Finally,insteadoflockingalasertoaWGM,theoppositewasrealizedin[76].AWGMofafused-silicamicrospherewaslockedtoafrequency-scanninglaser.Theresonancefrequencywasmodulatedbyaxialcompressionofthemicrosphere,andphase-sensitivedetectionofthe ber-coupledopticalthroughputwasusedforlocking.SuchasystemisparticularlyusefulinWGR-basedchemicalsensors,whichthefollowingsectionisdevotedto.
WGM谐振腔综述
ILCHENKOANDMATSKO:OPTICALRESONATORSWITHWGMs—PARTII:APPLICATIONS19
E.SpectroscopyandAnalysisofChemicalandBiologicalAgents
StartingfromliquidWGRsusedforresonator-enhancedspec-troscopy(see[77]forreview),solidstateWGRswereutilizetoenhancetheinteractionbetweenlightandatoms/molecules.Oneofthe rstexperimentsonthesubjectwasrealizedintheframeofcavity-QED[78].Theradiativecouplingoffreeatomstotheexternalevanescent eldofaWGMwasdetected.Thecouplingmanifesteditselfasanarrowabsorptionlineobservedintheres-onatortransmissionspectrum.Itwasproventhattheevanescent eldofthehigh-Q(5×107)andsmallmodevolume(10 8cm3)fusedsilicamicrosphereenablesvelocity-selectivein-teractionsbetweenasinglephotonintheWGMandasingleatominthesurroundingatomicvapor.Anultrasensitivespec-trometerbasedonastretchedsilicamicrospherewasproposedin[50],[79].
ThenextstageinthesensordevelopmentwasrelatedtoWGR-basedbiosensors[80]–[82].Opticalbiosensorsaretypi-callytransducersthatdetectthepresenceofmoleculesatasur-face.Theyhaveseveraldesirablefeatures,particularlyforthedetectionofbiologicalmolecules,thatinclude:1)highsensi-tivity(lessthannanomoles);2)non-destructivitytothesample;3)highselectivity;and4)applicabilitytovarioussubstances.Thetransductionprocessesinopticalbiosensorsgenerallytakeplaceonasurfaceandcanbetailoredtosensealmostanykindofmolecule,chemicalandprebiotic,aswellasbiological.WGRsensorsbelongtotheevanescentwavesensors,whichareamongthemostsensitiveclassofbiosensors[83],[84].Anevanescentwaveproducedbythetotalre ectionoflightwithinthewaveguideinteractswithanalytesonthewaveguidesurfaceintheevanescent eldsensors.Theevanescentwaveprotrudesabovethewaveguidesurfaceby～100nm(theactualdistancedependsontherelativeindexofrefractionofthewaveguideandthesamplemedium),andsamplesonlytheanalyteonthesur-face.Surfacetreatmentssuchasantibodiesoroligonucleotidestrandscanprovidespeci cityfortheanalyte;thesensorthendetectsonlythoseboundtothesurface.Transductionmech-anismsforboundanalyteinclude uorescence,masschangeintheevanescentregion[85],andchangeintheindexofre-fraction[86].Typicalsensitivityofevanescentwavebiosensorsbasedon beropticsensorsorplanarwaveguidesensorsisintherangeofnano-molestopico-moles.
ThebasicdetectionschemethatutilizesWGRsisthatbindingofmoleculestothemicroresonatorsurfaceinducesanopticalchangeproportionaltothequantityofboundmolecules.TheparadigmforthisprocessisachangeinthecavityQasthesurfaceboundmoleculesaffectthephotonstoragetime,eitherthroughincreasedscatteringorabsorption.Ineffect,theanalytespoilstheQ,andtheresultingchangecanbemeasured.
AnyproteinwilladheretoglasssurfaceofagenericWGR,andhence re-polishedspheresareentirelynonspeci c.Twoconditionsmustbemetforchemicalmodi cationofthemicro-spheresurface: rst,theglassmustbecoatedwithacompoundthatwillminimizenonspeci cbinding.Second,anantibodyorotherproteinwithsensitivitytoaparticularligandmustbelinkedtothesphereinsuchawaythatboththeprotein’sfunctionality
andthesphere’sQarepreserved.Athin lmofamaterialwiththicknesssmallerthantheWGM’sevanescent eldwillnotsig-ni cantlyaltertheQofthemicro-resonator;thus,athicknessof～10–100nmcanbeappliedtothemicrospherewhileretainingitshigh-Q.
Apossibilityofenhancementofthedetectionsensitivityofevanescent-waveopticalbiosensorswasdiscussedin[87]–[92].ItwasshownthattheresonantcouplingofpowerintotheWGRallowsforef cientuseofthelongphotonlifetimesofthehigh-QWGMstoincreasetheinteractionofthelightandtheparticlesunderthestudy.Thisenhancementresultsinstronger uores-cenceandinchangesoftheresonatorparameters.
Aspectroscopictechniqueforhigh-sensitivity,label-freeDNAquanti cationwasdevelopedin[93].ItwasdemonstratedthataWGMexcitedinamicron-sizedsilicaspherecanbeusedtodetectandmeasurenucleicacids.Thesurfaceofthesilicasphereistobechemicallymodi edwitholigonucleotides.A rst-orderperturbationtheorywasdevelopedforWGMsinadielectricmicrosphere[94],[95].Thetheorywereappliedtothreesensorapplicationsofthemicrospheretoprobethemediuminwhichthesphereisimmersed:arefractive-indexdetector,anadsorptionsensor,andarefractive-indexpro lesensor.
BiosensorsbasedontheshiftofWGMsinmicrospheresac-companyingproteinadsorptionweredescribedbyuseofaper-turbationtheoryin[94].Forrandomspatialadsorption,theorypredictsthattheshiftshouldbeinverselyproportionaltomicro-sphereradiusa,andproportionaltoproteinsurfacedensityandexcesspolarizability.
Hybridzincoxide/silicamicrodisklaserswereutilizedtosensevolatileorganiccompounds,suchastolueneandnitroben-zene[96].Nonspeci cadsorptionoftheseorganicmoleculesontotheWGRsurfacecausesanincreaseinthediskrefractiveindex,ultimatelyresultinginaredshiftoftheobservedlasingwavelengths.
ImprovementofphotonicWGMsensorsusingthefano-resonantlineshapewasproposedin[97].Polystyrenemicror-ingresonatorswerefabricatedbythenanoimprintingtechnique,andtheopticalspectraweremeasuredinglucosesolutionsofdifferentconcentrations.Theshiftinresonantwavelengthandvariationofthenormalizedtransmittedintensitywerelinearlyrelatedtotheconcentrationoftheglucosesolution.