SensorsandActuatorsA233(2015)374–389
ContentslistsavailableatScienceDirect
SensorsandActuatorsA:Physical
journalhomepage:www.99jianzhu.com(Y.Zhao)
http://dx.doi.org/10.1016/j.sna.2015.07.025
0924-4247/?2015ElsevierB.V.Allrightsreserved.
Y.-n.Zhangetal./SensorsandActuatorsA233(2015)374–389
375
1.Introduction
SinceYablonovitchandJohn?rstproposedtheconceptofpho-toniccrystal(PC)in1987[1,2],PC,whichpossessesaperiodicdielectricstructureandthecapabilityofguidingandmanipu-latinglightatthescaleofopticalwavelength,hasbeenstudiedextensivelybothintheoryandexperiment[3,4].OneofthebasicpropertiesofPCisthephotonicbandgap(PBG),andthepropaga-tionoflightwithinthefrequencyrangeofPBGwillbeforbidden[5].Nevertheless,theperiodicityofthisdielectricstructurewillbebrokenwhensomedefectsareintroducedinPC,whichmakesitpossibleforPCtopresentstrongelectromagnetic?eldcon?ne-ment,smallmodevolume,andlowextinctionloss[6].Ontheotherhand,byadjustingthestructuralparametersofPCorin?l-tratingsuitablematerialsintheairholesofPC,thepropagationoflightcanbemodi?edandengineeredatwill.Therefore,manyPCbaseddeviceshavebeenwidelyusedintheapplicationsoflight?owcontrol,suchas?lters[7,8],electro-opticalmodulators[9,10],switches[11,12],anddelaydevices[13].Specially,PCbasedsensorsseemtobemuchmorepopularduetotheirpromisingchar-acteristicslikeultra-compactsize,highmeasurementsensitivity,?exibilityinstructuraldesign,andmoresuitableformonolithicintegration[14–16].Besides,thePCbasedsensorscanalsoinheritthefavorablecharacteristicsofopticalsensors,suchassafetyin?ammableexplosiveenvironment,immunitytoelectromagneticinterference,long-distancemonitoring,andrapidresponsespeed.Therefore,duringthelastdecades,manyexcellentopticalsensorsbasedonPChavebeeninvestigatedanddevelopedinalargerangeofsensingapplications,suchasgassensors[17–19],liquidsensors[20],temperaturesensors[21],stresssensors[22],refractiveindex(RI)sensors[23,24],humiditysensors[25,26],andbiochemicalsen-sors[16,27].
Asatypicalstructuretype,PCcavity(PCC)isformedbyintroduc-ingpointdefectsintheorderlyarrangedlattices.Itexhibitsstrongspatialandtemporallightcon?nementandlongphotonlifetime(namely,highqualityfactorQ)[28],thusgreatlyenhancetheinter-actionstrengthbetweenoptical?eldandmaterialofdefectedregion.Asforsensingapplications,theenhancedinteractioneffectgivesrisetoanopticalmodeofPCCwitharesonantwavelengththatishighlysensitivetothelocalvariationsinitssurroundingmedium,andmakePCCapromisingbuildingblockforhigh-sensitiveopti-calsensors[29].Inaddition,theeffectivesensingareaofPCC
isontheorderofamicrometerorlessacross,whichprovidesanadvancedsensingplatformforin-situmonitoringwithsmartdesign.
Inthiswork,anoverviewofopticalsensorsbasedonPCCsisintroducedindetail,whereveravailable,togiveanewper-spectiveforfurtherresearchonothersensingapplicationsofPCCs.Therestofthispaperisorganizedasfollows:InSection2,theopticalpropertiesofPCCsandtheirbasicsensingprinci-plesareanalyzedanddiscussed.InSection3,theopticalsensorsbasedonPCCs,alongwiththeirstructuresandsensingproper-ties,arepresented.InSections4and5,thekeyproblemsandnewdirectionsofPCCsforsensingapplicationsareputforward,respectively.FinallyinSection6,wedrawabriefconclusionandprospect.
2.OpticalpropertiesandsensingprinciplesofPCCs
Fromtheviewofdefectedstructure,PCCcanbedividedintoLn(n≥3)cavity[30–32],Hm(m=0,1,2)cavity[33,34],mode-gapcavity[35],ringcavity[36,37],andshoulder-coupledcavity[38,39],asshowninFig.1.
Takingtheshoulder-coupledcavitythatpublishedinRef.[39]asanexample,wewillintroducetheresonantpropertiesandsensingprincipleofthisPCC.Tobeginwith,weanalyzethepropertyofPCwaveguide(PCW),whichisformedbyremovingthecentralrowofairholesfromtheperfectPCalongxdirection,asshowninFig.2(a),whereaisthelatticeconstant,ristheradiusofairhole,disthewaveguidewidth,andhistheslabthickness.ItsbasicpropertyisthatthelightlocatedinPBGcanonlybeguidedinthelinewave-guideasthelightiscon?nedhorizontallybyPBGofPCandverticallybytotalinternalre?ectionduetotheRIdifferencesbetweendiffer-entlayers.Fig.2(b)showsthecalculatedelectric?elddistributionofPCWwhentheworkingfrequencyoftransmissionlightislocatedinthePBG,whichissimulatedbyusingMIT’sfreelyavailablesoftwareMEEP[40].ItisfoundthattheTE-likepolarizedlightcanbestronglycon?nedinwaveguideregionbothin-planedirection(horizon-tally)andout-planedirection(vertically),andtheleakageoflightisverysmall.However,whenthefourairholesareintroducedatthecenteroftheabovewaveguidetoformashoulder-coupledcavity,mostofthelightenergywillstronglylocalizedinthecentralpartofthePCC,asshowninFig.3(a).Thecorrespondingtransmission
Fig.1.Schematicstructuresof(a)L4PCC,(b)H0PCC,(c)mode-gapPCC,(d)ringPCC,and(e)shoulder-coupledPCC.
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Fig.2.SchematicstructureofPCW(a)anditscorrespondingelectric?elddistribution(b)[36]
.
Fig.3.(a)Electric?elddistributionofashoulder-coupledPCC,and(b)transmissionspectraofW1PCWandshoulder-coupledPCC[36].
spectraofW1PCWandshoulder-coupledPCCareshowninFig.3(b),fromwhichwecan?ndthatthePCWpossesseshightransmittanceandwideworkingrange,whilethetransmissionspectrumofPCCbehavesaverynarrowLorentziancurvewithacer-tainresonantfrequency.Namely,onlythelightenergyatresonantfrequencycouldleakoutfromthecentralpointdefect.
Itwasdemonstratedthattherelationshipbetweenthenormal-izedtransmissionintensityTofshoulder-coupledPCCandworkingfrequenciesωcanbeexpressedapproximatelyasaLorentzianfunc-tion[39]:
??
T(ω,ω0)=
ω0/2Q
??2
molecule,gasconcentration,andmechanicaleffect,areinvadedtochangetheRIofdefectedholes,theycanalsoinducetheshiftofresonantwavelength.Thus,varioushigh-sensitiveandultra-compactopticalsensorsbasedonPCCsaredesignedandproposed[33,36,38,39].
Inaddition,thedetectionlimit(C)ofthePCCbasedRIsensorcanbecalculatedbythemeasurementsensitivity(S)andtheminimalresolvablewavelengthshiftofPCC(????min),anditcanbegivenby:C=????min/S.ForPCC,therelationshipbetween????minandqualityfactorQfollows:????min=??0/(10Q)[41,42].Therefore,wehave
(ω?ω0)2+ω0/2Q
C=
??0(10QS)
whereω0istheresonantfrequency,QisthequalityfactorofthisPCCandcanbegivenas:
Q=
??1
ω
+
??1?1
r
Fromthisequation,wecanseethathighRIsensitivityandhigh
qualityfactorarerequiredforPCCtoimprovethesensingproper-tiesofPCCbasedsensors.
=
QωQrωr
whereQωisthelifetimeoflighttodecayfromthecavityintothewaveguide,andQristhelifetimeoflighttoradiatefromcavityintothesurroundingair.
Duetotheuniquepropertiesofstrong?eldcon?nementandhighQfactorofPCC,theresonantwavelengthofPCCishighlysen-sitivetotheambientvariations.Specially,theresonantwavelength??0(??0=a/ω0)ofPCCwillshiftwiththeRIvariationofdefectedholesofPCC,asshowninFig.4.SomewhatliketheFabry–Perotcavity,theshiftofresonantwavelengthsatis?es
????0=S×??n
whereSisnearlyconstantwhentheresonantwavelengthchangeswithinasmallrange,anditisalsothemeasurementsensitivityofPCCbasedRIsensor.Ifsomeotherparameters,suchasbiochemical
Fig.4.TransmissionspectraofPCCwhentheRIofdefectedholesischangedfrom1.330to1.350withanintervalof0.005[36].
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TableI
ComparisonofdifferentPCCsthatusedforRIsensorsandtheirsensingproperties.
Ref.
Schematicstructure
Qualityfactor
Sensitivity(nm/RIU)
DetectionlimitC(RIU?1)
Experiment/simulation
Publishedyear
[43]400
200
0.002
Experiment
2004
[44]
3820
330
0.001
Simulation
2008
[45]
400
155
0.018
Experiment
2008
[45]3000
63
0.006Experiment
2008
[46]17,890
500
0.0001
Simulation
2013
[47]2966
131.7
3.797×10?6
Simulation
2014
[48]
107330
1.24×10?5
Simulation
2014
[49]
107160
8.75×10?5
Simulation
2015
[54]
50,000
1500
7.8×10?6
Experiment
2009
[55]
7500
370
2.3×10?5
Experiment
2013
[56]
25,000
235
1.25×10?5
Experiment
2014
3.OpticalsensorsbasedonPCCs
3.1.Refractiveindexsensor
Asdiscussedabove,PCCexhibitsstrong?eldcon?nementandhaslongphotonlifetime,whichgiverisetoanopticalmodewitharesonantwavelengththatishighlysensitivetoRIperturbationattributedtomediumthatin?ltratedintheairholesofPCC,andthusallowsustoimplementvariousRIsensorsbasedonPCC[43–49].ThemeasurementofRIchangeofglycerol–watermix-turebymonitoringtheresonantwavelengthshiftsinvariousratioswas?rstdemonstratedin2004byChowetal.[43].TheproposedPCCwasformedbyreducingtheradiusofonecentralhole,andhadaQfactorofabout400.ForRImeasurement,theproposedultra-compactsensor(sensingareaofabout10?m2)demonstratedameasurementsensitivityof200nm/RIU(refractiveindexunit)anddetectionlimitof0.002RIU.Besides,itwasconcludedthatthedetectionlimitcanbefurtherimprovedbyincreasingthemeasure-mentsensitivityandtheQfactorofPCCandreducingthenoiselevelofthemeasurement.Accordingtothisprinciple,variousoptimizedPCCshavebeendesignedtofurtherimprovethesensingpropertiesofRIsensor.TableIsummarizedthestructuralschematicsofPCCsandtheircorrespondingsensingproperties.
Atthesametime,anewtypeofwaveguide,i.e.slotphotoniccrystalwaveguide(SPCW),wastheoreticallyproposedandexper-imentallydemonstrated[50].Itisawaveguideformedbyopening
aslotalongthelinewaveguideofPCW,andithastheuniquecharacteristicofguidingandcon?ninglightinthelowRInarrowslotwithstrong?eldenhancement[51].Duetotheelectric-?elddiscontinuityandhighRIdifferenceattheinterfaceofsiliconandlowRIslot,thecavitymodeinsidetheslotcanbegreatlyenhanced[52].In2008,Yamamotoetal.[53]proposedanovelPCCinSPCW,inwhichthecavitywasformedbylocallymodifyingafewofairholesthatadjacenttothewaveguide.ThesimulationresultsdemonstratedthatthehighQfactorashighas2×105couldbeobtained.Thenin2009,DiFalcoetal.[54]haveexperimentallydemonstratedthatthehighQfactorofupto50,000andstrongRIsensitivityof1500nm/RIUcouldbeobtainedintheslotPCC,whichwasformedbyvaryingthepitchofthesurroundingPCalongtheslotwaveguideaxis.Lateron,someotherslotPCCs[55,56]wereproposedanddemonstratedfortheirbetterapplicationsinrefrac-tiveindexsensors,andtheircorrespondingsensingpropertiesarealsosummarizedinTableI.
TheaboveresultshavedemonstratedthatPCCswithstrongopti-calcon?nementandhighQcanbewellusedforRImeasurement,alongwiththewavelengthshiftofresonantpeak.Themeasure-mentresultscanalsoprovideguidanceforsomeothersensors,suchasbiosensors,chemicalsensors,mechanicalsensorsandgassen-sors,inwhichthechangeofthesemeasurementparameterscanallbeconvertedintoRIvariations.AsforRIsensor,therearealsosomeotheropticalsystems,suchassurfaceplasmaresonance(SPR)[57],modalinterference[58],evanescentwave[59],andFabry–Perot
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TableII
ComparisonofthepresentedopticalsystemsthatusedforRImeasurement.
Opticalsystem
Detectionlimit(RIU?1)
Sensitivity(nm/RIU)
Advantages
Disadvantages
Ref.
PCC
7.8×10?6
1500
SPR
5×10?6
2000
Compactness;Integration;
Easytodemodulate;
FlexibleinstructuraldesignHighsensitivity;Good?exibilityandextensibility
Largecouplingloss;
Temperaturecross-sensitivity;Dif?cultyinfabrication
[54]
Modalinterference
1.74×10
?6
580
Evanescentwave
10?6
700
Lowcost;
Easytofabricate;SimplestructureGood?exibilityandextensibility
F-Pcavity
1.64×10?5
670,000
Lowcost;
Simplestructure
Dif?culttofabricate;
Temperaturecross-sensitivity;Workinginnon-communicationwavelengths(most)
Temperaturecross-sensitivity;Interferencesofmultiplemodes(non-linearoutput).Lackofrobustness;Lowtransmittance;
In?uencetolightintensity?uctuationsUneasytocontrolcavitylength;Temperaturecross-sensitivity;
Lacksof?exibilityandextensibility;Dif?culttodemodulate
[57]
[58]
[59]
[60]
(F–P)cavity[60].InTableII,wehavesummarizedthebestvaluesofdetectionlimitandcomparedtheadvantagesanddisadvantagesoftheseopticalsystemsfortheirapplicationsinrefractiveindexmeasurement.
3.2.Biochemicalsensor
AstheconcentrationofbiochemicalsampleisdirectlylinkedtotheRIoftargetanalyte,nowthecommerciallyavailablebio-chemicalsensorsusuallyexploittheRIchangeinducedbytheanalyteinteractionwiththeoptical?eldasthesensingmechanism.Throughthismeans,thetargetanalytescanbedetectedintheirnat-uralformswithoutanymodi?cations.AlongwiththedevelopmentofPCCbasedRIsensors,manyPCCbasedbiochemicalsensorsweresubsequentlyproposedanddemonstrated.In2005,themeasure-mentsofcationandanionconcentrationsweredemonstratedby
usingaL4PCCcoatedwithion-selectivepolymer[61].Besides,itwasshowedthatincreasingthelengthofcavitywouldenhancetheQofcavitybyanorderofmagnitudeandthenwouldimprovetheshiftofresonantwavelengthwhileretainingcompactsizecharac-teristic[62,63].Thenin2007,itwasreportedthataPCCresonatorwithonlyonespotdefectcouldabletodetectproteinmoleculeassmallas2.5fg,whiletheactivesensingvolumecouldbedownto0.15?m2[64].Besides,thisstructurecouldalsobeusedtodetectagoldnanoparticlewith10nmindiameter[65]andanti-biotinwithconcentrationof20pM(correspondingtolessthan4.5fgofboundmaterialonthesensorsurfaceandfewerthan80moleculesinthemodalvolumeofthecavity)[66].Latterin2010,Hsiaoetal.[67]?rstdemonstratedthattheringPCC(seeFig.5(a))couldalsobeusedforhigh-sensitivemonitoringofreactionkineticsandproteinconcentrationwiththeminimumdetectablebiomoleculeweightofonly0.2fg,whichshowedpromisingapplicationswhen
Fig.5.Schematicstructuresof(a)ringPCC[60]and(b)cascadedringPCC[61]thatusedforbiochemicalsensors,andSEMimagesof(c)H0cavity[63]and(d)slotPCC[65]thatusedforbiochemicalsensors.
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Fig.6.StructureofheterostructurePCC(a),thecorrespondingtransmissionspectraforvacuum,nitrogen,SF6gases(b),andtherelationshipbetweenresonantwavelengthshiftandsurroundingpressure(c)[66].
thedetectionofbiomoleculedowntothelevelofsinglecopyofDNAwasneeded.Inthisdesign,theringPCCwasformedbyinte-gratingterminalwaveguides,i.e.,linedefects,andahexagonalringwaveguide,i.e.,ahexagontracedefect,ina220nmthickdevicelayerofsiliconwafer.Thelatticeconstantwasde?nedas410nmandtheradiusofholeswassetas120nm.Inaddition,whentworingPCCswerecascadedtogether(seeFig.5(b)),itwouldbepos-sibletodetecttwokindsoftargetDNAmoleculesorrealizeatemperaturecompensatedbiosensor[68].Amajoradvantageofthisstructurewasthatitallowsthemeasurementsofmultiplebiomoleculesatthesameinputportthroughtheuseofappro-priatesensingholesandoffersthepossibilitytoimplementthecorroborationmechanismbyexchangingtheinputandoutputports[69].Beyondthesestructures,Paletal.proposedaH0PCC(seeFig.5(c)),whichcouldbeusedforthemeasurementoflgGmoleculewithdetectionlimitof1.5fg[70]andHumanPapillo-mavirusvirus-likeparticles(VLPs)withdetectionlimitof1.5nM[71].Inthedesign,thedefectholehadaradiusof0.2a,andthesur-roundingairholeradiiwere?xedat0.3a.AseachPCCsensorcouldpotentiallybefunctionalizedwithdifferentreceptormolecules,itwasalsopossibletodetectmultiplepathogenicviruses,or,alter-natively,differentstrainsofthesamevirus,onthesamechip.Combinedthespatialcon?nementofoptical?eldprovidedbyslotwaveguidewiththetemporalcon?nementofoptical?eldinPCC,Scullionetal.[72]?rstdemonstratedthepossibilityofslotPCC(seeFig.5(d),latticeconstant490nm,cavityperiod460nm,holeradius135nmandslotwidth120nm)inthedetectionofdissolvedavidinconcentrationaslowas15nM,withthesensingareaofonly2.2?m2.
Fromtheaboveresults,wecan?ndthatwiththeappropri-atechoiceofreceptorthatin?ltratedintheairholesofPCC,thePCCsensorcouldbeadaptedtodetectanybiochemicalmolecules,whoseperformancescanbefurtherimprovedbydesigningcavitieswithhigherQfactorsandbylocalizingthetargetmolecularrecog-nitionprocessesinthedefectregion.Thesmallsizeofthedevice,combinedwithstrongsuccessfulimplementationofmultiplePCCs,providesastrongpotentialforlargearraysofindependentsensorsonacentimetersizedchip.
3.3.Gassensor
TheperiodicairholemicrostructureofPCCisanaturalcandi-dateforhousinggasanalytes,thus,theresonantwavelengthofPCC,aswellastheRIofairhole,wouldchangewiththeconcentrationvariationofin?ltratedgasorthevariationofambientpressure.ThisisalsothemeasurementprincipleofgassensorbasedonPCC.Com-paringwiththetraditionalopticalgassensor,thesizeofPCCbasedgassensorcouldbedrasticallyreduced.
Oneexampleisthemeasurementofgasconcentrationinagasmixtureoftwogaseswithdifferentrefractiveindicesorrelativegaspressures[73,74].InRef.[73],aheterostructurePCCwasformedbymodulatingtheradiiofthe?rstrowofairholesadjacenttothewaveguide(seeFig.6(a)),whichresultedinasensitivityof80nm/RIUandqualityfactorof380,000.AsshowninFig.6(b),thisPCCstructurecouldbewellusedtoidentifyvacuum,nitrogen,andSF6.Besides,whenthepressureforSF6atmospherewaschangedinastepof0.5×104Pa,anobviouswavelengthshiftofthePCCwouldbeobservedasshowninFig.6(c).Inthisdesign,theradiiofsomeholeswereenlargedto0.27aalongtheW1waveguidetocreatemirrorregions(markedwithA).Theactualcavity(markedwithC)isenclosedbetweenthosemirrorregions,whoseradiiwere0.25a.BetweenregionAandregionC,theradiioftheholeswereincreasedlinearly(regionB).
Thenin2010,Jágerskáetal.[74]improvedthemeasurementsensitivityofgasRIbyintroducingaheterostructureslotPCC.Thecavitylengthwasde?nedtobeL=3a,andtheresonantstateofwhichwasfoundbetweenthecutoffsofthe120and100nmwideslotwaveguides,asshowninFig.7(a).Asforthemeasurementsofhelium,nitrogen,andcarbondioxide,anexperimentalsensitivityupto510nm/RIUandthedetectionlimithigherthan1×10?5RIUweredemonstrated.However,theheterostructurePCCneedstobecarefullyoptimizedand?nelytunedinordertoachieveultrahighQ,andasaresultithasalowtolerancetofabricationdeviations.Recently,Lietal.[75]proposedandexperimentallydemonstratedaseriesofLnslotPCCs(seeFig.7(b)),whichoperatedasgassen-sors.Finally,thequalityfactorexceeding30,000,sensitivityupto421nm/RIU,anddetectionlimitdownto1×10?5RIUwere
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Fig.7.StructuresofheterostructureslotPCC[67]andLnslotPCCs[68]forgassensing.
experimentallydemonstrated.Thesimplestructureandhighfab-ricationtoleranceofthisPCCextendeditsapplicationsinopticalsensors.ButitshouldbementionedthattheRIoftargetgasisalwayssmall(～1.0)andthecorrespondingRIvariationduetocon-centrationchangeisusuallylowerthan10?4RIU,sotheabovemethodscannotbeusedtoidentifytheconcentrationoftargetgas.Besides,asanyothergasesandenvironmentparameterscanallresultintheRIvariationofairhole,theywouldbringmanyunpre-dictableerrorstothemeasurementsystem,andevencausethesystemunabletowork.
Toresolvetheseproblemsandplaytheadvantagesofultra-compactandhigh-sensitivityofPCCbasedgassensor,Zhangetal.[19]?rstproposedagasconcentrationsensorwithacryptophaneEin?ltratedPCC(seeFig.8(a)).Inthisdesign,thelatticeconstantwasa=351nm,theradiusofbulkairholewasr=0.3a,andthethicknessofthePCslabwash=0.6a,thedefectedradiuswasr1=0.45a.TheconcentrationvariationofmethanewouldchangetheRIofcryp-tophaneEthatin?ltratedinthedefectedholesofPCC,andtheninduceashiftofresonantwavelength,allowingprecisionmeasure-mentofmethaneconcentration.BycombingselectiveadsorptionpropertyofcryptophaneEtomethaneandexcellentresonantprop-ertiesofPCC,theresonantspectrumofPCCwouldshiftsharplywiththeconcentrationchangeofmethanegas,asshowninFig.8(b).Asaresult,atheoreticaldetectionlimitof697.35ppmformethanesensingcouldbeachieved,whichprovidesanewdirectionforthegassensorbasedonPCC.
3.4.Mechanicalsensor
TheoperationprincipleofmechanicalsensorbasedonPCCisthephotoelastic,piezoelectric,andelectroopticeffectsofthematerialsconstitutingthePCCstructure.Whencertainmechanical
actionisappliedtothePCC,itwillinducetheeffectiveRIvariation,deformation,orde?ectionofthePCCstructure,whichwillthenmodifythetransmissionspectrumofthePCC,andthusshiftthecorrespondingresonantwavelength.Theamountofsuchspectralshiftcanbethereforeexploitedtomeasuretheappliedmechanicalaction.
In2007,Stomeoetal.[76]proposedaH1PCCforpressuresen-sor.Thedroppeakthatcorrespondedtotheresonantwavelengthofthemodelocalizedinthecavityshifteditsspectralpositionwithalinearitysensitivityof5.82nm/GPaforpressurerangingbetween0.25Gpaand5GPa.Consideringtheeffectiveactionareaoftheappliedpressurewasequalto1mm2,thedetectionlimitofabout0.3mNcouldbeobtained.Then,Leeetal.demonstratedthattheshoulder-coupledPCCinasuspendedsiliconbridgestructurecouldalsobeusedforpressuremeasurement[77].AsshowninFig.9(a),thelatticeconstantwas500nm,theradiusofallholeswas180nm,andtheinitialdefectlengthwas640nm.LongitudinaldeformationofairholesandachangeindefectlengthofthecavitycausedbyappliedpressurewouldallshifttheresonantwavelengthofPCC.Itwasconcludedthattheminimumdetectableforceandthemin-imumdetectableverticalde?ectionwere0.25Nand20–25nm,respectively.Besides,itwasdemonstratedthattheshapechangeoftheairholesinthedeformedPCCstructurehasjustalittleeffectontheoutputresonantbehavior.Moreimportantly,therelativeposi-tionshiftoftheseairholesinthedeformedPCCplaysamajorroleincontributiontotheoutputresonantbehavior.Uponthisstruc-ture,Leeetal.investigatedthemechanicalpropertyofthisPCC,byusingasiliconcantilever(seeFig.9(b))[78,79].Inthegraphofstrainversusresonantwavelengthshift,aratherlinearrelationshipwasobservedevenfordifferentcantilevers.Fora30?mlongand15?mwidecantilever,thedetectionlimitsforstain,verticalde?ec-tionatthecantileverend,andforceloadwere0.0133%,0.37?m,and0.0625N,respectively.Latterin2011[36,80],theyfurther
Fig.8.SchematicstructureandthecorrespondingtransmissionspectraofcryptophaneEin?ltratedPCC[69].
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Fig.9.Structuresofshoulder-coupledPCCin(a)suspendedsiliconbridge[71]and(b)siliconcantilever[72]formechanicalsensing.
investigatedsiliconcantileverwithringPCCasthemechanicalsen-
sor.TheringPCCwasformedbyaregularhexagonalarrayofair
holeswithlatticeconstantof410nmandholesradiiof120nm.
InRef.[80],itwasdemonstratedthatthesensingsensitivityof
ringPCC(seeFig.10(a))couldbegreatlyimprovedwhencom-
paredwithshoulder-coupledPCC.Finally,theminimumdetectable
forceandstrainforringPCCwere75.7nNand0.0023%,respec-
tively.TheninRef.[36],tworingPCCswereintegratedonthesiliconcantilever(seeFig.10(b))formechanicalsensing.ByinvestigatingthesensingcharacteristicsofdualringPCCsatvariouspositionsadjacenttothejunctionofthecantileverandthesubstrate,themin-imumdetectableforceaslowas7.58nNwasobtained.Inthesameyear,theyproposedcircularSidiaphragmintegratedwithtripleringPCC(seeFig.10(c))asthemechanicalsensor[81],whichgivenminimumdetectableforceof0.847?Ninthewideforcerangeof10?Nto20?
N.
Fig.10.StructuresofoneringPCC[74],dualringPCC[33],andtripleringPCC[75]formechanicalsensing.
382Y.-n.Zhangetal./SensorsandActuatorsA233(2015)374–389
Fig.11.(a)StructureofH0slotcavityfordisplacementsensing[77]and(b)structureofPCCintegratedwithcantileverforstresssensing[78].
Atthesametime,manydifferentPCCstructureswereproposed
bysomeotherresearchteamstostudythemechanicalproperties
[82–84].InRef.[82],Tungetal.demonstratedthatthetheoretical
minimumdetectablestrainforaL3PCCwas8.5n?.InRef.[83],Yang
etal.studiedthepropertiesofH0slotcavity(seeFig.11(a)).With
thestructuralparametersofradiusr=0.32a,slotwidthwslot=0.45a,andlatticeshiftsx=0.2a,thesimulationresultsdemonstratedaquasilinearmeasurementofmicrodisplacementwithasensitivityof1.0a?1.InRef.[84],Maoetal.realizedtheminimumsurfacestressof0.8mN/mbyintegratingacantileverinsideaPCC(seeFig.11(b)).AndinRef.[38],Yangetal.proposedashoulder-coupledPCC,asshowninFig.12(a),whoselatticeconstantwas385nm,radius
Fig.12.Structuresofshoulder-coupledPCC[35],piston-typedPCC[79],andmodularPCC[80]forforcesensing.
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383
Fig.13.Schematicstructureandthecorrespondingsensingpropertyofpolymerin?ltratedPCC.
ofairholeswas0.3a,thicknesswas0.56a,andshiftofholeswas0.2a.SimulationresultsdemonstratedthattheproposedPCCcouldbeusedforthestressdetectioninbothhorizontalandverticaldirections,withdetectionlimitsof58nNand44nN,respectively.However,ifthemechanicalactionsaretoolarge,thedeforma-tionorde?ectionofPClatticestructurewillbeserious[38].Asaresult,theresonantconditions(suchasshapeofresonantspec-trum,Qfactor,andtransmittance)ofPCCwillbechanged,whichwillin?uencethesensingproperties(sensitivity,linearity,detec-tionlimit,stability,sensingrange,etc.)ofthemechanicalsensorandevencausethesensorunabletowork[77].Toresolvethisprob-lem,in2013,Yangetalproposedatorsion-freepressuresensorbasedonapiston-typedH0cavitystructure,asshowninFig.12(b)[85].Here,theH0cavitywasachievedbyaddingananoholeinthecenterofdefectedhole.Inthedesign,theradiusofairholeswasr=0.32aandthelatticeconstantwasa=425nm.Finally,thesensitivityashighas0.50nm/nNwasobservedandthedetectionlimitwasestimatedtobeassmallas0.68nN.Combiningtheadvan-tagesofshoulder-coupledPCCinRef.[38]andpiston-typedH0PCCinRef.[85],Yangproposedathreedimensionalforcesensor(seeFig.12(c))[86],inwhichtheshoulder-coupledPCCwasusedtodetecttheforcesinthehorizontalandverticaldirections,andthepiston-typedH0PCCwasusedtodetecttheforceintheuprightdirection.Finally,bydesigningandoptimizingthisnovelPCCstruc-ture,highsensitivitiesof8.2nm/?N,12.5nm/?N,and10.9nm/?Nwereobtainedinthehorizontaldirection,verticaldirectionanduprightdirection.Correspondingly,thedetectionlimitsforthreedirectionswere24nN,16nN,and18nN,respectively.ThisnovelsensingmechanismcreatesanewvisionofPCCbasedmechanicalsensors.
ofpolymerandmagnetic?eldsensorbasedonmagneto-opticaleffectofmagnetic?uid.In2011,Yangetal.[87]proposedanelectro-opticsensorbyusingaH0cavityin?ltratedwithnonlin-earopticalpolymer.AsshowninFig.13(a),thetriangularlatticePCwasrealizedonasiliconslabwithairholesin?ltratedwithpoly-mer(npolymer=1.6),andhavinglatticeconstantof403nm,radiusof128.96nm,slabthicknessof221.65nm,andlatticeshiftof80.6nm.Besides,thereweretwomicro-electrodesthatwereplacedoneachsideofPCslab.Ifthedrivingvoltagevaried,theRIofpolymerwouldbechangedaccordinglybecauseofthePockel’seffect,whichwouldinturnshifttheresonantdropofH0cavity,asshowninFig.13(b).Finally,thelinearmeasurementsensitivityof31.9nm/Vwasobtained.Coincidentally,in2015,Zhaoetal.[88]demonstratedamagnetic?eldsensorbyusingacascadedH0cavityin?ltratedwithmagnetic?uid.AsshowninFig.14,twoPCcavitieswithr1=0.32aandr2=0.30awereintegratedonamonolithicsiliconsubstrateandside-coupledtoasinglePCWtoformcascadedPCcavities,inwhichthetwodefectedregionswerein?ltratedwithtwodifferenttypesofmagnetic?uid.ThetworesonantdipsoftwoPCcavitieswereindependentofeachother,andashiftinoneofthemdidnotperturbtheother.Thisallowedtheimplementationoftwoindividualsensorsunderthesameenvironment,andeventu-allyrealizedthesimultaneousmeasurementofmagnetic?eldandtemperature.Theimplicationwhichwecandrawfromtheabovetwosensorsisthattheapplication?eldofPCCbasedsensorcanbefurtherextendedbyin?ltratingsomesensitivematerialsintoairholesofPCC.
4.KeyproblemsofPCCsforsensingapplications
3.5.Othersensors
Inadditiontotheabovesensingapplications,thePCCcanalsobeenusedforelectricvoltagesensorbasedonelectro-opticaleffect
AlthoughtherapiddevelopmentandgreatpotentialofPCCbasedopticalsensor,therestillexistsomekeyproblemsforitsprac-ticalapplications,suchasfabricationerrors,couplingproblem,andtemperaturein?uence.
Fig.14.SchematicstructurecascadedH0PCCwithmagnetic?eldin?ltration[88].
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4.1.Fabricationerrors
Inactualimplementations,thestabilityofresonantpropertiesofPCCagainstfabricationerrorsisasigni?cantfactor.AtthecurrenttechnologyofPCfabrication,thepositionandsizeofairholecanbecontrolledinthelevelof1nm[89]and2–4nm[90],respectively.Pergandeetal.[91]demonstratedthattheoveralltransmissionofbulkPCwaslimitedby?uctuationsoftheholesradii.Besides,aporeradius?uctuationof1%wouldleadtotheattenuationinthetransmissionof15dB/mm.Sincetheinteractionstrengthbetweenoptical?eldandmaterialsofdefectedregionisrelativelystrong,thefabricationerrorsofcavityregionofPCCwillhavemuchmoresigni?cantimpactsontheresonantpropertiesofPCC.Forexample,Haginaetal.[92]demonstratedthattheultimateQofheterostruc-turePCcavitywouldreducetooneeighthofidealonewhenonly1nmerrorwasintroducedtotheholesradii.Besides,thefabrica-tionerrorsofPCWarecomplicated,random,unpredictable,andimmeasurable,sohowtoeaseoffabricationandobtainachievabletolerancesareimportantconsiderationsinfuture.
4.2.Couplingproblem
BeforethepracticalapplicationsofPCC,onesubsequentchal-lengeisthedif?culttoef?cientlycouplelightfromconventionalsinglemode?berintoPCCdevice[63].Todecreasethecouplingloss,themostcommonmethodistointroducePCWsonbothsidesofPCC[38].Then,thelightis?rstlyemittedfromconventionalsin-glemode?bertoPCW,andthentransmittedthroughthePCWtoPCC.However,itremainschallengingbecausethetypicalwave-guidecross-section(<1?mwidthand<500nmthickness)makesitdif?culttocouplelightinandfromthe?bercoreofconventionalsinglemode?ber(8–10?mdiameter).Additionaldif?cultiesareencounteredduetothedifferenttransmissionprinciplesofPCW(basedonPBGtheory)andconventionalsinglemode?ber(basedontotalinternalre?ection).Ingeneral,thecouplinglosscanbegreatlydecreasedbyaccuratealigningofconventional?berwiththePCdevicethroughanadjustablemechanicaldeviceandproperdesigningofthecouplinginterface.
4.3.Temperaturein?uence
Asweknow,therefractiveindexofsiliconisthermaldepend-ence,whichmeansthattheresonantpropertiesofsiliconbasedPCCdeviceareeasilyin?uencedbyexternaltemperature[21].Toprohibittheundesiredvariation,precisiontemperaturecontrolsystemisessentialinpracticalapplication,whichwillthenaddthesizeandcostofthePCCbasedsensors.Thiseffecthadnotbeentakenintoaccountinthepreviousresearchesuntil2009,whenKarnutschetal.[93]proposedatemperature-insensitivePCCbasedontheopto?uidictechnology.Bydesigningsuitabledimensionsandusingaliquidwithanappropriatethermo-opticcoef?cient,itwasdemonstratedthatthethermo-opticeffectofthein?ltratedopto?uidiccouldminimizeoreveneliminatethetemperaturedependenceofthedeviceforadesiredworkingrange.However,theutilizationofopto?uidicin?ltratedPCCmightalsodecreasethe?exibilityofPCCinstructuraldesign.Therefore,howtodecreasethetemperaturein?uenceisalsoacriticalprobleminfuture.
5.NewdirectionsofPCCsforsensingapplications
5.1.Opto?uidiccontrolledPCC
Asdiscussedabove,thesensingpropertiesofPCCbasedsen-sorsareincloserelationwiththeQfactorofPCC.Currentmethodstorealizehigh-QPCCstypicallyrelyonextremelyprecisecontrolofsizeandpositionofairholesontheorderofnanometer,bothof
whicharehardtoachieveprecisely.Therefore,thenanometer-scaleprecisionrequiredtorealizesophisticatedandoptimizedstruc-tureseventuallybecomesalimitingfactorinachievinghigh-QPCCs,aspointedoutbyAsanoetal.[94].Besides,theworkingwavelengthofPCCcanonlybelocatedatacertainvalueandcannotbevar-iedoncefabricated,whichwilllimittheapplicationsituationsandfurtherdevelopmentsofPCCbasedsensors.
Veryrecently,thereisanewphotonicbranchtointegratenanophotonicsonthemanipulationofphotonsatthescaleofopti-calwavelengthwithmicro?uidicsonthecontrolof?uidsatthemicronscale,whichde?nesamajorpartofopto?uidics?eld[95].Besides,thein?ltrated?uidspossessawiderangeofrefractiveindices(from1.33forwaterbasedsolutionsto1.5forsilicaoilmatching?uidsandtoabove1.8forCargille?uids),whichareespe-ciallyeffectiveintuningphotonicstructuresbeyondthataccessiblethroughin?ltrationofsolidmaterials.Demonstrationsofopto?u-idicdevicesexploitthecharacteristicsof?uidstoachievedynamicmanipulationofopticalpropertiesandrevealthepromisefortheirwidespreaduse[96].Thisprovidesapotentialtechnologytoreal-izehigh-sensitiveopticalsensors,andoffersa?exiblemeanstowrite,tuneorrecon?gurephotonicdevicesforaswatheofapplica-tions[97].Inthisregard,ithasbeenshownthatPCsingeneralandPCCsinparticularcanbeadvantageouslyexploitedwithinopto?u-idicarchitectures[98–100].Inaddition,boththeamountofliquidandthelocationoftheselectivelyin?ltratedareacanbeaccuratelycontrolledbyusinganintegratedopto?uidiccircuitbondedontothelithographicmasking[101],aconfocallaserscanningmicro-scopeequippedwithamicro-in?ltrationsystem[96],oracomputercontrolledmicropipette[102]whosesizeiscomparabletotheairholes.Byexploitingtheinherent?exibilityof?uidin?ltration,theopto?uidicin?ltrationschemesnotonlyofferthepotentialforrealizingtunableandrecon?gurablePCCsatwillwithnoneedofstructuralvariation,butalsoprovidethe?exibilitytocreatespa-tiallyprogrammablePCCsaccordingtopracticalrequirements.Inthiscase,thein?ltrationof?uidsintotheairholesofPCCdeviceshasbeenpopularlyinvestigatedanddemonstrated.
Tomljenovic-Hanicetal.[103]designedaheterostructurePCC(seeFig.15(a))withoutchangesofanystructuralparameters.ItwasnumericallydemonstratedinFig.15(b)thattheQfac-torvalueofthisdesignachievablebysubstitutingtheairintheholeswithpolymermaterialsorliquidcrystalswashigherthanQ=9.7×105.Thisapproachrepresentsanoveltechniqueforcre-atingultrahigh-QPCCsandfurthermoreopensthepossibilityofpost-processinginPCCs.Thenin2007[102],thismethodwasexperimentallyinvestigatedviaevanescentprobingfromatapered?berattelecommunicationwavelengths.ResultsdemonstratedaPCCwithQfactorof4300,whichdidnotrequirenanometer-scalealterationsinstructuralgeometriesandmaybeundertakenatanytimeafterthePCCfabricated.Thespectralandspatialrecon-?gurabilityoftheproposedheterostructurePCCswerefurtherdemonstratedin2008[104],whichshowedhighQfactor(≈10,000)resonancesforabroadrangeofcavitylengths.Atthesametime,Bogetal.[105]alsodemonstratedapost-processedheterostructurePCCbyselective?uidin?ltrationofairholesusingaglassmicrotip,whichresultedinahigherintrinsicQfactorof57,000inexperi-ment.Besides,Bedotaetal.[106]haveexploitedthein?ltrationandevaporationdynamicsoftheliquidcrystalwithinthisheterostruc-turePCCbyusingaFabry–Perotmodelthataccountedforthejointeffectsofliquidvolumereductionandcavitylengthvariationduetoliquidevaporation.Itwasdemonstratedthattheevaporationtimeisproportionaltothevolume-to-surfaceratio,andthereforeroughlyscaleswiththelineardimensionofthesystem.Besidestheaboveheterostructurecavities,in2009,Intontietal.[107]studiedthespectraltuningmechanismofpoint-defectedPCCbycontrolledremovaloflocallyin?ltratedwater.Themicro-in?ltrationwithwaterofoneorfewcavityholesanditssubsequentcontrolled
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Fig.15.StructureandresonantpropertiesofheterostructurePCCformedbyopto?uidicin?ltration[90].
evaporationprovidedthepossibilitytolocalandcontinuoustunethecavityresonancesinaspectralrangelargerthan20nm.Besides,itwasalsodemonstratedthattheadditionofwaterinthemicrocav-ityregioncouldimproveitsQfactor.Andin2012,Speijckenetal.[100]demonstratedtheinsituopto?uidiccontrolofrecon?gurablePCC,inwhichanextremelylowvaporpressureoilwasusedtoavoidevaporationissueatroomtemperature,andthein?ltratedoilcouldbeselectivelyremovedfromthedefectbyincreasingthepoweroftheexcitationlaserandsystematicallymovingthefocusposi-tion.Beyondthat,Kamutschetal.[93]havedemonstratedthatthispost-fabricationtechnologycanalsobeusedforthetemperaturestabilizationofPCCdevice.Thekeyprinciplebehindthisopto?u-idictemperaturestabilizationwastheconceptthatasubstancewithnegativethermo-opticcoef?cientbalancedthethermaldriftofthehostPCmaterial.Thistemperature-stablecavityconstitutesamajorbuildingblockinthedevelopmentofhigh-sensitivesensorsystemsforchemicalandbiomedicalapplications.
Fromtheabovestudies,wecanconcludethatthereversibil-ityofopto?uidicin?ltrationcombinedwiththedegreeoffreedominthe?uidchoiceownedthefollowingthreeadvantages:First,theopto?uidicPCCsarerealizedduringthein?ltrationstep,whichcanrelaxtheconstraintonthefabricationprecisionascomparedtopreviousstructure-basedschemesforpropertyoptimizationofPCCs.Second,thesecavitiescanbecon?guredina?exibleman-nerbyalteringthesizeorpositionofthein?ltratedregionaswellastheRIoftheliquid,offeringa?exible,ef?cientandversatilemethodforthepost-engineeringandrecon?guringofPCCs.Third,thesecavitiesexhibithighQfactorsdespitethepresenceofa?uid.Theversatilityand?exibilityoftheopto?uidictechnologypresentgreatpotentialforthecontrolsofPCCbaseddevices,andtheresultsofferperspectivesforincorporatingPCCdevicesinsensingcircuits.Ifthestructureis?lledwithliquidcrystal,electro-opticornonlin-earpolymer,thereisalsothepossibilityoftuningthesestructureswhenexternalvoltageisapplied.
5.2.CascadedPCC
Asdiscussedabove,PCCsensorswithhighQfactorandsmallvolumecanenhancetheinteractionbetweentheanalyteandinci-dentlightandimprovethesensitivitytobulkproperties.However,mostofthesesensorstypicallyoperateaspointorsinglesensorandthenumberoftargetswhichcanbesensedatonetimeisrelativelysmall.Toovercomethesedrawbacksandrealizemul-tiplesensingsites,manysensorarraysbasedoncascadedPCCshavebeendeveloped[33,70,108–112].In2011,Paletal.[70]pro-posedamultiplexedlgGsensor,inwhichthreenanocavitycoupledwaveguideswereplacedinseries.Butthesensorvolumewastoo
largeandnotsuitableforsensingapplication.Whilein2012,Wangetal.[108]developedatheoreticalmodeloftheintegratedpar-allelself-collimationsensorarray.Butonlythreesensorscouldbeintegratedonthemonolithicplatform,whichresultsinalowintegrationdensity.Atthesametime,Yangetal.[109]theoreti-callyinvestigatedtheperformancesofnanoscalePCintegratedRIsensorarrayonmonolithicsubstratebyusingsomeH0cavitiesside-coupledresonantcavities(seeFig.16(a)).Theoutputresonantspectraofdifferentcavitieswereindependentwitheachother.In2013[85],thisstructurewasalsowellusedformultiplepressuresensors.Besides,Olyaeeetal.[110]havealsodemonstratedthatthesensorarraycouldalsoberealizedwhensomeH1cavitieswereside-coupledtoaPCW.However,themaindrawbacksoftheseside-coupledresonantcavityarrayswerethatthelargerthenumberofPCCsintegratedonthemonolithicplatform,thenarrowerthespac-ingoftheresonantpeaksofadjacentcavitieswas.Therefore,thesensingsignalofeachcavitymightinteractwitheachotherduetothecrosstalkinmulti-cavityparallelsensors.Andifthevaria-tionofoneoutputsignalcausedbythetargetparameterchangewastoolarge,theshiftofresonantwavelengthmightbegreaterthanthespacingofadjacentresonantcavities.Thiswillresultindif?cultiesinrecognizingthesensingsignalsfromdifferentcavities.
Toef?cientlyenhancetheintegrationdensityofsensorarrayandrestraincrosstalkofadjacentPCCs,Yangetal.[33]furtherpro-posedaPCparallelresonantcavitiesdesignin2014.ThedevicewascomposedofsomeH0cavitiesside-coupledtoparalleloutputwaveguidesofanoptimizedPCbeam-splitter.Atlast,theextinc-tionratioofthewell-de?nedsinglenotchexceeded30dB,whichallowedtheimplementationofsimplebutfunctionalPCintegratedsensorarray,andeventuallyofmorecomplexsensornetworks.Atthesametime,Liuetal.[111]proposedaradius-gradedPCsensorarray,wheretwoL3cavitiesandtwoH1cavitiesweremultiplexedandinterlacedonbothsidesofaW1PCWontheradius-gradedPCslab(seeFig.16(b)).AndHuangetal.[112]demonstratedalowcrosstalkring-slotarraystructureusedforlabel-freemultiplexedsensing.Theproposedsensorarraywasbasedonanarrayofthreering-slotandinput/outputlinedefectcouplingwaveguides(seeFig.16(c)).Eachring-slotcavityhadslightlydifferentcavityspac-inganddifferentresonantfrequency.Aboveall,thecascadedPCCcanfurtherimprovethecompactnessandintegrationofPCCbasedsensor.
5.3.SlowlightassistedPCC
Recently,slowlightwitharemarkablylowgroupvelocityhasrecentlyattractedwideattention,asitisregardedasapromising
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Fig.16.StructuresofcascadedPCCthatpublishedinRefs.[98–101].
approachfortime–domainprocessingofopticalsignalandspatialcompressionofopticalenergy[113].Inpracticalapplications,asthelight-matterinteractionsrelyonthestrengthoftheinterac-tionbetweentheoptical?eldandthematerial,manynonlinearphenomenawillbeenhancedunderthepresenceofslowlight,whichallowsustodesignminiaturizedandhigh-sensitivedevicesbasedonthis?eldenhancement[114–116].In2012,Hosseinibalametal.[117]proposedaslowlightassistedPCCforultracompact,lowpower,andhigh-sensitivebiosensor.Theproposedbiosensorwascomposedofahalf-ringcavitythatisside-coupledtoanopto?u-idicslowlightPCW(seeFig.17(a)).SimulationresultsinFig.17(b)
demonstratedthatthesensitivitytoRIchangescouldbeincreasedfrom77nm/RIUto293nm/RIUwhenslowlightwasintroduced.Thenin2013,Laietal.[118]demonstratedthatinphotoniccrys-talsensorswithaside-coupledcavitywaveguidecon?guration,groupvelocityofthepropagatingmodeinthecoupledwaveguideatthefrequencyoftheresonantmodeplayedanimportantroleinenhancingthesensitivity.InlinearL13PCCs,withnearlysameQfactorof7000,thesensitivitycouldbeincreasedfrom57nm/RIUto66nm/RIUwhenthegroupindexofthecoupledwaveguideincreasedfrom10.2to13.2.Therefore,itwasconcludedthatinside-coupledPCcavity-waveguidesensors,inadditiontotheQof
Fig.17.StructuresandthecorrespondingRIsensingpropertyofringPCCwhenslowlightisintroduced(byin?ltratingopto?uidicinthe?rsttworowsofairholes)andnotintroduced[106].
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theuncoupledPCCandtheopticalmodeoverlapwithanalyte,slowlightinthecoupledPCWalsocontributedtotheenhancedsensitiv-itiesofresonancemodes.ThisisapromisingmethodforenhancingthesensitivitiesofPCCbasedsensors.
5.4.FunctionalPCCwithopticalcoating
Inrecentyears,thedepositionofopticalcoatingwithnano-metricthicknesshasbeenshowntosigni?cantlyenhancethesensitivityandselectivityofanumberofopticalsensingsystemstocertainexternalparameters,suchasrefractiveindex[119],pH[120],gasconcentration[121],temperature[24],biochemicalmolecule[27],andhumidity[122].Wecanpredictthattheappli-cationrangesofPCCsensorcanbegreatlyextendedbycombiningopticalcoatingwithPCCtorealizethefunctionalPCC[121].Besides,thetechnologyofopticalcoatingcanalsoimprovethesensitivityandselectivityofopticalsensorsbasedonthefunctionalPCCwithopticalcoating.
6.Conclusion
ThereviewforthereportedworksandtheircorrespondingresultsdemonstratedthatthePCCshaveplayedveryimportantrolesintheopticalsensor?eldsandwillproduceasigni?-cantindustrialvalue.Foreachsensortype,thesensingprinciple,structureofPCC,andthecorrespondingsensingpropertiesweredescribedindetail.Besides,thenewdirectionsofPCCsforsensingapplicationswerealldiscussed.Fromwhich,thereaderswhoareinterestedinthis?eldcouldnotonlyseetheuniquepropertiesand?exibilitiesinstructuraldesignofPCCs,butalsobroadentheirthoughtsandburstoutsomenewsolutionstofurtherexploitthepotentialsofPCCsinultra-compactandhigh-sensitiveopticalsen-sors.WiththetechnologydevelopmentofPCfabrication,muchbetterdesignschemesofPCCswillbepresentedandmuchmorePCCbasedopticalsensorswillbeproposed.Inthefuture,thekeytechnologiesofPCCbasedopticalsensorswillbethecontrollability,network,integration,all?ber,real-timemeasurementsin?uidicenvironment,andtheexplorationsonnewmechanismsandnewmethods.
Acknowledgments
ThisworkwassupportedinpartbytheNationalScienceFoun-dationforDistinguishedYoungScholarsofChinaunderGrant61425003,theNationalNaturalScienceFoundationofChinaunderGrant61273059,theFundamentalResearchFundfortheCentralUniversitiesunderGrantN130604006andN140404021,andStateKeyLaboratoryofSyntheticalAutomationforProcessIndustriesunderGrant2013ZCX09.
References
[1]E.Yablonovitch,Inhibitedspontaneousemissioninsolid-statephysicsand
electronics,Phys.Rev.Lett.58(1987)2059–2062.
[2]S.John,Stronglocalizationofphotonsincertaindisorderedphysicsdielectric
superlattices,Phys.Rev.Lett.58(1987)2486–2489.
[3]A.Scherer,O.Painter,J.Vuckovic,etal.,Photoniccrystalsforcon?ning,guid-ing,andemittinglight,IEEETrans.Nanotechnol.1(1)(2002)4–11.
[4]J.D.Joannopoulos,P.R.Villeneuve,S.Fan,Photoniccrystals:puttinganew
twistonlight,Nature386(6621)(1997)143–149.
[5]J.D.Joannopoulos,S.G.Johnson,J.N.Winn,etal.,PhotonicCrystals:Molding
theFlowofLight,seconded.,PrincetonUniversityPress,Princeton,2008.[6]M.Notomi,Stronglightcon?nementwithperiodicity,Proc.IEEE99(10)
(2011)1768–1779.
[7]M.Arjmand,R.Talebzadeh,Optical?lterbasedonphotoniccrystalres-onantcavity,Optoelectron.Adv.Mater.RapidCommun.9(1–2)(2015)32–35.
[8]X.Ge,Y.Shi,S.He,Ultra-compactchanneldrop?lterbasedonphotoniccrystal
nanobeamcavitiesutilizingaresonanttunnelingeffect,Opt.Lett.39(24)(2014)6973–6976.
[9]J.M.Brosi,C.Koos,L.C.Andreani,etal.,High-speedlow-voltageelectro-optic
modulatorwithapolymer-in?ltratedsiliconphotoniccrystalwaveguide,Opt.Express16(6)(2008)4177–4191.
[10]Y.Gao,R.J.Shiue,X.Gan,etal.,High-speedelectro-opticmodulatorintegrated
withgraphene-boronnitrideheterostructureandphotoniccrystalnanocav-ity,NanoLett.15(3)(2015)2001–2005.
[11]K.Fasihi,High-contrastall-opticalcontrollableswitchingandroutinginnon-linearphotoniccrystals,J.LightwaveTechnol.32(18)(2014)3126–3131.[12]K.Cui,Q.Zhao,X.Feng,etal.,Thermo-opticswitchbasedontransmission-dip
shiftinginadouble-slotphotoniccrystalwaveguide,Appl.Phys.Lett.100(20)(2012)201102(1–4).
[13]C.Y.Lin,H.Subbaraman,A.Hosseini,etal.,Siliconnanomembranebasedpho-toniccrystalwaveguidearrayforwavelength-tunabletrue-time-delaylines,Appl.Phys.Lett.101(5)(2012)051101(1–4).
[14]Y.Zhao,Y.Zhang,Q.Wang,Researchadvancesofphotoniccrystalgasand
liquidsensors,Sens.Actuators,B:Chem.160(1)(2011)1288–1297.
[15]R.V.Nair,R.Vijaya,Photoniccrystalsensors:anoverview,Prog.Quantum
Electron.34(3)(2010)89–134.
[16]C.Fenzl,T.Hirsch,O.S.Wolfbeis,Photoniccrystalsforchemicalsensingand
biosensing,Angew.Chem.Int.Ed.53(13)(2014)3318–3335.
[17]W.C.Lai,S.Chakravarty,X.Wang,etal.,On-chipmethanesensingbynear-IR
absorptionsignaturesinaphotoniccrystalslotwaveguide,Opt.Lett.36(6)(2011)984–986.
[18]Y.Zhang,Y.Zhao,Q.Wang,Multi-componentgassensingbasedonslotted
photoniccrystalwaveguidewithliquidin?ltration,Sens.Actuators,B:Chem.184(2013)179–188.
[19]Y.Zhang,Y.Zhao,Q.Wang,Measurementofmethaneconcentrationwith
cryptophaneEin?ltratedphotoniccrystalmicrocavity,Sens.Actuators,B:Chem.209(2015)431–437.
[20]W.C.Lai,S.Chakravarty,Y.Zou,etal.,Multiplexeddetectionofxyleneand
trichloroethyleneinwaterbyphotoniccrystalabsorptionspectroscopy,Opt.Lett.38(19)(2013)3799–3802.
[21]Y.Chang,Y.Jhu,C.Wu,Temperaturedependenceofdefectmodeinadefective
photoniccrystal,Opt.Commun.285(6)(2012)1501–1504.
[22]T.Lu,P.T.Lee,Ultra-highsensitivityopticalstresssensorbasedondouble-layeredphotoniccrystalmicrocavity,Opt.Express17(3)(2009)1518–1526.[23]Y.Liu,H.W.M.Salemink,All-opticalon-chipsensorforhighrefractiveindex
sensinginphotoniccrystals,EPL107(3)(2014)34008(1–5).
[24]S.Zheng,B.Shan,M.Ghandehari,etal.,Sensitivitycharacterizationofcladding
modesinlong-periodgratingsphotoniccrystal?berforstructuralhealthmonitoring,Measurement72(2015)43–51.
[25]A.Casas-Bedoya,S.Shahnia,D.DiBattista,etal.,Chipscalehumiditysensing
basedonamicro?uidicin?ltratedphotoniccrystal,Appl.Phys.Lett.103(18)(2013)181109(1–4).
[26]S.Zheng,Y.Zhu,S.Krishnaswamy,Fiberhumiditysensorswithhighsensi-tivityandselectivitybasedoninteriornano?lm-coatedphotoniccrystal?berlong-periodgratings,Sens.Actuators,B:Chem.176(2013)264–274.
[27]S.Zheng,Y.Zhu,S.Krishnaswamy,Nano?lm-coatedphotoniccrystal?ber
long-periodgratingswithmodaltransitionforhighchemicalsensitivityandselectivity,in:ConferenceonSmartSensorPhenomena,Technology,Networks,andSystemsIntegration,Mar.12,2012–Mar.14,2012,SPIE,SanDiego,CA,2012,p.8346(83460D).
[28]P.Lalanne,C.Sauwan,J.P.Hugonin,Photoncon?nementinphotoniccrystal
nanocavities,LaserPhotonicsRev.2(6)(2008)514–526.
[29]S.Chakravarty,A.Hosseini,X.C.Xu,etal.,Analysisofultra-highsensitivity
con?gurationinchip-integratedphotoniccrystalmicrocavitybio-sensors,Appl.Phys.Lett.104(19)(2014)191109(1–5).
[30]Y.Akahane,T.Asano,B.S.Song,etal.,High-Qphotonicnanocavityinatwo-dimensionalphotoniccrystal,Nature425(6961)(2003)944–947.
[31]Y.Zhang,D.Li,C.Zeng,etal.,Siliconopticaldiodebasedoncascadedphotonic
crystalcavities,Opt.Lett.39(6)(2014)1370–1373.
[32]A.Majumdar,J.Kim,J.Vuckovic,etal.,Electricalcontrolofsiliconphotonic
crystalcavitybygraphene,NanoLett.13(2)(2013)515–518.
[33]D.Yang,H.Tian,Y.Ji,Nanoscalelowcrosstalkphotoniccrystalintegrated
sensorarray,IEEEPhotonicsJ.6(1)(2014)4200107(1–7).
[34]Y.Fu,Y.Lee,S.Lin,Designanddemonstrationofhighquality-factor
H1-cavityintwo-dimensionalphotoniccrystal,Opt.Lett.38(22)(2013)4915–4918.
[35]C.Ca?r,X.LeRoux,E.Cassan,High-Qsilicon-on-insulatorslotphotoniccrystal
cavityin?ltratedbyaliquid,Appl.Phys.Lett.103(25)(2013)251106(1–4).[36]B.Li,C.Lee,NEMSdiaphragmsensorsintegratedwithtriple-nano-ringres-onator,Sens.Actuators,A:Phys.172(1)(2011)61–68.
[37]D.Wang,Z.Yu,Y.Liu,etal.,UltrasmallmodalvolumeandhighQfactor
optimizationofaphotoniccrystalslabcavity,J.Opt.15(12)(2013)125102(1–7).
[38]Y.Yang,D.Yang,H.Tian,etal.,Photoniccrystalstresssensorwithhighsensi-tivityindoubledirectionsbasedonshoulder-coupledaslantnanocavity,Sens.Actuators,A:Phys.193(2013)149–154.
[39]Y.Zhang,Y.Zhao,D.Wu,etal.,Fiberloopring-downrefractiveindexsensor
basedonhigh-Qphotoniccrystalcavity,IEEESens.J.14(6)(2014)1878–1885.[40]A.F.Oskooi,D.Roundy,M.Ibanescu,etal.,Meep:a?exiblefree-software
packageforelectromagneticsimulationsbytheFDTDmethod,Comput.Phys.Commun.181(3)(2010)687–702.
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