4)WGMLasersWithSemiconductorGainMedia:WGM-basedlaserscanbecreatedwithsemiconductorquantumdotscoupledtotheWGMs.Oneofthemostimportantproblemshereisfabricationofasinglequantumdotmicrolaser.SuchamicrolasermadebycapturingthelightemittedfromasingleInAs–GaAsquantumdotintheWGMofaglassmicrospherewasproposedtheoreticallyin[197].Amasterequationmodelofasinglequantumdotmicrospherelaserwasdescribedin[198].Theoperationofasinglequantum-dot-microspherelaserandasemiconductormicrospherebistableelementwastheoreticallystudiedin[199].
Aquantumdot-microcavitysystemconsistingofCdTenanocrystalsattachedtoamelamineformaldehydelatexmi-crospherewasrealizedexperimentally[200].Thehighopticaltransparency,andthermalandmechanicalstabilityofmelamineformaldehyde,makeitinterestingasapotentialcandidateinop-ticalapplications.Therefractiveindexofmelamineformalde-hydeinthevisibleregion(n=1.68)isgreaterthanthatofsilica(n=1.47)orotherglassmaterials(n≈1.5).Photolumines-cencespectraofthemicrospherescoveredbyathinshellofCdTenanocrystalswerestudiedinordertoexaminetheemis-sionintensityasafunctionofexcitationpower.
Ultralow-threshold(thepumpwaslessthan2µW)CWlasingwasachievedatroomtemperatureinafused-silicamicrospherethatwascoatedwithHgTequantumdots(colloidalnanoparticles)[201].
WGRscansigni cantlyimproveoperationofsemiconductorquantumwelllasers.Amicrolaserdesignbasedonthehigh-re ectivityWGMsaroundtheedgeofathinsemiconductormicrodiskwasdescribed,andinitialexperimentalresultswerepresentedin[202].ItwasshownthatopticallypumpedInGaAsquantumwellsprovidesuf cientgainwhencooledwithliq-uidnitrogentoobtainsingle-modelasingat1.3and1.5µmwavelengthswiththresholdpumppowersbelow100µW.
WGM谐振腔综述
ILCHENKOANDMATSKO:OPTICALRESONATORSWITHWGMs—PARTII:APPLICATIONS23
ArealizationofanInGaAs–InGaAsProomtemperaturequantumwelldisklaser1.6µmindiameterand0.18µminthickness,operatingat1.542µmandusing0.85µmopticalpumping,wasreportedin[204].Methodsfordirectionalcou-plingoflightoutputfromandtoWGRmicrodisklasersweredescribedin[203].
Anopticallypumped,pulsedGaNmicrodisklaseroperatingatroomtemperaturewascreated[205].WGMsofthediskhadlinewidthasnarrowas0.1nm.WGRswithdiameterscoveringtherange25750µmweretested.Opticalpumpingwasperformedperpendiculartothediskplanebythethirdharmonic(355nm)orthefourthharmonic(266nm)ofaQ-switchedNd:YAGlaser.Theoutputlightemissionfromthesestructureswascollectedbyare ectingobjectivelocated80 fromthesurfacenormal,Quantum-cascadeWGMdisklasersemittingat9.5-and11.5-µmwavelengthswerereportedin[206].Takingadvantageofthehigh-qualityresonator(Q～200),thethresholdcurrentdensityofdisklasersemittingat9.5µmwasreducedtobelowthevalueofthecorrespondingridgewaveguidegeometry.
A“microgear”lasercomposedofamicrodiskandarota-tionallysymmetricBragggratingwasdescribedin[207].AGaInAsP-InPdevicewithmicronsizewasfabricated,andtheroomtemperatureCWoperationwasobtainedby17-µWpump-ing.
AnopticallypumpedmicrodiscGaN-basedlaserwasdemon-stratedin[208].TheopticallypumpedWGRshaddistinctmodesatexcitationpowersrangingfromabout8to16W·cm 2.Qualityfactorsforthemicrodiskswereoftheorderof4600.Theobservedlasingthresholdwas12.1W·cm 2.B.ResonatorModi edScattering
Thereareatleastthreescatteringprocessesplayingsigni -cantrolesinWGRs.TheyareBrillouin,Rayleigh,andRamanscattering.
1)BrillouinScattering:StimulatedBrillouinScattering(SBS)wasdemonstratedinliquiddroplets[209]–[218],thoughnoSBSinhigh-QsolidWGRswasregisteredbecauseofselec-tionrules[215].
2)RayleighScattering:Rayleighscatteringleadstothelim-itationoftheQ-factorofWGMsaswellastotheinter-modecoupling.Thescatteringislargelysuppressedinhigh-QWGRsbecauseofrestrictionsimposedonscatteringanglesbycavitycon nement,soveryhigh-QWGMsarefeasible[219].Thescattering,ontheotherhand,couplesinitiallydegeneratecoun-terpropagatingmodesintheWGRsandcreatestheintracavityfeedbackmechanisminstrumentalforthelaserfrequencylock-ingapplication[74].Rayleighscatteringmediatedintracavitybackscatteringreaches100%,aswasshowntheoretically[219]anddemonstratedexperimentally[220].Inthefrequencydo-main,intracavitybackscatteringisobservedasthesplittingofinitiallydegenerateWGMresonancesandtheoccurrenceofcharacteristicmodedoublets[221],[222].In uenceofRayleighscatteringonQ-factorsofhighrefractiveindexcontrastWGRsfabricatedfromsilicon-on-insulatorwaferswasstudiedusinganexternalsilica bertaperwaveguide[223],[224].
3)RamanScattering:Substantialopticalpowerenhance-mentwithinahigh- nesseopticalcavityhasrecentlyyieldedCWRamanlaserswithlowthresholdandlargetunability(see,e.g.,[225],[226]).Suchpropertiesmakecavity-enhancedCWRamanlasersattractiveforhighresolutionspectroscopy,remotesensing,atomicphysics,andtelecommunications.Reducingthecavitysizemayfurtherimprovetheperformanceofthelasers.Opendielectricsphericalmicrocavitiesarepromisingforthosepurposes.
AnenhancementofstimulatedRamanscattering(SRS)isoneoftheeffectsdemonstratedinsphericalmicrocavities.LowthresholdSRSwasobservedwithpulsed[213],[227]–[233]andCW[234],[235]opticalpumpinginmicrometer-sizeliquiddroplets.Theoreticaldescriptionoftheprocesswaspresentedin[236]–[239].