SRSwasinvestigatedinaliquidparahydrogendropletchar-acterizedwithWGMhavingQ-factorexceeding109[240].TheSRSwasregisterednotonlyforvibrationaltransitionbutalsoforrotationaltransitionasinthegas-phaseH2system,leadingtomultiorderSRSsidebandscoveringthewholevisiblespectralrange.
SRSinultrahigh-Qsurfacetension-inducedsphericalandchip-basedtoroidmicrocavitiesisconsideredboththeoreti-callyandexperimentallyin[241].ThesefusedsilicaWGRsexhibitsmallmodevolume(typically103µm3)andpossesswhispering-gallerytypemodeswithlongphotonstoragetimes(intherangeof100ns),signi cantlyreducingthethresholdforstimulatednonlinearopticalphenomena.
ThestudiesofRamangaininisolatedhigh-QWGRsareimportanttounderstandcavityQEDpropertiesofRamanlasing.Previously,microcavityQEDenhancementofRamangainhasbeeninferredastheresultofmeasurementsofadependenceoftheSRSthresholdonthesizeandmaterialofthemicrodroplets,anditscomparisonwiththevaluesofSRSthresholdreportedforliquidcore bershavingequivalentinteractionlengthandcorecomposition[234],[235].Thisenhancementhasbeenlinkedtothecavitymodi cationofthepropertiesofausuallaser.AtheoryoftheRamangainmodi cationthatexplainstheexperimentalresultswasdeveloped[242],[243].Recentexperimentswithsilicamicrosphereshavenotshownanysigni cantchangeinSRSgainwhichmightbeattributedtoquantumeffects[241],[244].Thisissuewasaddressedin[245],whereitwasshownthatnocavityQED-associatedRamangainenhancementexists,unlikethecavityenhancementofthespontaneousemission.C.SwitchesandModulators
1)WGMSwitches:WGRscanbeusedasef cientandcompactopticalswitchesandmodulators.NonlinearopticalswitchesbasedonWGMsareprimarilyconsideredinrelationwiththeirapplicationstoall-opticalcomputing.ApossibilityofsuchswitchingandapplicationsofWGRtocreateaquantummechanicalcomputerwas rstrecognizedin[246].
ThemajorityofstudiesofopticalswitchesthatutilizeWGMsaretheoretical.ItwasshowntheoreticallythatWGRmicrodisklasersarestableandswitchreliably[247],andhencearesuitableasswitchingelementsinall-opticalnetworks.
WGM谐振腔综述
24IEEEJOURNALOFSELECTEDTOPICSINQUANTUMELECTRONICS,VOL.12,NO.1,JANUARY/FEBRUARY2006
Anintegratedall-opticalswitchbasedonahigh-Qnonlin-earcylindricalmicrocavityresonatorwasproposed[248].TheswitchconsistsoftwoplanarwaveguidescoupledtoaWGR.Itwasarguedthatduetothehigh-Qfactorandthesmalldi-mensions,fastswitchingatlowpowerisfeasibleforthedevicesbasedonpresentlyavailablenonlinearpolymersastheactivematerial.
Ageneralelectrodynamicaltheoryofahigh-Qopticalmicro-sphereresonatorinanexternalalternatingmagnetic eldwasreportedin[249].ItwasshownthatthatsuchasystemcanchangeapolarizationstateoftheWGMphotonscon nedinthesphereduetotheFaradayeffect.Thispropertywasproposedforuseinall-opticalswitchesandlogicaldevices.
Numericalevaluationofanopticalresponseofaprism-couplednonlinearmicrospherewasdiscussedin[250].Thenumericalresultshaveshownthatthecontroland/orthesig-nallightscaninducetheopticalswitching-likevariationinthelightre ectance.ThiseffectwasinterpretedbythevariationinthedielectricconstantofthesphereduetoitsKerrnonlinearity.CoupledWGRspossessdifferentandfrequentlymoread-vancedpropertiescomparedtoasingleWGR.Sequencesofop-ticalmicroresonatorscanbeusedtoconstructintegratedstruc-turesthatdisplayslowgroupvelocityoflight,ultrahighorlowdispersionofcontrollablesign,enhancedself-phasemodulation,andnonlinearopticalswitching[251].
Itwaspointedoutthatthereshouldbeareductioninswitchingthresholdfornonlinearopticaldevicesincorporating berringresonators[252],[253].ThecirculatingpowerinsuchWGRsismuchlargerthantheincidentpower,andthephaseofthetransmittedlightvariesrapidlywiththesingle-passphaseshift.Itwasshownthatthecombinedactionoftheseeffectsleadstoa nesse-squaredreductionintheswitchingthreshold[252],allowingforphotonicswitchingdevicesthatoperateatmilli-wattpowerlevelsinordinaryoptical bers.AsetofcoupleddifferentialequationsthatdescribeKerrnonlinearopticalpulsepropagationandopticalswitchinginsystemscoupledbyafewmicroresonatorswasderivedin[254].Gap-solitonswitchinginasystemcomposedoftwochannelwaveguidescoupledbymicroresonatorswasstudiedin[255].
Anumericaldemonstrationofthefeasibilityofconstructinganall-optical“AND”gatebyusingamicroresonatorstructurewithKerrnonlinearitywaspresentedin[256].Itwasshownthatthegatecanbemuchsmallerthansimilar“AND”gatesbasedonBragggratings,andhaslowerpowerrequirements.
Thereareafewexperimentalstudiesofall-opticalswitchesthatutilizeWGMs.Forinstance,laser-inducedmodi cationofcavityQ’swasachievedinamicrodropletcontainingasat-urableabsorber[257].Theelastic-scatteringspectrafromsuchdropletsforhigherincidentintensitiesshowthatcavityQ’sareincreasedwhentheabsorptionisbleached.Thelasingspectrafromadropletcontainingasaturableabsorberandlaserdyeweremodi edwhenanintensebleaching eldwasinjectedintothedropletcavityafterthepump eldhadinitiatedthelasing.All-opticalnonlinearswitchingincompactGaAs–AlGaAsmicroringresonatorsatthe1.55-µmwavelengthwasdemon-stratedin[258].Switchingwasaccomplishedinthepump–probecon gurationinwhichthepump–probesignalswere
tunedtodifferentresonancewavelengthsofthemicroring.Re-fractiveindexchangeinthemicroringduetofreecarriersgen-eratedbytwophotonabsorptionwasusedtoswitchtheprobebeaminandoutofresonance.