超快光学-第01章-入门

UltrafastOptics:IntroductionThebirthofultrafastopticsUltrahighintensityTheuncertaintyprincipleandlongvs.shortpulsesGenericultrashort-pulselaserMode-lockingandmode-lockingtechniquesGroup-velocitydispersion(GVD)CompensatingGVDwithapulsecompressorContinuumgenerationMeasuringultrashortpulsesTheshortesteventevercreatedUltrafastspectroscopyMedicalimagingProf.RickTrebinoGeorgiaTech“GallopingHorse”ControversyPaloAlto,CA1872ThebirthofultrafasttechnologyLelandStanfordEadweardMuybridgeBarbet:Doallfourhoovesofagallopinghorseeversimultaneouslyleavetheground?Timeresolution:1/60thofasecondIfyouthinkyouknowfast,thinkagain.Ultrashortlaserpulsesaretheshortesteventsevercreated.Ultrafastopticsvs.electronics1960197019801990200010–610–910–1210–15Timeresolution(seconds)YearElectronicsOpticsNooneexpectselectronicstoevercatchup.ThemetricsystemPrefixes:Milli(m)10-3Micro(µ)10-6Nano(n)10-9Pico(p)10-12Femto(f)10-15Atto(a)10-18Kilo(k)10+3Mega(M)10+6Giga(G)10+9Tera(T)10+12Peta(P)10+15SmallBigWe’llneedtoreallyknowthemetricsystembecausethepulsesareincrediblyshortandthepowersandintensitiescanbeincrediblyhigh.Exa(E)10+18TimescalesIt’sroutinetogeneratepulses1picosecond(10-12s).Researchersgeneratepulsesafewfemtoseconds(10-15s)long.Suchapulseistooneminuteasoneminuteistotheageoftheuniverse.Suchapulseistoonesecondas5centsistotheUSnationaldebt.1minute10fslightpulseAgeofuniverseTime(seconds)ComputerclockcycleCameraflashAgeofpyramidsOnemonthHumanexistence10-1510-1210-910-610-31001031061091012101510181femtosecond1picosecondHaroldEdgertonMIT,1942“HowtoMakeApplesauceatMIT”1964“SplashonaGlass”JuniorHighSchoolstudent1996StrobephotographyTimeresolution:afewmicroseconds'65'70'75'80'85'90'95101001000ShortestPulseDuration(fs)YearActivemodelockingPassivemodelockingCollidingpulsemodelockingIntra-cavitypulsecompressionTi-SapphireCurrentrecord:60attoseconds!UltrafastTi:sapphirelaserUltrafastlasersTheelectricfieldofa4.5-fspulse-20020Time(fs)ElectricfieldUltrafastset-upscanbeverysophisticated.Thehighestintensitiesimaginable1kHz“Chirped-PulseAmplification(CPA)”systemattheUniversityofColorado(MurnaneandKapteyn)0.2TW=200,000,000,000watts!Evenhigherintensities!NationalIgnitionFacility(underconstruction)192shapedpulses;1.8MJtotalenergyNovaLongvs.shortpulsesoflightTheuncertaintyprinciplesaysthattheproductofthetemporalandspectralpulsewidthsisgreaterthan~1.LongpulseShortpulseIrradiancevs.timeSpectrumtimetimefrequencyfrequencyUltrafastlasermediaSolid-statelasermediahavebroadbandwidthsandareconvenient.LaserpowerAgenericultrashort-pulselaserAgenericultrafastlaserhasabroadbandgainmedium,apulse-shorteningdevice,andtwoormoremirrors:Pulse-shorteningdevicesinclude:SaturableabsorbersPhasemodulatorsDispersioncompensatorsOptical-KerrmediaGeneratingshortpulses=Mode-lockingLockingvs.notlockingthephasesofthelasermodes(frequencies)RandomphasesLightbulbIntensityvs.timeUltrashortpulse!LockedphasesTimeTimeIntensityvs.timeGroupvelocitydispersion(GVD)broadensultrashortlaserpulsesDifferentfrequenciestravelatdifferentgroupvelocitiesinmaterials,causingpulsestoexpandtohighlychirped(frequency-swept)pulses.Longerwavelengthsalmostalwaystravelfasterthanshorterones.InputultrashortpulseAnymediumChirpedoutputnot-so-ultrashortpulsePulsecompressorThisdevicehasnegativegroup-velocitydispersionandhencecancompensateforpropagationthroughmaterials(i.e.,forpositivechirp).It’sroutinetostretchandthencompressultrashortpulsesbyfactorsof1000.Thelongerwavelengthstraversemoreglass.Ultrafastopticsisnonlinearoptics.Athighintensities,nonlinear-opticaleffectsoccur.Allmode-lockingtechniquesarenonlinear-optical.Creatingnewcolorsoflaserlightrequiresnonlinearoptics.Second-harmonic-generationofinfraredlightyieldsthisbeautifuldisplayofintensegreenlight.Generally,small-scaleself-focusingoccurs,causingthebeamtobreakupintofilaments.ContinuumgenerationContinuumGeneration:focusingafemtosecondpulseintoaclearmediumturnsthepulsewhite.Recentlydevelopedtechniquesinvolvingopticalfibers,hollowfibers,andmicrostructurefibersproduceverybroadbandcontinuum,over500THz(1000nm)inspectralwidth!TheDilemmaInordertomeasureaneventintime,youneedashorterone.Tostudythisevent,youneedastrobelightpulsethat’sshorter.Butthen,tomeasurethestrobelightpulse,youneedadetectorwhoseresponsetimeisevenshorter.Andsoon…So,now,howdoyoumeasuretheshortestevent?Usingthepulsetomeasureitself:TheIntensityAutocorrelatorCrossingbeamsinanonlinear-opticalcrystal,varyingthedelaybetweenthem,andmeasuringthesignalpulseenergyvs.delay,yieldstheIntensityAutocorrelation,A(2)(t).TheIntensityAutocorrelation:(2)()AItItdtttNonlinearcrystalPulsetobemeasuredVariabledelay,tDetectorBeamsplitterE(t)E(t–t)Esig(t,t)ThesignalfieldisE(t)E(t-t).SothesignalintensityisI(t)I(t-t)Frequency-ResolvedOpticalGating(FROG)“PolarizationGate”GeometryNonlinearmediumPulsetobemeasuredVariabledelay,tCameraBeamsplitterE(t)E(t-t)Esig(t,t)=E(t)|E(t-t)|2FROGinvolvesgatingthepulsewithavariablydelayedreplicaofitselfinaninstantaneousnonlinear-opticalmediumandthenspectrallyresolvingthegatedpuls