Title: 7 μm, ultrafast, sub-millijoule-level mid-infrared optical parametric chirped pulse amplifier pumped at 2 μm Authors: D. SANCHEZ, M. HEMMER, M. BAUDISCH, S. L. COUSIN, K. ZAWILSKI, P. SCHUNEMANN, O. CHALUS, C. SIMON-BOISSON, AND J. BIEGERT

Final manuscript The original publication may be found at: Journal: Optica, Vol. 3, Issue 2, pp. 147-150 (2016) DOI: http://dx.doi.org/10.1364/OPTICA.3.000147

7-micron, ultrafast, sub-mJ-level mid-IR OPCPA pumped at 2 micron D.SANCHEZ1,*,M.HEMMER1,#,M.BAUDISCH1,S.L.COUSIN1,K.ZAWILSKI2,P.SCHUNEMANN2,O.CHALUS3,C.SIMON-BOISSON3ANDJ.BIEGERT1,41ICFO-InstitutdeCienciesFotoniques,TheBarcelonaInstituteofScienceandTechnology,08860Castelldefels(Barcelona),Spain2BAESystems,MER15-1813,P.O.Box868,Nashua,NewHampshire03061,USA3THALESOptroniqueS.A.S.,LaserSolutionsUnit,2avenueGay-Lussac,78995ElancourtCedex,France4InstitucióCatalanadeRecercaiEstudisAvançats(ICREA),PasseigLluisCompanys23,Barcelona08010,Spain#presentaddress:CenterforFree-ElectronLaserScience,DeutschesElektronenSynchrotron(DESY),Hamburg,Germany

*Correspondingauthor:daniel.sanchez@icfo.eu

ReceivedXXMonthXXXX;revisedXXMonth,XXXX;acceptedXXMonthXXXX;postedXXMonthXXXX(Doc.IDXXXXX);publishedXXMonthXXXX

Wepresentanovelall-fiberpumpedOPCPAarchitectureto generate self-CEP stable, sub-8optical cycledurationpulses at 7-micron wavelength approaching millijoule-levelpulseenergyat100Hzrepetitionrate.Thesystemyields a peak power of 1.1 GW and, if focused to thediffraction limit,would reachapeak intensityof7x1014W/cm2. The OPCPA is pumped by a 2-micron Ho:YLFchirped pulse amplifier to leverage the highly efficientand broadband response of the nonlinear crystal ZGP.The7-micronseedat100MHzisgeneratedviaDFGfroman Er:Tm:Ho multi-arm fiber frequency comb and afraction of its output optically injects the Ho:YLFamplifier. While the pulse bandwidth at 7 micron isperfectly suited for nonlinear and spectroscopicapplications,currentparametersoffer,forthefirsttime,the possibility to explore strong field physics in anentirely new wavelength range with a ponderomotiveforce77 times larger than froman800nmsource.Theoverall OPCPA system is very compact and provides anew tool for investigations directly in the molecularfingerprintregionoftheelectro-magneticspectrumortodrive high harmonic generation to produce fullycoherent X-rays in the multi-keV range and possiblyzeptosecondtemporalwaveforms.

©2015OpticalSocietyofAmerica

OCIScodes:(140.3070)Infraredandfarinfraredlasers;(190.4970)Parametricoscillatorsandamplifiers;(320.7090)Ultrafastlasers;

http://dx.doi.org/10.1364/optica.99.099999

Thedevelopmentofcoherentlightsourceswithemissioninthemid-IRiscurrentlyundergoingaremarkablerevolution.Themid-IRspectralrange has always been of tremendous interest, mainly to

spectroscopists,duetotheabilityofmid-IRlighttoaccessrotationaland vibrational resonances of molecules which give rise to superbsensitivity upon optical probing [1-3]. Previously, high energyresolution was achieved with narrowband lasers or parametricsources,buttheadventoffrequencycombsourceshasrevolutionizedspectroscopybyprovidinghighenergyresolutionwithinthefrequencycomb structure of the spectrum and at the same time broadbandcoverage and short pulse duration [4-6]. Such carrier to envelopephase (CEP)controlled lightwaveforms,whenachievedatultrahighintensity,giverisetoextremeeffectssuchasthegenerationofisolatedattosecondpulses in thevacuumtoextremeultravioletrange(XUV)[7].Motivatedlargelybythevastpotentialofattosecondscience,thedevelopment of ultraintense few-cycle and CEP stable sources hasintensified[8],anditwasrecognizedthatcoherentsoftX-rayradiationcould be generated when driving high harmonic generation (HHG)withlongwavelengthsources[9-11].Recently,basedonthisconcept,the highest waveform controlled soft X-ray flux [12] and isolatedattosecondpulseemissionat300eV[13]wasdemonstratedviaHHGfroma1850nm,sub-2-cyclesource[14].Withinstrongfieldphysics,longwavelengthscalingmayleadtofurtherinterestingphysicssuchasthedirectreshapingofthecarrierfield[15],scalingofquantumpathdynamics [16], the breakdown of the dipole approximation [17] ordirect laseracceleration[18].Theexperimentaldevelopmentof longwavelengthlightsourcesthereforeholdsgreatpromiseinmanyfieldsofscienceandwillleadtonumerousapplicationsbeyondstrongfieldphysicsandattosecondscience.Here, we present the first mid-IR optical parametric chirped pulseamplifier (OPCPA) operating at a center wavelength of 7 μm withoutputparameterssuitablealreadyforstrong-fieldexperiments.It isalsothefirstdemonstrationofOPCPAusinga2μmlaserpumpsourcewhichenables theuseofnon-oxidenonlinearcrystalswith typicallylimitedtransparencyat1μm wavelength.ThisnewOPCPAsystemisall-optically synchronized and generates 0.55mJ energy, CEP stableopticalpulses.Thepulsessupportasub-4cyclepulsedurationandarecurrentlycompressed tosub-8opticalcyclesdue touncompensated

optica Letter

higher order phase from the grating compressor which will beaddressedinthefuture.AschematicrepresentationoftheOPCPAisshowninFig.1andfollowsthepreviouslyprovenconceptofour3µmOPCPAsystem[19].Thefront-end of the system, which seeds both the pump and opticalparametricamplifier(OPA)stages,isamulticolorall-fibrelaser(MenloSystemsGmbH)[20].Theseederisbasedonamode-lockedEr:fibreoscillator and power amplifier which deliver 1.55-μm wavelength,nanojoule-levelpulsesatarepetitionrateof100MHz.Theoutputissplitintotwoarms,thefirstofwhichiscompressedinfibreto72fswith3nJpulseenergy.Thesecondoutputiscoupledintoahighlynon-linearfibrewherethespectrumisbroadenedtoanopticaloctaveviasuper-continuumgeneration.Theportionofthebroadenedspectrumaround2μmissubsequentlysplitintotwoarms.Thefirstisusedtoseedachainofbroad-bandTm:Hofibreamplifiersanddelivers3nJenergypulsescenteredat2040nm,whichwecompressinfree-spaceto 130 fs. The second arm is spectrally narrowed to 1.5 nmbeforeseedingachainofTm:Hofibreamplifiers.Attheoutputweobtain4nJ,fewpicosecondpulses.

Fig.1.a)LayoutoftheOPCPAconceptfeaturinganall-fibreseeder,aDFGstage,a2µmHo:YLFpumplaser,achainofthreeconsecutiveZGPOPAs and a reflective grating-based compressor; HNLF, highlynonlinearfibre;NBF,narrow-bandwidthfilter,Stretch.,stretcher;PM,pick-off mirror; Comp., compressor. b) Layout of the Ho:YLF CPAfeaturing a CVBG stretcher, a pulse picker, a ring regenerativeamplifier, a single-pass cryogenically cooled booster amplifier and aCVBGcompressor;QWP,quarter-waveplate;CVBG;chirpedvolumeBragggrating;PC,Pockelscell;TFP,thin-filmpolarizer.

Fig.2.Spectrumofthemid-IRpulsesamplifiedintheZGPOPAs(blackline,shaded)measuredusingascanningmonochromatorandseedspectrumfrom the CSPDFG stage (grey line)measuredwith a Fourier transforminfrared spectrometer. The spectral width of the OPCPA output at the13.5%pointis1360nm–correspondingto85fstransformlimit.

Themid-IR seed for theOPA chain is generated through differencefrequencygeneration (DFG)of the femtosecondoutputsof the fibrelaser.SeedgenerationusingDFGwasdemonstratedtogeneratelargebandwidthsinthemid-IR[21]andfurthermore,iftheinputpulsesforthe DFG originate from a common oscillator, the output will beintrinsicallyCEPstable[22,23].PassiveCEPstabilizationhasproventoberobustandreducestheoverallcomplexityofthelasersystem[24].WechosethenonlinearcrystalCdSiP2(CSP)asthegenerationmediumdue to its unique combinationof high effectivenonlinear coefficient(deff~84.5pm/V),broadphase-matchingcharacteristicsinthemid-IRandwidetransmissionwindow[25].UsinganARcoated1.5mmthickCSPcrystal(BAESystems),theDFGstageproducesCEPstableopticalpulseswithup to150pJofenergyat100MHz[26].Thegeneratedspectrum,whichisshownasthegreylineinFig.2,iscentredaround6.5μmandspanstherangebetween5.5and8μm.The pump laser is a central component of an OPCPA as it largelydetermines the performance of the overall system. Currently, themajorityofhigh-intensitymid-IRsourcesrelyonparametricamplifierspumped by laser systems operating at 1 μm [19,27,28]. However,extendingtheoperatingwavelengthofOPCPAtolongerwavelengthshas so farbeenhinderedby an increasinglyunfavourablepump-to-idler photon ratio as well as a scarcity of nonlinear crystals withsuitable optical and mechanical properties or transparency [29].Scaling the pump laser to longer wavelengths circumvents theseobstacles and has recently been achievedwith pump laser systemsbasedonHo-dopedgainmediadeliveringhighenergy, femtosecond[30]andpicosecond[31,32]pulsesaround2µm,suitableforpumpingOPAs.ThepumplaseroftheOPCPAdescribedinthisworkreliesonchirpedpulseamplification(CPA)usingHo:YLFasgainmedium[31].TheCPAlineisseededwiththenarrow-band,2μmpulsesfromthefibrefront-end.Thefewpicosecond2μmpulsesaretemporallystretchedusingachirpedvolumeBragggrating(CVBG;OptiGrateCorp.)toameasuredduration of 170 ps, suitable for seeding the high gain amplifierswithoutriskofopticaldamage.Uponstretching,thepulsesarepickedat 100 Hz repetition rate and directed towards a Ho:YLF ringregenerativeamplifier (RA). Injectionandejection fromthecavity isachieved using a rubidium titanyle phosphate (RTP) Pockels celloperatedathalf-wavevoltage.ThenJseedpulsesareamplifiedto4mJ

intheRAandthensenttoasinglepass,cryogenicallycooledHo:YLF[33]amplifier.BoththeRAandsinglepassamplifierarepumpedbyaCWTm:fibrelaseroperatingatawavelengthof1940nm.AttheoutputoftheCPAwemeasure40mJenergywithpowerfluctuationsoflessthan0.8%rmsandpeak-to-peakfluctuationsoflessthan3%overhalfa million of shots. The compressor is based on a CVBG (OptiGrateCorp.),withan25x27mm2aperture.Thecompressorhasanoverallefficiencyof85%andisusedtotemporallycompresstheoutputtoameasureddurationof11ps.Thetemporalseedstretchfactor,whichdeterminesthepumptoseedpulsedurationratio,isanimportantdesignaspectofanOPCPAsinceithas a large impact on efficiency, amplifiable bandwidth andsuperfluorescence generation. There is a trade-off, since in generalone-to-onematchingofthepumpandseedpulsedurationsleadstoagreater efficiency, while a shorter seed pulse leads to broaderamplification bandwidths. We used a nonlinear wave propagationcode[34]tonumericallysimulatetheDFGandOPAprocessesinordertochoosethestretchfactorfortheseed.Oursimulationssuggestthataseedpulsedurationof6pstogetherwithan11pspumppulsepermitsamplifying sufficient bandwidth to support a sub-100 fs pulse; notethatoneoptical cycleat7μmequals23 fs.WechosebulkBaF2 forstretchingdueitslowFresnellossesandhighGVDinthemid-IR(700fs2/mmat6.5μm).Accordingtoourcalculations, themid-IRseed isstretchedto6psafterpropagatingthrough12cmofBaF2.Thestretched,picojoule-levelseedpulsesareamplified inachainofthree consecutive non-collinear OPA stages. Due to the largebandwidthsandhighintensitiesinvolved,thenonlinearcrystalfortheamplifiersneedstobechosencarefully.Theuseofa2μmpumplaserallowsustousethehighlynon-linearcrystalZnGeP2(ZGP)(deff~75pm/V), which for transparency reasons cannot be pumped at awavelengthshorterthan2μm[29].Thefirstparametricamplificationstagesconsistsofa5mmARcoatedZGPcrystal(BAESystems).Usingapproximately200μJofpumpenergy, focused toan intensityof20GW/cm2,andseedingwith100pJ,thefirstamplifierexhibitsagainof~104, yielding2μJ of energy in themid-IR.An identical5mmZGPcrystalisusedinthesecondamplificationstage,whichispumpedwith4mJofenergyandproduces100μJpulses.Finally,theoutputofthesecondOPAstageiscollimatedandlooselyfocusedintothethirdOPA,which consists of a 3mmZGP crystal and is pumpedwith 8mJ ofenergy.Thislastamplificationstagebooststhemid-IRenergyto0.55mJ.ByblockingtheseedoftheOPAchainwewereabletoestablishanupper limit of superfluorescence contributing to parametricbackgroundofmaximally5%.

Fig. 3. Autocorrelation trace from a background free intensityautocorrelation.Shownisthecompressedpulses(bluecircle)withthebestfittedpulseshapewhichisaLorentzianprofile(solidredline).Thedeconvolutionresultsinapulsedurationof180fs.Measuredspatialprofile(inset)ofthecompressedpulseattheoutputoftheOPCPA.

Fig.4. Thegraph shows theOPCPAoutputpulseenergymeasuredsingle shot and over a period of 30 minutes. We find an excellentstabilitycorrespondingto2.5%rms.

TheamplifiedspectrumattheoutputoftheOPCPAisshowninFig.2andwasmeasuredusingascanningmonochromatorequippedwithaliquid N2 cooled mercury cadmium telluride (MCT) detector. Thespectrum is centredat7μmwithabandwidthof1360nmat1/e2whichsupportsa85fspulseduration(sub-4cycles).Consideringthattheamplificationisultrabroadband,themeasuredenergyconversionefficiencyof18%intosignalplusidlerinthelastOPAstageisexcellent.Afteramplification,forafirsttestofcompressibility,themid-IRpulsesare directed towards a Martinez-style compressor consisting of areflectiveAl-coateddiffractiongratingwith150l/mmandanuncoatedCaF2 lens. The total energy throughput of the compressor is 35%whichyieldsacompressedenergyof200µJ.Weexpecttosignificantlyenhance the low energy throughputwith custommade and coatedcompressor components which should result in throughputefficienciesof60to80%.Upon compression, the temporal profile of the optical pulses wascharacterizedusingbackground free intensity autocorrelationbasedonaZGPcrystalandallreflectiveoptics.Figure3showsthemeasuredtrace,whichexhibitsanautocorrelationwidthof360 fs. Inorder toevaluatethepulseduration,wefitvariouslineshapes(sech2,Gaussianand Lorentzian) to the measured autocorrelation trace for itsdeconvolution.WefindthattheautocorrelationtracesarebestfittedwithaLorentziantemporalprofile,whichissensibleconsideringthatpulse propagation suffers from residual absorption of small watercontentinthepurgedboxwhichwouldgiverisetomodificationofthesech2autocorrelationprofileinfavorofaLorentzianshape.Underthisassumption, theAC tracecorresponds toapulsedurationof180 fs,which isshorter than8opticalcycles.Ascomparison, themeasuredpulsedurationwouldcorrespondtothesamenumberofopticalcyclesasa20-fs-durationpulseattheTi:Sapphirewavelengthof800nm.Inorder to leveragethe fullpotentialof thesource togenerate few-cycle pulses, advanced pulse diagnostics capabilities are warrantedthatcanmeasurethepulseamplitudeandphaseat7micronrapidly.AstraightforwardimplementationofmethodssuchasFROGorSPIDERis however stymied by the absence of mid-IR array detectors withsufficientspatialorspectralresolutionthusresultinginunrealistically

long scan times or insufficient resolution.We believe thatmethodssuchaselectrooptical samplingwillbe ideally suited fordirect andrelatively fast waveformmeasurements in themid-IR [35]. Furtherreduction inpulsedurationshouldbestraightforwardto implementbased on methods using spectral broadening and subsequentcompression[36,37]ordirectpulsecompressioninthemid-IR[38].Lastly,wemonitored the stability of the compressed outputwith apyroelectricdetectorandwemeasuredanexcellent systemstabilitywithenergyfluctuationsoflessthan2.5%rmsover30minutes(Fig.4).Inconclusion,wehavedemonstratedthefirstultrafastmid-IROPCPAdirectlyoperatingatacentrewavelengthof7μmwithanewOPCPAarchitectureleveragingnon-oxidecrystalswhicharedirectlypumpedat2μm.Theentiresystemisfiberpumpedandbasedonamulti-armfiber front-endwith different emissionwavelengthswhich providestheopticalseedandopticalsynchronizationwithoutanyelectronics.TheOPCPAseedisderivedviaDFGfromtwoofthosefiberoutputswhichleadstoapassivelyCEP-stablemid-IRfrequencycomboutputat100MHzrepetitionrate.ThepumplaserisdirectlyseededbythesamefiberfrontendandconsistsofaHo:YLFregenerativeamplifierfollowedby a cryogenically cooled Ho:YLF single pass amplifier. The 200 µJoutputat180fsleadstoapeakpowerof1.1GWand,iffocusedtothediffractionlimit,toapeakintensityof7x1014W/cm2whichissufficientfor strong field experiments. Simulations indicate that the OPCPAarchitecture should be scalable to pump energies close to the joulelevelandtopulseenergiesinthetensofmillijoulerangeat7micron.Furtherongoingstepsincludeincreasingpulseenergythroughscalingof the Ho:YLF amplifier, careful tailoring of pump and seed pulsedurationsandchirpoftheOPCPAstages,andfurtherreductionofthepulsedurationat7micron.WebelievethatthedemonstratedpumpandOPCPAarchitecturesareattractiveduetothecompactnessoftheoverallsystemandthefactthatitisentirelyfiberlaserpumpedwhichreducesoperatingcostsdrastically.Moreover,thesystem’soperatingcharacteristicsnowenableaccesstothefullmid-IRspectralrangetoultrafast,nonlinearandstrongfieldphysics.Funding. We acknowledge financial support from the SpanishMinistryofEconomyandCompetitiveness,throughFIS2014-56774-R,FIS2014-51478-ERC, the “Severo Ochoa” Programme for Centres ofExcellence in R&D (SEV-2015-0522), the Catalan Agencia de Gestiód’Ajuts Universitaris i de Recerca (AGAUR) with SGR 2014-2016,Fundació Cellex Barcelona, the European Union’s Horizon 2020research and innovation programme under grant agreement No654148 Laserlab-Europe, the Marie Sklodowska-Curie grantagreements No. 641272 and 264666, and the German BMBF forfundingundertheEurostarsprojectMIRANDUS(E!6698)

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