固體材料的宏觀光學性質

第四章固體材料的宏觀光學性質固體材料的宏觀光學性質共60頁，您現(xiàn)在瀏覽的是第1頁!OverviewThestudyoftheopticalpropertiesofmaterialsisahugefieldandwewillonlybeabletotouchonsomeofthemostbasicpartsSowewillconsidertheessentialpropertiessuchasabsorption/reflection/transmissionandrefractionThenwewilllookatotherphenomenalikeluminescenceandfluorescenceFinallywewillmentionapplications,inparticularopticalfibresandlasers固體材料的宏觀光學性質共60頁，您現(xiàn)在瀏覽的是第2頁!NatureoflightLightisanelectromagneticwave:withavelocitygivenbyc

=

1/(00)=3x108m/sInviewofthis,itisnotsurprisingthattheelectricfieldponentofthewaveshouldinteractwithelectronselectrostatically.astronomynotes./light/emanim.gif固體材料的宏觀光學性質共60頁，您現(xiàn)在瀏覽的是第3頁!GeneraldescriptionofabsorptionBecauseofconservationofenergy,wecansaythatI0=IT+IA+IRIoistheintensity(W/m2)ofincidentlightandsubscriptsrefertotransmitted,absorbedorreflectedAlternativelyT+A+R=1whereT,A,andRarefractionsoftheamountofincidentlightT=IT/I0,etc.Somaterialsarebroadlyclassedastransparent:relativelylittleabsorption

andreflectiontranslucent:lightscatteredwithin

thematerial(seeright)opaque:relativelylittletransmission.tekano.pwp.blueyonder.co.uk/tekano/translucent.jpg固體材料的宏觀光學性質共60頁，您現(xiàn)在瀏覽的是第4頁!Thus,ifwecanplot-ln(I)againstx,weshouldfindfromthegradientDependingonthematerialandthewavelength,lightcanbeabsorbedbynuclei–allmaterialselectrons–metalsandsmallband-gapmaterials固體材料的宏觀光學性質共60頁，您現(xiàn)在瀏覽的是第5頁!Ifwethinkofouratom-on-springsmodel,thereisasingleresonancepeak:Butthingsaremoreplexwhentheatomsareconnected–phononsrecalltransverseandlongitudinalopticalphononsf0fabsorption固體材料的宏觀光學性質共60頁，您現(xiàn)在瀏覽的是第6頁!MolecularmaterialsMaterialssuchasorganic(carboncontaining)solidsorwaterconsistofmoleculeswhicharerelativelyweaklyconnectedtoothermoleculesHence,theabsorptionspectrumisdominatedbyabsorptionsduetothemoleculesthemselvese.g.watermolecule:.sbu.ac.uk/water/images/molecul5.jpg固體材料的宏觀光學性質共60頁，您現(xiàn)在瀏覽的是第7頁!Sincethebondshavedifferent“springconstants”,thefrequenciesofthemodesaredifferentwhentheincidentilluminationisofawavelengththatexcitesoneofthesemodes,theilluminationispreferentiallyabsorbedThistechniqueallowsustomeasureconcentrationsofdifferentgasspeciesin,forexample,theatmospherebyfittingspectraofknowngasestothemeasuredatmosphericspectra,wecanfigureoutthequantitiesofeachofthegases固體材料的宏觀光學性質共60頁，您現(xiàn)在瀏覽的是第8頁!ThisstructureformetalsmeansthatalmostanyfrequencyoflightcanbeabsorbedSincethereisaveryhighconcentrationofelectrons,practicallyallthelightisabsorbedwithinabout0.1μmofthesurfaceMetalfilmsthinnerthanthiswilltransmitlighte.g.goldcoatingsonspacesuithelmetsPenetrationdepths(I/I0=1/e)forsomematerialsare:water:32cmglass:29cmgraphite:0.6μmgold:0.15μm固體材料的宏觀光學性質共60頁，您現(xiàn)在瀏覽的是第9頁!Themetalappears“silvery”sinceitactsasaperfectmirrorOKthen,whyaregoldandcoppernotsilvery?becausethebandstructureofarealmetalisnotalwaysassimpleaswehaveassumedtherecanbesomeemptylevelsbelowEFandtheenergyre-emittedfromtheseabsorptionsisnotinthevisiblespectrumMetalsaremoretransparenttoveryhighenergyradiation(x-&-rays)whentheinertiaoftheelectronsthemselvesisthelimitingfactor固體材料的宏觀光學性質共60頁，您現(xiàn)在瀏覽的是第10頁!Electronicabsorptioninnon-metalsDielectricsandsemiconductorsbehaveessentiallythesameway,theonlydifferencebeinginthesizeofthebandgapWeknowthatphotonswithenergiesgreaterthanEgwillbeabsorbedbygivingtheirenergytoelectron-holepairswhichmayormaynotre-emitlightwhentheyrelaxECEVEGhole固體材料的宏觀光學性質共60頁，您現(xiàn)在瀏覽的是第11頁!Semiconductorscanappear“metallic”ifvisiblephotonsareallreflected(likeGe)butthosewithsmallerEg,suchasCdSlookcolouredyellowforCdSwhichabsorbs540nmandaboveTheabovepictureisgoodforpurematerialsbutimpuritiescanaddextraabsorptionfeaturesECEVphononhf1hf2固體材料的宏觀光學性質共60頁，您現(xiàn)在瀏覽的是第12頁!Refractioninnon-metalsOneofthemostimportantopticalpropertiesofnon-metallicmaterialsisrefractionThisreferstothebendingofalightbeamasitpassesfromonematerialintoanothere.g.fromairtoglassWedefinetheindexofrefractiontobe n=c/vwherecisthespeedoflightinavacuumandvisthespeedoflightinthematerial(whichisingeneralwavelength-dependent)Afamiliarexampleistheprismwherethedifferentamountsofbendingseparatesoutthewavelengths固體材料的宏觀光學性質共60頁，您現(xiàn)在瀏覽的是第13頁!Sincelightisanelectromagneticwave,theconnectionwithboththedielectricpermittivity()andthemagneticpermeability(μ)isnotsurprisingTheindexofrefractionisthereforeaconsequenceofelectricalpolarization,especiallyelectronicpolarizationHence,theradiationlosesenergytotheelectrons+–固體材料的宏觀光學性質共60頁，您現(xiàn)在瀏覽的是第14頁!Reflectioninnon-metalsReflectionoccursattheinterfacebetweentwomaterialsandisthereforerelatedtoindexofrefractionReflectivity,R=IR/I0,wheretheI’sareintensitiesAssumingthelightisnormallyincidenttotheinterface:wheren1andn2aretheindicesforthetwomaterialsOpticallensesarefrequentlycoatedwithantireflectionlayerssuchasMgF2whichworkbyreducingtheoverallreflectivitysomelenseshavemultiplecoatingsfordifferentwavelengthsn1n2固體材料的宏觀光學性質共60頁，您現(xiàn)在瀏覽的是第15頁!ColoursSmalldifferencesinpositioncanleadtolargedifferencesinappearanceForexample,high-puritysingle-crystalAl2O3iscolourlesssapphireIfweaddonly0.5-2.0%ofCr2O3wefindthatthemateriallooksredrubyTheCrsubstitutesfortheAlandintroducesimpuritylevelsinthebandgapofthesapphireTheselevelsgivestrongabsorptionsat:400nm(green)and600nm(blue)leavingonlyredtobetransmitted固體材料的宏觀光學性質共60頁，您現(xiàn)在瀏覽的是第16頁!TranslucencyEvenafterthelighthasenteredthematerial,itmightyetbereflectedoutagainduetoscatteringinsidethematerialEventhetransmittedlightcanloseinformationbybeingscatteredinternallysoabeamoflightwillspreadoutoranimagewillbeeblurredInextremecases,thematerialcouldbeeopaqueduetoexcessiveinternalscatteringScatteringcanefromobviouscauses:grainboundariesinpoly-crystallinematerialsfineporesinceramicsdifferentphasesofmaterials固體材料的宏觀光學性質共60頁，您現(xiàn)在瀏覽的是第17頁!Rayleighscatteringalsocausestheskytobeblue.Thereasonforthisisthewavelength-dependenceofRayleighscatteringscatteringgoesasl-4sosincelred~2lbluebluelightisscattered~16timesmorethanredlightThismechanismisofgreattechnologicalimportancebecauseitgovernslossesinopticalfibresformunicationButbeforewegetontofibres,wewillmentionacouplemorebasiceffects固體材料的宏觀光學性質共60頁，您現(xiàn)在瀏覽的是第18頁!DispersionDispersionisageneralnamegiventothingswhichvarywithwavelengthForexample,thewavelength-dependenceoftheindexofrefractionistermedthedispersionoftheindexAnotherimportantcasearisesbecausethespeedofthewavedependsonitswavelengthIfapulseofwhitelightistransmittedthroughamaterial,differentwavelengthsarriveattheotherendatdifferenttimesthisisalsocalleddispersion固體材料的宏觀光學性質共60頁，您現(xiàn)在瀏覽的是第19頁!LuminescenceisfurtherdividedintophosphorescenceandfluorescenceFluorescenceandphosphorescencearedistinguishedbytheelectrontransitionsrequiringnochangeorachangeofspin,respectivelyhencefluorescenceisafasterprocessbecausenochangeofspinisrequired,around10-5–10-6sphosphorescencetakesabout10-4–101sThustheenergydiagrammightbelike:E2E1E3phosp.phosp.fluor.incidentflipflip固體材料的宏觀光學性質共60頁，您現(xiàn)在瀏覽的是第20頁!Influorescentlights,theplasmageneratesUVlight,andafluorescentcoatingonthewallsofthetubeconvertsthistovisiblelighttheselightshaveavisibleflickerbecause(60Hz)-1>10-5sRatherconfusingly,materialsthatdothisaregenerallycalledphosphorsToobtainawhitelight,amixtureofphosphorsmustbeused,eachfluorescingatadifferentwavelengthTVtubesusuallyusematerialsdopedwithdifferentelementstogivethecolours:ZnSdopedwithCu+givesgreenZnS:AggivesblueYVO4:Eugivesred固體材料的宏觀光學性質共60頁，您現(xiàn)在瀏覽的是第21頁!Obviously,thelossinthecableisimportantbecauseisdeterminesthemaximumuninterruptedlengthofthefibreWeknowthatlossesdependonthewavelengthofthelightandthepurityofthematerialrecallthepenetrationdepthforglasswas~30cmIn1970,1kmoffibreattenuated850nmlightbyafactorof100By1979,1kmoffibreattenuated1.2μmlightbyafactorofonly1.2thislightisinfraredNow,over10kmofopticalfibresilicaglass,thelossisthesameas25mmofordinarywindowglass!固體材料的宏觀光學性質共60頁，您現(xiàn)在瀏覽的是第22頁!TheRayleighscatteringresultsfromminutelocaldensityvariationswhicharepresentintheliquidglassduetoBrownianmotionandbeefrozenintothesolidThereallycleverpartaboutopticalfibresisthatthelightisguidedaroundbendsinthefibreThisisachievedbytotalinternalreflectionattheboundaryofthefibre固體材料的宏觀光學性質共60頁，您現(xiàn)在瀏覽的是第23頁!ThelightistransmittedinthecoreandtotalinternalreflectionismadepossiblebythedifferenceintheindexofrefractionbetweenthecladdingandthecoreAsimpleapproachisthe“step-index”design:Themainproblemwiththisdesignisthatdifferentlightraysfollowslightlydifferenttrajectoriesn固體材料的宏觀光學性質共60頁，您現(xiàn)在瀏覽的是第24頁!Suchbroadeningislargelyeliminatedbyusinga“graded-index”design:ThisisachievedbydopingthesilicawithB2O3orGeO2parabolicallyasshownaboveNow,waveswhichtravelintheouterregions,dosoinalowerrefractiveindexmaterialandtheirvelocityishigher(v=c/n)n固體材料的宏觀光學性質共60頁，您現(xiàn)在瀏覽的是第25頁!AconvenientfactisthatpoundsemiconductorlaserscanemitIRlightclosetothe1.55μmwavelengthwherethefibreabsorbsleastReferringbacktothesystemdiagram,itwouldbeadvantageoustointegratetheencoderandtransmittersothecircuitsandthelightemittercanbeintegratedThisiswhythereissomuchinterestingettinglightoutofporoussiliconorSipoundswherethinstrandsofmaterialexhibitquantum-mechanicaleffectswhichadjusttheSibandstructuretofacilitateefficientlightemission固體材料的宏觀光學性質共60頁，您現(xiàn)在瀏覽的是第26頁!LasersLASERstandsforLight

AmplificationbytheStimulated

EmissionofRadiationThekeywordhereis“stimulated”Allofthelightemissionwehavementionedsofarisspontaneousithappenedjustduetorandomlyoccurring“natural”effectsStimulatedemissionreferstoelectrontransitionsthatare“encouraged”bythepresenceofotherphotonsEinsteinshowedthatanincidentphotonwithE≥Egwasequallylikelytocausestimulatedemissionoflightastobeabsorbed.007sdomain./gf_laser.jpg固體材料的宏觀光學性質共60頁，您現(xiàn)在瀏覽的是第27頁!Sincewegetmorephotonsoutthanweputin,thisisopticalamplificationhencelAserthissystemwasfirstusedtoamplifymicrowavesformunications(maser)SuchaconditioniscalledapopulationinversionThisstimulatedemissioniswhatgivesthelaseritscoherentoutputwhichiswhatmakesitusefulforholography,forexampleClearly,randomspontaneousemission“wastes”electrontransitionsbygivingincoherentoutputsoweminimisethembyusingtransitionsforwhichthespontaneousemissionsareoflowprobabilityso-calledmetastablestates固體材料的宏觀光學性質共60頁，您現(xiàn)在瀏覽的是第28頁!Soallweneedtomakealaseristoachieve(i)apopulationinversion(ii)enoughphotonstostimulateemissionThefirstisachievedbyfillingthemetastablestateswithelectronsgeneratedbylightfromaxenonflashlampThesecondconditionisachievedbyconfiningthephotonstotravelbackandforthalongtherodofrubyusingmirroredendsnextslideTherubylaserhasanoutputat694.3nm固體材料的宏觀光學性質共60頁，您現(xiàn)在瀏覽的是第29頁!Inordertokeepthecoherentemission,wemustensurethatthelightwhichpletestheroundtripbetweenthemirrorsreturnsinphasewithitselfHencethedistancebetweenthemirrorsshouldobey2L=NlwhereNisaninteger,listhelaserwavelengthandListhecavitylengthSemiconductorlasersworkinjustthesamewayexceptthattheyachievethepopulationinversionelectricallybyusingacarefullydesignedbandstructure固體材料的宏觀光學性質共60頁，您現(xiàn)在瀏覽的是第30頁!SummaryWehavelookedathowtheelectronicstructureofatomsandtheirbondingleadstovaryingopticalbehavioursinmaterialsInparticular,propertiessuchasabsorptionandemissionarecloselyrelatedtotheelectronsApplicationsofthisknowledgeincludeanti-reflectivecoatingsforlensesfibre-opticmunicationslasers固體材料的宏觀光學性質共60頁，您現(xiàn)在瀏覽的是第31頁!Manyoftheelectronicpropertiesofmaterials,informationonthebonding,materialpositionetc.wasdiscoveredusingspectroscopy,thestudyofabsorbedoremittedradiationevidenceforenergylevelsinatomsevidenceforenergybandsandband-gapsphotoelectriceffect固體材料的宏觀光學性質共60頁，您現(xiàn)在瀏覽的是第32頁!Ifthematerialisnotperfectlytransparent,theintensitydecreasesexponentiallywithdistanceConsiderasmallthicknessofmaterial,xThefallofintensityinxisIsoI=-a.x.Iwhereistheabsorptioncoefficient(dimensionsarem-1)Inthelimitofx0,wegetThesolutionofwhichisI

=

I0

exp(–x)Taking“l(fā)n”ofbothsides,wehave:whichisknownasLambert’sLaw(healsohasaunitoflightintensitynamedforhim)固體材料的宏觀光學性質共60頁，您現(xiàn)在瀏覽的是第33頁!ATOMICABSORPTIONHowthesolidabsorbstheradiationdependsonwhatitis!Solidswhichbondionically,showhighabsorptionbecauseionsofoppositechargemoveinoppositedirectionsinthesameelectricfieldhencewegeteffectivelytwicetheinteractionbetweenthelightandtheatomsGenerally,wewouldexpectabsorptionmainlyintheinfraredbecausethesefrequenciesmatchthethermalvibrationsoftheatoms固體材料的宏觀光學性質共60頁，您現(xiàn)在瀏覽的是第34頁!ElectronicabsorptionA/~kieran/reuhome/vizqm/figs/hydrogen.gif固體材料的宏觀光學性質共60頁，您現(xiàn)在瀏覽的是第35頁!Thespectrumofliquidwater.sbu.ac.uk/water/images/watopt.jpg固體材料的宏觀光學性質共60頁，您現(xiàn)在瀏覽的是第36頁!OpticalpropertiesofmetalsRecallthattheenergydiagramofametallookslike:EFistheenergybelowwhich,at0K,allelectronstatesarefullandabovewhichtheyareemptythisistheFermiEnergyForT>0,EFistheenergyatwhichhalfoftheavailableenergystatesareoccupiedSemiconductorsalsohaveaFermilevelforanintrinsicmaterialEFisinthemiddleofthebandgapnearerEcforn-type;nearerEvforp-typefulllevelsempty

levelsT=0KEF固體材料的宏觀光學性質共60頁，您現(xiàn)在瀏覽的是第37頁!Sowhathappenstotheexcitedatomsinthesurfacelayersofmetalatoms?theyrelaxagain,emittingaphotonTheenergylostbythedescendingelectronisthesameastheoneoriginallyincidentSothemetalreflectsthelightverywell–about95%formostmetalsmetalsarebothopaqueandreflectivetheremainingenergyisusuallylostasheatIntermsofelectrostatics,thefieldoftheradiationcausesthefreeelectronstomoveandamovingchargeemitselectromagneticradiationhencethewaveisre-emitted=reflected固體材料的宏觀光學性質共60頁，您現(xiàn)在瀏覽的是第38頁!Reflectionspectraforgoldandaluminumare:blueredgoldreflectslotsofredwavelengthsaluminumspectrumisrelativelyflat.thermo./eThermo/CMA/Images/Various/109Image_12275.gif固體材料的宏觀光學性質共60頁，您現(xiàn)在瀏覽的是第39頁!Hence,theabsorptioncoefficientsofvarioussemiconductorslooklike:固體材料的宏觀光學性質共60頁，您現(xiàn)在瀏覽的是第40頁!ImpuritylevelsdivideupthebandgaptoallowtransitionswithenergieslessthanEg

Rebinationcanbeeitherradiative(photon)ornon-radiative(phonon)dependingonthetransitionprobabilitiesPracticalp-ndiodesusuallycontainasmallamountofimpuritytohelprebinationbecauseSihasarelativelylowrebination“efficiency”forthesamereasonthatSiisinefficientatgeneratinglight固體材料的宏觀光學性質共60頁，您現(xiàn)在瀏覽的是第41頁!Refractionisalsovitalforotherapplications,suchas:opticalfibres–keepsthelightinsemiconductorlaser–keepsthelightintheamplifyingcavityofthelaserGiventhatwhereμandμ0(=μrμ0)arethepermeabilityofthematerialandfreespace,respectively(amagneticproperty)andeande0(=ere0)arethepermittivityofthematerialandfreespace,respectively(anelectrostaticproperty)Wefindthatn=√(μrer)(≈√erformanymaterials)固體材料的宏觀光學性質共60頁，您現(xiàn)在瀏覽的是第42頁!SinceE=hv/,anddoesn’tchange,thevelocitymustbesmallerinthematerialthaninfreespacesinceweloseEtotheatoms,vmustalsodecreaseElectronicpolarizationtendstobeeasierforlargeratomssonishigherinthosematerialse.g.glass:n~1.5leadcrystal:n~2.1(whichmakesglassesandchandeliersmoresparkly!)ncanbeanisotropicforcrystalswhichhavenon-cubiclattices固體材料的宏觀光學性質共60頁，您現(xiàn)在瀏覽的是第43頁!SpectraSowehaveseenthatreflectionandabsorptionaredependentonwavelengthandtransmissioniswhat’sleftover!Thusthethreeponentsforagreenglassare:CallisterFig.21.8固體材料的宏觀光學性質共60頁，您現(xiàn)在瀏覽的是第44頁!Thespectraforrubyandsapphirelooklike:Asimilartechniqueisusedtocolourglassesorpotteryglazebyaddingimpuritiesintothemoltenstate:Cu2+:blue-green,Cr3+:greenCo2+:blue-violet,Mn2+:yellow.valleydesign./images//~jtalbot/glossary/photopumping.gif固體材料的宏觀光學性質共60頁，您現(xiàn)在瀏覽的是第45頁!Inhighlypurematerials,scatteringstilloccursandanimportantcontributionesfromRayleighscatteringThisisduetosmall,randomdifferencesinrefractiveindexfromplacetoplaceInamorphousmaterialssuchasglassthisistypicallyduetodensityorpositionaldifferencesintherandomstructureIncrystals,latticedefects,thermalmotionofatomsetc.alsogiverisetoRayleighscattering固體材料的宏觀光學性質共60頁，您現(xiàn)在瀏覽的是第46頁!固體材料的宏觀光學性質共60頁，您現(xiàn)在瀏覽的是第47頁!LuminescenceLuminescenceisthegeneraltermwhichdescribesthere-emissionofpreviouslyabsorbedradiativeenergyCommontypesarephoto-,electro-,andcathodo-luminescence,dependingonwhethertheoriginalincidentradiationwaslightofadifferentwavelength–e.g.fluorescentlightelectricfield–e.g.LEDelectrons–e.g.electronguninacathoderaytube(CRT)Thereisalsochemo-luminescenceduetochemicalreactionswhichmaketheglowingringsseenatfairgrounds!固體材料的宏觀光學性質共60頁，您現(xiàn)在瀏覽的是第48頁!Iftheenergylevelsareactuallyarangeofenergies,then:Sothelightemittedbyfluorescenceisoflongerwavelengththantheincidentlightsincetheenergyissmallerandphosphorescentlightistypicallylongerwavelengththanfluorescentlightphononemission

~10-12sperhopfluorescence,~10-5s固體材料的宏觀光學性質共60頁，您現(xiàn)在瀏覽的是第49頁!OpticalfibresFibre-optictechnologyhasrevolutionisedtelemunicationsowingtothespeedofdatatransmission:equivalentto>3hrsofTVpersecond24,000simultaneousphonecalls0.1kgoffibrecarriessameinformationas

30,000kgofcoppercableOwingtoattenuationinthecable,transmissionisusuallydigitalandthesystemrequiresseveralsections:encoderconversiontoopticalrepeaterdetectiondecoder