumberofbeams,andsecondly,thecombinationofthousandsoflaserswould berequiredwhileeightchannelshavebeendemonstrated[28].Theapproach couldbealsotransferredtoanyotherlaserarchitecture,andhence,thescalabilityisinthatsenseinferiortodiskorinnoslabtechnologywherekWultrafast sourcesweredemonstratedwithoutcombininglaserbeams[32–34].Ultrafast fiberoscillators havereachedaveragepowersof66W[35]whichisakinto theTDoscillatorthatwasmostlyusedintheexperimentspresentedhere. However,resortingtothepositivedispersionregimewasnecessarytonot sustainpeakpowerinduceddamageswhichindicatesthatrelyingontheTD conceptisbeneficialforpowerscaling.Nevertheless,all-fiberlasersexhibit certainlymanyadvantagesintermsofapplicabilitywhicharehighwall-plug efficiency,robustnessduetotheabsenceoffree-standingoptics,compactness,
1.1WhatDoes“Generation”Referto?3
lowvulnerabilitytothermalload(uptomoderatefiberdiameters)andcostefficiency[31].Consequently,theirdevelopmenthasbeenalsodrivenbya varietyofcompaniesandnotonlybyresearchinstitutions[26].
Theterm“generation”hasbeenadaptedfromthehistoricalreviewsprovided inRefs.[36, 37].Theyrefertogenerationsof mode-lockedlasers.Mode-locking isthefundamentalprincipleofeverytable-topultrafastlaser.Itwillbebriefly explainedinSect. 1.4.Theclassificationofgenerations,whichbothpapersuse,is showninTable 1.1.SomeimportantmilestonesoftheevolvingTDtechnology havebeenadded.The firstgeneration ofmode-lockedlaserswascalledthegenerationofpicosecond(ps)lasersandmaybealsoreferredtoasthepioneeringeraof
Table1.1 Generationsofmode-locked oscillators -anoverview
Time Architecture
1964–1970 Multiple
1970–1990 Organicdyes
1990-?a
Bulksolid-state
?a Thin-disk
Selectedmilestones References
Firstactive mode-locking [14]
Firstpassive mode-locking [15, 16]
FirstCW mode-locking [17]
Sub-10fspulseswith externalcompression [18]
Demonstrationof KLMTi:sapph oscillator [9, 10]
DemonstrationofCEP stabilization [19]
Firstfew-cyclepulse oscillatorwith octave-spanning spectrum [20]
Firstmode-lockedTD oscillator [21]
First >100Wfs oscillator [22]
Demonstrationof KLMTDoscillator [23]
CEPstabilizationand few-cyclepulse generationbyexternal compression [24]
CW-continuouswave,KLM-Kerrlensmode-locked,CEP-carrier-envelopephase,TD-thin-disk a AquestionmarkissetherebecausethebreakthroughofTDoscillatorsinascaleofprevious generationshasnotyetcome.Inthefinalchapterofthisdissertationtherecentprogresswillbe reviewedinordertoconcludeonhowthetechnologywilladvance.
ultrashortpulsegenerationsincepslasersarestillquitecommon.Manygainmaterialswereexplored.Fundamentalresearchonmode-lockingwasconductedandits principlesweredescribedforthefirsttime.The secondgeneration wascharacterized byfsdyelasers.Thisperiodledtothefirstdemonstrationoffew-cyclepulses,i.e.the ultimatedurationlimitsofvisiblepulseswereapproached.AhmedZewail’s“studies ofthetransitionstatesofchemicalreactionsusingfemtosecondspectroscopy”,which wererewardedwiththe1999Nobelprizeinchemistry[38, 39],wereinitiallybased ondyelasers,butalsostronglybenefitedfromthefollowinggenerationof solid-state femtosecondlasers whichenabledfurtherbreakthroughsincreatingnature’sshortesteventsoutsidetheatomicnucleus.Attosecond(as)pulses[40–42]andlightfield transients[43, 44]weredemonstratedforthefirsttime.However,theseconcepts donotnecessarilyrequiresolidlasergainmediasincetheyoriginatefromoptical nonlinearities.Theyratherrequirelightsourceswhicharehighlyreliableandfairly easytohandle.Thatiswhatreallymadethedifferencebetweendyeandsolid-state femtosecondlasers.NotbycoincidencetheKerr-lensmode-locked(KLM)Ti:sapph lasershavebeencommercializedonlyoneyearaftertheywerereportedforthefirst timein1990[9, 45].ContrarytoKrauszetal.[36],French[37]evenintroducesa fourthgenerationwhichhecalls“useful ultrafastlasers”.Hederivesthisnamefrom thefollowingconsideration:
“Theextremelyrapiddevelopmentoftunablefemtosecondsolid-statelasershasbrought thefieldtothepointwhereitisnotalwaysnecessarytodesignanexperimentaroundthe availablelasersource:rather,itisnowreasonabletoexpectthatasuitableultrafastlaserwill existforaparticularapplication.”
ThecategorizationthatKrauszetal.proposedshallbeutilizedheresincethefourth generationofFrenchdoesnotintroduceanewlaserarchitecture.Nevertheless,the quotationpointsoutanadditionalimportantpropertyofthethirdmode-lockedlaser generation:Thisistunabilityor,inmoregeneralwording,thefreedomtoadaptthe centralwavelengthandthebandwidthofthelightpulsestothetargetedapplication. Althoughdyelasersinprinciplealreadycoveredthespectralrangefrom320nmto 1800nm,theywerelackingstabilityinparticularatnearinfrared(near-IR)wavelengths[37].Solid-stategainmaterialsallowlasingfrom700nmtoabout3 µm[36]. Furthermore,nonlinearfrequencyconversionandsupercontinuumgenerationgive accesstofrequenciesfromseveralTHzuptothedeepultraviolet(UV)regionand grantaccesstopartiallyevenoctave-spanningbandwidths[43, 46–49].
Anotherkeyfeaturehasmassivelyincreasedtheimportanceoffslasers.Itemerged shortlyaftertheturnofthelastmillennium.Thephase,whichdescribestheshift betweenthepeaksofthecarrierwaveandpulseenvelopewascontrolledforthefirst time[19, 50].Thishaslinkedthedomainofopticalfrequencies(THz-PHz)tothe domainofradio-frequencies(MHz-GHz)andrevolutionizedmetrology.Therefore, thethirdgenerationofmode-lockedlasersisalsocloselylinkedtothe2005Nobel prizesinphysicsforJohnL.Hall’sandTheodorW.Hänsch’s“contributionstothe developmentoflaser-basedprecisionspectroscopy,includingtheopticalfrequency combtechnique”[51].Moreover,theabilitytopreciselycontrolthecarrier-envelope phase(CEP)openedthedoortoexplorenonlinearopticaleffectswhichdonotonly
1.1WhatDoes“Generation”Referto?5
dependonthecycle-averagedintensityofthelight-fieldbutalsoonitstemporal phase[52].Theseareso-calledextremeorfield-sensitiveopticalnonlinearities.Most prominentamongthemisprobablythegenerationofphase-coherenthighharmonics [53, 54].
Sincethesenewavenuesofexploitingextremenonlinearopticssetahugedemand onbothpeakpowertodrivetheeffectsandaveragepowertomaintainhighdatarates despitelowefficienciesinthenonlinearconversionprocesses,thebulksolid-state laserarchitecturecameupagainstitslimitations[11].Despitetheeffortstoextend thelasercavitylengthsinordertoincreasethepulseenergyandthepeakpower, resp.whilekeepingtheaveragepowerataWatt-level[55]andtooperatethelaser inthepositivedispersionregime,i.e.withtemporallystretchedpulses,toavoid detrimentalnonlinearities,Ti:sapphoscillatorsremainedatsub-μJpulseenergyand sub-10Waverageoutputpowerlevels[56, 57].Bycontrast,thefirstmode-lockedTD oscillator,demonstratedin2000[21],alreadydeliveredmorethan16Wofaverage poweratabout0.5 µJpulseenergy.Today,roughly18yearslater,averagepowers ofmorethan250W[58, 59],multipletensof μJ[59–61]andpeakpowersofmore than60MW[59, 61]aredirectlyextractedfromtheoscillators.Moreover,upto 230MWafterexternalpulsecompression[62]havebeenobtained.Inspiteofthese highlyimpressivenumbers,mode-lockedTDlasershaveonlybeenemployedina fewspectroscopicapplications[13, 63, 64]whichdidnotevenclearlyrevealthe advantagesoftheoscillators.Areasonisthatuntilrecentlythekeyfeaturesofthe secondandthirdgenerationsofmode-lockedlasershavenotbeenrealizedwiththe new,power-scalabletechnology. However,toconstituteanovelgenerationoffemtosecondoscillators,itisessentialthatallmainadvantagesoftheoldertechnologies aretransferredtothenewone. Otherwise,thedevelopmentmaybeinterestingfor specificapplicationsbutwillnotbecapableofreplacingatechnologywhichhasbeen establishedfordecades.Thekeyelementsofthefourmode-lockedlasergenerations aredisplayedinFig. 1.1
Fig.1.1 Keyelementsofthefourgenerationsofmode-lockedlasersdescribedinthemaintext. Thegraphicindicatesthatallfeaturesoftheprecedinggenerationsmustbeembeddedintothenew ones.Thethesiscoversthekeyelementsprintedinboldtype
Averyimportantsteptowardsapplicabilityofmode-lockedTDoscillatorswas madethroughthefirstdemonstrationofKLMin2011[23].Maybesimilartothe replacementofslowsaturableabsorbersindyelaserswithfastsaturableabsorbersin bulklasers,theadvancefromsemiconductorsaturableabsorbermirror(SESAM)to Kerrmediaasprimarymode-lockeryieldedahugegaininreliability[65].In-fact,the firstKLMoscillatorhasbeenutilizedfortheresearchpresentedinthisdissertation. Ithasbeenrunningforaboutfiveyearsonadailybasiswithouttheneedforany majorreplacements.
ThreekeyelementsofFig. 1.1 remainedtoberealizedinordertoproclaima nextgenerationofmode-lockedoscillators.Theyarehighlightedinboldtype,being firstly,thedemonstrationoffew-cyclepulseoperation,secondlywaveformstabilization,andthirdly,theabilitytotransferalltheadvantageouspropertiestoother wavelengthregions.Thisthesispresentstherealizationofthefirsttwofeaturesand showsexamplesoftransferringallpropertiesfromthenear-IRtothe(mid-IR)which isextremelyattractiveforfrequencyandtime-domainapplications[41, 66, 67].
Thenextsubsectionwilltakealookattheaddressedkeyelementsfromahistoricalperspective.Thiswillmotivatetheexperimentalmethodspresentedinthis dissertation.
1.2AShortHistoryofShortPulses In1961,alreadyoneyearafterthefirstdemonstrationofthelaser,thecontrolled pulsedoperationofalaserwasusedtogeneratelightofunprecedentedintensity[68]. Thisso-called“GiantOpticalPulsation”wasestimatedtobe0.12 µslong,coming witha“totalpeakoutputintensity”ofabout600kW.Thestillverycommontechnique ofquality-,orbriefly,Q-switchingbymeansofaPockelscellwasemployedto achievethepulsedoperation.Temporallyincreasingthelosses(loweringthequality) ofthelaserresonatorallowsanenhancedinversionbuild-upintheactivemedium whichisdepletedquicklyiftheresonatorqualityishigh.ThePockelseffectdescribes adirectcurrent(DC)-fielddependentpolarizationrotationofthelaserfield[69],i.e. activelyswitchingahighstaticvoltageledtothepulseformation.Furtherinformation aboutQ-switchingcanbefoundinlasertextbooks,forinstanceinRefs.[70, 71].
Bytailoringthelossmodulationfrequencytothelongitudinalmodespacingofthe resonator,theformationofmuchshorterpulsesisachieved.Theperiodicmodulation leadstoside-bandformationofthelasinglongitudinalmodesandthustoabroad emissionspectrum.Moreover,allmodesoscillateinphase,theyarelocked,and henceinterfereconstructivelyatacertaintime t0 whichconsequentlyleadstothe formationofultrashortpulses(cf.Sect. 1.4).Themoremodesoscillateandthebetter theirrelativetiming,theshorterthegeneratedpulses.In1964,thisprincipleof mode-lockingwasdescribedandrealized,resp.forthefirsttimebyW.E.Lamb [72]andL.E.Hargroveetal.[14].ThelatterpaperdescribesaHe-Ne-laserwithan acousto-optic-modulator(AOM)insidetheresonator.Theopticalwaveisdiffracted fromtheacousticwavewhichisdrivenatthedoubledlaserrepetitionrate.Atthe
1.2AShortHistoryofShortPulses7
zero-crossingtimeoftheacousticwave,theresonatorlossesareminimal,andhence photonspreferablypassthemodulatoratthisinstantoftime.Consequently,the modesareforcedtooscillateinphasewhichenabledthegenerationof5nspulsesin thefirstdemonstrationofthemethod[14].
Remark:Pulseduration Thedefinitionof“pulseduration”isnotunique.Unlessexplicitlyspecified differently,itreferstothefullwidthathalfmaximum(FWHM)ofthepulse intensityenvelope.
Uptosixordersofmagnitudeshorterpulsescanbegeneratedifthemode-locking isnotachievedbyanactivedevicesuchasanAOMbutthroughapassivemechanismwhichisingeneraltriggeredbysome(sloworfast)saturableabsorber[73]. Thesaturationbehaviorresultsinlowerresonatorlossesforintensepulsesthan forwavescontinuouslyspreadintime.Sincethemostintensepulsesaregeneratedbyaconstructiveinterferenceofabroadbandofresonantcavitymodes,the laserseeksthemode-lockingregimeifsomechaoticlightislaunchedinthecavity (cf.Sect. 1.4).Whiletheactiveapproachrequiresthesynchronizationoftherepetitionrateandthedrivingfrequencyofthemode-locker,thepassiveapproachis intrinsicallysynchronized.Mode-lockingis,forinstance,explicitlydescribedinthe textbooks[74, 75].Firstpassivelymode-lockedlasersweredemonstratedin1965. AQ-switchedrubylaserwasusedandanadditionaldyewasinsertedwhichacted asthesaturableabsorberandinitiatedthemode-locking[15].Adyealsoserved aspassivemode-lockerinthefirstfemtosecondlaserwhichutilizedflowingRhodamine6Gasactivemedium.Itwasreportedin1974[76].Thesamegainmedium wasemployedinthefirstsub-100fslaser[77].Contrarytotheirpreviouswork, thegrouparoundCharlesV.Shankseparatedthegainmediumfromthesaturable absorberandarrangedtheminthecollidingpulsegeometry.Thetechniqueis,for instance,explainedinRef.[78].Moreover,allbandwidthlimitingelementsinside thecavitywereremoved.Additionalbalancingofintracavitygroupdelaydispersion (GDD)andself-phasemodulation(SPM)aswellassaturableabsorptionandgainled totheshortestpulseswhichhavebeendirectlygeneratedfromadyelaser.Apulse intensityautocorrelationmeasurement[79, 80]revealedadurationof27fs[81].
Ingeneral,theminimalpulsedurationachievablefromanoscillatorislimited bythegainbandwidthofitsactivemedium(althoughafewexceptionshavebeen demonstrated,e.g.Refs.[82, 83]).Therefore,externalspectralbroadeningisutilized toextendthespectralwidthofthepulses,andconsequentlytocompressthemto shorterdurations.Inthemajorityofexperiments,thisisachievedbyapplyingSPM incombinationwithnegativeGDDtothepulses.Firstbroadeningexperimentswere conductedinfusedsilicafiber[84, 85]andnegativedispersionwasappliedthrough gratingorfiberpairs[86, 87].Theprinciplewasalsousedtogeneratethefirstsub10fspulses[18].In1987,theshortestpulsesofthesecondgenerationofmode-locked
oscillatorswereachieved.Theyexhibitedadurationofonly6fs,correspondingto threeopticalcyclesatthecentralwavelengthof620nm[88].
Remark:Dispersion Theterm“negativedispersion”isambiguoussincegroupvelocitydispersion (GVD)andthe“dispersionparameter”,whichismorecommoninfiberoptics, haveoppositesigns.Inthisdissertation,positivemeansnormaldispersion, whilenegativemeansanomalousdispersion.
Bythistime,theuniquepropertiesforultrashortpulsegenerationofTi:sapphcrystalshavealreadybeendiscovered[89].Ofcourse,manyoftheopticaltechniques andmodelswhichhavebeenstudiedwithdyelasers,suchasbalancingofGDD andSPMinthelasercavity(e.g.describedbytheso-calledmasterequationforfast saturableabsorbers[90])orexternalpulsecompression,couldbetransferredtothe solid-statearchitecture.Forexample,theshortestpulserecordofthesecondmodelockedlasergenerationwasbeaten10yearslaterforthefirsttimewithasimilar fiber-prism-gratingpulsecompressionscheme[91].Therefore,itisrathersurprising thatprobablythemostcommonsolid-statelasermode-lockingtechnique,KLM,was notdiscoveredbeforetheadventofTi:sapphoscillators.Ratherbycoincidence,itwas firstlyexperimentallyobserved[9, 10]andbasedonearlierproposals[92]explained shortlyafteritsrealization[93–95].Nowadays,theextremelybroadgainbandwidth ofTi:sapphhassupportedoctave-spanningspectragenerateddirectlyfromtheoscillator[20].Broadbandpulses,exhibitingaFouriertransform-limit(FTL)ofonly 3.7fs,couldbecompressedto4.4fs,lessthantwoopticalcycles[96].Togenerate suchultrashortpulses,thecontrolofhigher-orderphasetermsbecomesinevitable [54],andthustheachievementisinherentlylinkedtotheadvancesofdielectric multi-layercoatings[97].Theinventionofchirpedmirrors[54, 98]hasprovideda hugedegreeoffreedomintailoringthephaseofthelightfields,beingnotrestricted anymoretothecontrolofGDDandthirdorderdispersion(TOD)whichwaspossible withgrating-prism-typecompressors.Furtherreductionsofpulsedurationsevento sub-cycleregime[43, 44]havebeenachievedthroughthesynthesizeofmultiple ultrashortpulsesbyspectralbroadeningofnearlymJpulseswithkHzrepetitionrate inahollow-corecapillaryfollowedbythreeorfourparallelchirpedmirrorcompressionstages.
Theshortestisolatedlightpulseswhichhavebeengenerateduptonow,relyon thetechniquecalledhighharmonicgeneration(HHG)[53, 99–102].In2001,the methodopenedthegatetothesub-fsregime[40–42].Nowadays,evenpulsesof lessthan100ashavebeendemonstrated[103–105].Attosecondpulsegeneration originallyrequiredthelightfieldcausingHHGtofulfillthreeconditions[42]:
(i)Itspeakintensitiesmustbeontheorderof1013 W/cm2 -1015 W/cm2 toinitiate theextremenonlineareffect.
(ii)Itsdurationmustbeinthefew-cycleregimetogenerateonlyasingleaspulse pershot.
(iii)Itswaveformmustbecontrolledtocontroltemporalandspectralshapeofthe highharmonics.
Itistonotethatmeanwhilemodifiedattosecondgenerationtechniqueshavebeen developedthatsomewhatsoftenthesecondcondition,butthosearelessefficientthan theoriginaltechnique[106].
Condition(iii)wasrecognizedearly,[52]butthemeanstoachieveprecisecontrol camewiththerevolutionoffrequencymetrology,[107]namelythefullstabilization ofthefrequencycombemittedbyamode-lockedlaser.ThereviewofRef.[50] describesthattherealizationofastablefrequencycomb,asaclockworkforlinking apreciselyknownfrequencyreferencetoanatomicormoleculartransitionofinterest, hadbeenenvisionedfordecades.However,onlyaftertheemergenceoftheTi:sapph technologyinthe1990sandtheinventionofmicrostructuredphotoniccrystalfibers (PCFs),thetechnicalpreconditions,namelytheroutinegenerationofultrabroadband (octave-spanning)spectra,werefulfilledtoactuallyrealizeanuniversal,preciselink. Whileforfrequencydomainapplications,thenJpulsesemittedfrombulk solid-stateorfiberoscillatorsaresufficientforconductingexperiments,reaching >1013 W/cm2 (condition(i))forfield-sensitivenonlinearopticswashardlypossible withultrafastoscillatorsofthefirstthreegenerations.Afewfield-sensitiveexperimentsingaseswithoscillatorsweredemonstrated.Theyexhibit,however,verylow conversionefficienciesanddonotfulfillconditions(ii)and(iii),[57, 108, 109]i.e. can,forinstance,notbeemployedinaspulsegenerationwithMHzrepetitionrates. Forfrequencycombspectroscopyapplicationsinthevacuumultraviolet(VUV)and extremeultraviolet(XUV),enhancementcavitieswereutilizedtoboostthepulse energiesandaveragepowersinsidearesonatorinordertomakeupforthelowconversionefficienciesinHHG[110].Thisapproachhasallowedtogeneratefrequency combswithuptoabout100eVphotonenergies[111, 112]andpowerlevelsoftens tohundredsof μWperharmonic[113–115].Buttheconceptexhibitsalsoseveral drawbacks.Firstly,thesetupisrathercomplexandexpensive.Secondly,outputcouplingoftheUVwhilemaintaininghighenhancementfactorsofthenear-IRisa highlycomplexissuewhereno“ideal”solutionforawideparameterrangehasbeen found,yet[116–119].Thirdly,enhancinglargebandwidthscomeswithincreasing difficultyandreducedenhancementfactors.Theshortestenhancedpulsesexhibited adurationofabout20fs[120],beingstilltoolongtosatisfycondition(ii).
Forextracavityexperimentsexploitingfield-sensitivenonlinearopticaleffects, multi-passorregenerativeamplifiersareutilizedtoboostthepulseenergiesofthe oscillatorpulsesbyseveralordersofmagnitude[121–123].Toavoidnonlineareffects intheadditionalgainmediachirpedpulseamplification(CPA)[124]hasbeenestablished.ItstretchespulseswithfsFTLstonsinordertostronglyreducetheirpeak power.RecompressionafteramplificationhasallowedtoreachPWpeakpowerlevels[125, 126].However,duetostorageofpumppowerinthegainmaterialsofthe amplifiersandtheconsequentthermalload,thereisatrade-offbetweenrepetition rateandpeakpower,inparticularinTi:sapphbasedCPAsystems[11].Thistrade-off
iseliminatedbyswitchingfromrealtoparametricgainmaterials,i.e.crystalswith quadratic(χ(2) )nonlinearities,exploitingso-calledopticalparametricchirpedpulse amplification(OPCPA)[127].Inthiscase,thequantumdefectbetweenpumpand laserphotonenergyisnotthermallydissipatedbutstoredinathird,idlebeam,i.e.in opticalpower.ThebandwidthofOPCPAsisdeterminedbythespectrumofthelow powerbeamseedingtheamplifierandthephase-matchingbandwidthofthenonlinearcrystals,whichcanbeverybroad,particularlyinanoncollineargeometry[46]. Therefore,thepowerfulpulsespumpingtheOPCPAprocesscanexhibitpsdurations [11]whicharetypicallyreachedbypower-scalablelaserarchitectures.Withfiber, [28]innoslab,[128]andthin-diskamplifiers,[34]kWaveragepowerlevelswith pulsedurationsbetween0.26to1.1pshavealreadybeendemonstrated.
Nevertheless,CPAandOPCPAsystemsarehighlycomplex,expensiveandoften alsonoisy.Theywillbeneededforapplicationsinparticleacceleration,relativistic opticsandplasmaphysics[125]but not necessarilytofulfillthethreeconditions whichhavebeenstatedabove.Instead,power-scalableultrashortpulseoscillators couldbeutilizedtogenerateCEP-stabilizedfew-cyclepulseswhichcanbefocused topeakirradiancesof1015 W/cm2 .ApartfromemployingsuchsourcesinHHG, time-domainapplicationsinvolvingfreechargecarrierswouldhighlybenefit.For instance,timeresolvedphotoemissionelectronmicroscopy(PEEM)aimsforhigh temporalandspatialresolution,butifmultipleelectronsarereleasedthroughasingle lasershot,thespatialresolutionwillbestronglyreducedduetospacechargeeffects [129].Hence,highrepetitionratesandmoderateintensities,justenoughtofreea singleelectronpershot,wouldbeidealfortime-resolvedPEEM.Similararguments holdforultrafastelectrondiffractionwheretheshortestelectronpulsescontainonly asingleparticle[130, 131].Moreover,highrepetitionratelasersemployedincold targetrecoilionmomentumspectroscopy(COLTRIMS)neartheionizationthresholdhaveproventodetectimprobableeventswhichcanhardlybeinvestigatedwith kHzsystems[132].However,thetargetedstudiesofcomplexdynamicslikenonsequentialdoubleionizationwouldstronglybenefitfromfew-(orbettersingle-)cycle pulsesandCEPcontrol[133].TheTDtechnologyismostpromisingforrealizing thestatedapplications[13].Itwillbeintroducedinthenextsection.
1.3TheThin-DiskConcept-Power-ScalableUltrashort PulseOscillators Inordertoscaletheaveragepowerachievablefromsolid-statelasers,AdolfGiesen suggestedtoutilizeonlyverythingainmediawhichactasactivecavitymirrors [134].Forthisideaanditsrealization,AdolfGiesenhasreceivedthe2017Charles HardTownesAwardoftheOpticalSocietyofAmerica.Thesetupofathin-diskmoduleissketchedinFig. 1.2.Thedisksareusuallyverythin(0.1–0.4mm)andcanbe cooledveryefficientlyandhomogeneouslyacrossthebeamplane(Fig. 1.2a).Therefore,thermaleffectsinthegainmediumcanbestronglysuppressed.Consequently,
Fig.1.2a SchematicsideviewonaTD.Thediskisanti-reflection(AR)coatedatthefrontsideand highreflection(HR)coatedontherearside,andthusactsasaturningmirror(withgain)insidethe laserresonator.Theyellowarrowsshowthecoaxialheatflowwhichensureslowthermalgradients acrossthelaserbeam.Inthiscase,thegainmediumisytterbiumdopedyttrium-aluminumgranate (Yb:YAG),butotheractivemediaareutilizedaswell.ThesketchshowsthattheTDisbondedtoa diamondheatsink.ThisisusuallydonebythedisksupplierTRUMPFLaserGmbH.Inmostofthe experimentspresentedinthisthesis,theuseddiskisbondedtoacopperheatsink(diskwasprovided byDausinger + GiesenGmbH).Bothmaterialsexhibitextraordinaryheatconductivity,andthus thethermalpower(≈150Wfortheoscillatorusedinmostoftheexperiments)caneffectivelybe dissipatedinthecoolingwatercircuit. b SketchofasimpleTDlaser.Thepumplightisentering thediskheadfromtherearsideandpassesthediskmultipletimesbyvirtueofanimagingsystem consistingofparabolicmirrors.Multiplepassesarenecessarytoletthethindiskabsorbalargeshare ofthepumplight.Thesimpleresonatorconsistsonlyofthegainmediumandanoutputcoupler (OC)whichislocatedinfrontofthediskhead.(Reprinted,withpermission,fromRef.[11], c 2014OSA)
thelaserarchitectureis average powerscalable.Moreover,thediskdiametersof >20mm2 allowlargelaserspotsizes.Hence,thefluenceinthegainmediumcan bekeptsufficientlylowtoavoiddetrimentalnonlinearities,andthustheconceptis also peak powerscalable.Theadvantagesofthetechnology,statedinRef.[135],are summarizedinTable 1.2.ThesuccessofTDmanifestsitselfbythedevelopment ofcontinuouswave(CW),ns,psandfslasers,thecommercializationofthearchitecturebycompanieslikeDausinger + GiesenGmbH,JenoptikAGorTRUMPF LaserGmbH + Co.KGandthepowerrecordswhichhavebeenachievedwiththe technology.TheaveragepowersofthefirstTDoscillatorwas4.4W[134].Today, CWdisklasersroutinelyreachtensofkWofaveragepower[136, 137]inmultitransversalmodeoperationandupto4kWinnearfundamentaltransversalmode operation[138].Ultrashortpulseamplifiersemittingpspulseswithmorethan1kW andgoodbeamqualityhavebeendemonstratedaswell[33, 34].Theyreachpeak powersofmorethan150GW,exceedingtheintracavitypeakpoweroftheoscillator presentedinSect. 2.1 bymorethan4000times.
Thefirstmode-lockingofaTDlaserwasdemonstratedin2000[21].Anaverage powerof16.2Wwasreported.Thepulseenergywas0.47 µJandtheduration730fs. Mode-lockingwasrealizedwithSESAM,i.e.aslowsaturableabsorber.Thesetypes
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Title: The poetic Edda
Translated from the Icelandic with an Introduction and notes
Editor: Henry Adams Bellows
Author: Saemund
Release date: May 4, 2024 [eBook #73533]
Language: English
Original publication: New York: The American-Scandinavian Foundation, 1923
Credits: Jeroen Hellingman and the Online Distributed Proofreading Team at https://www.pgdp.net/ for Project Gutenberg (This file was produced from images generously made available by The Internet Archive/Canadian Libraries) *** START OF THE PROJECT GUTENBERG EBOOK THE POETIC EDDA ***
[Contents]
[Contents] A PAGE FROM THE CODEX REGIUS COMPRISING VERSES 31 TO 45 OF THE VOLUSPO [Contents]
THE POETIC EDDA TRANSLATED FROM THE ICELANDIC WITH AN INTRODUCTION AND NOTES
BY HENRY ADAMS
BELLOWS TWO
VOLUMES IN ONE FOUNDATION LONDON: HUMPHREY MILFORD OXFORD UNIVERSITY PRESS 1923 [Contents]
Copyright, 1923, by The American-Scandinavian Foundation
C. S. Peterson, The Regan Press, Chicago, U. S. A.
[Contents]
This series of S C is published by The American-Scandinavian Foundation in the belief
that greater familiarity with the chief literary monuments of the North will help Americans to a better understanding of Scandinavians, and thus serve to stimulate their sympathetic coöperation to good ends.
[Contents]
[Contents]
To George Lyman Kittredge
[Contents]
SCANDINAVIAN CLASSICS VOLUMES XXI AND XXII THE POETIC EDDA ESTABLISHED BY NIELS POULSON THIS VOLUME IS ENDOWED IN PART BY CHARLES S. PETERSON OF CHICAGO [vii]
[Contents]
ACKNOWLEDGEMENT The General Introduction mentions many of the scholars to whose work this translation owes a special debt. Particular reference, however, should here be made to the late William Henry Schofield, Professor of Comparative Literature in Harvard University and President of The American-Scandinavian Foundation, under whose guidance this translation was begun; to Henry Goddard Leach, for many years Secretary of The American-Scandinavian Foundation, and to William Witherle Lawrence, Professor of English in Columbia University and Chairman of the Foundation’s Committee on Publications, for their assistance with the manuscript and the proofs; and to Hanna Astrup Larsen, the Foundation’s literary secretary, for her efficient management of the complex details of publication. [xi]
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GENERAL INTRODUCTION There is scarcely any literary work of great importance which has been less readily available for the general reader, or even for the serious student of literature, than the Poetic Edda. Translations have been far from numerous, and only in Germany has the complete work of translation been done in the full light of recent scholarship. In English the only versions were long the conspicuously inadequate one made by Thorpe, and published about half a century ago, and the unsatisfactory prose translations in Vigfusson and Powell’s Corpus Poeticum Boreale, reprinted in the Norrœna collection. An excellent translation of the poems dealing with the gods, in verse and with critical and explanatory notes, made by Olive Bray, was, however, published by the Viking Club of London in 1908. In French there exist only partial translations, chief among them being those made by Bergmann many years ago. Among the seven or eight German versions, those by the Brothers Grimm and by Karl Simrock, which had considerable historical importance because of their influence on nineteenth century German literature and art, and particularly on the work of Richard Wagner, have been largely superseded by Hugo Gering’s admirable translation, published in 1892,
and by the recent two-volume rendering by Genzmer, with excellent notes by Andreas Heusler, 1914–1920. There are competent translations in both Norwegian and Swedish. The lack of any complete and adequately annotated English rendering in metrical form, based on a critical text, and profiting by the cumulative labors of such scholars as Mogk, Vigfusson, [xii]Finnur Jonsson, Grundtvig, Bugge, Gislason, Hildebrand, Lüning, Sweet, Niedner, Ettmüller, Müllenhoff, Edzardi, B. M.
Olsen, Sievers, Sijmons, Detter, Heinzel, Falk, Neckel, Heusler, and Gering, has kept this extraordinary work practically out of the reach of those who have had neither time nor inclination to master the intricacies of the original Old Norse.
On the importance of the material contained in the Poetic Edda it is here needless to dwell at any length. We have inherited the Germanic traditions in our very speech, and the Poetic Edda is the original storehouse of Germanic mythology. It is, indeed, in many ways the greatest literary monument preserved to us out of the antiquity of the kindred races which we call Germanic. Moreover, it has a literary value altogether apart from its historical significance. The mythological poems include, in the Voluspo, one of the vastest conceptions of the creation and ultimate destruction of the world ever crystallized in literary form; in parts of the Hovamol, a collection of wise counsels that can bear comparison with most of the Biblical Book of Proverbs;
in the Lokasenna, a comedy none the less full of vivid characterization because its humor is often broad; and in the Thrymskvitha, one of the finest ballads in the world. The hero poems give us, in its oldest and most vivid extant form, the story of Sigurth, Brynhild, and Atli, the Norse parallel to the German Nibelungenlied. The Poetic Edda is not only of great interest to the student of antiquity; it is a collection including some of the most remarkable poems which have been preserved to us from the period before the pen and the printing-press replaced the poet-singer and oral tradition. It is above all else the desire [xiii]to make better known the dramatic force, the vivid and often tremendous imagery, and the superb conceptions embodied in these poems which has called forth the present translation.
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WHAT IS THE POETIC EDDA? Even if the poems of the so-called Edda were not so significant and intrinsically so valuable, the long series of scholarly struggles which have been going on over them for the better part of three centuries would in itself give them a peculiar interest. Their history is strangely mysterious. We do not know who composed them, or
when or where they were composed; we are by no means sure who collected them or when he did so; finally, we are not absolutely certain as to what an “Edda” is, and the best guess at the meaning of the word renders its application to this collection of poems more or less misleading.
A brief review of the chief facts in the history of the Poetic Edda will explain why this uncertainty has persisted. Preserved in various manuscripts of the thirteenth and early fourteenth centuries is a prose work consisting of a very extensive collection of mythological stories, an explanation of the important figures and tropes of Norse poetic diction,—the poetry of the Icelandic and Norwegian skalds was appallingly complex in this respect, and a treatise on metrics. This work, clearly a handbook for poets, was commonly known as the “Edda” of Snorri Sturluson, for at the head of the copy of it in the Uppsalabok, a manuscript written presumably some fifty or sixty years after Snorri’s death, which was in 1241, we find: “This book is called Edda, which Snorri Sturluson composed.” This work, well known as the Prose Edda, Snorri’s Edda or the [xiv]Younger Edda, has recently been made available to readers of English in the admirable translation by Arthur G. Brodeur, published by the AmericanScandinavian Foundation in 1916.
Icelandic tradition, however, persisted in ascribing either this Edda or one resembling it to Snorri’s much earlier compatriot, Sæmund the Wise (1056–1133).
When, early in the seventeenth century, the learned Arngrimur Jonsson proved to everyone’s satisfaction that Snorri and nobody else must have been responsible for the work in question, the next thing to determine was what, if anything, Sæmund had done of the same kind. The nature of Snorri’s book gave a clue. In the mythological stories related a number of poems were quoted, and as these and other poems were to all appearances Snorri’s chief sources of information, it was assumed that Sæmund must have written or compiled a verse Edda—whatever an “Edda” might be —on which Snorri’s work was largely based.
So matters stood when, in 1643, Brynjolfur Sveinsson, Bishop of Skalholt, discovered a manuscript, clearly written as early as 1300, containing twenty-nine poems, complete or fragmentary, and some of them with the very lines and stanzas used by Snorri. Great was the joy of the scholars, for here, of course, must be at least a part of the long-sought Edda of Sæmund the Wise. Thus the good bishop promptly labeled his find, and as Sæmund’s Edda, the Elder Edda or the Poetic Edda it has been known to this day.
This precious manuscript, now in the Royal Library in Copenhagen, and known as the Codex Regius
