凝聚态光物理学3.ppt

文档介绍：discreteenergylevelSolidband(delocalizedstate)Forsemiconductororinsulator,photonexciteselectronfromfilledvalencetotheemptyconductionband,thetranstionenergyisThereisacontinuousrangeoffrequency;Thereisathresholdh>Eg=(Ef—Ei)min;Creationofanelectron-holepair;,uratthezonecentrewherek=0;Inaindirectbandgapmaterial,uratk=0,Wi->ftransitionrate,,thedensityofstatesg(h).(Fermi’sgoldenrule)thematrixelementM,(semiclassicalapproach):Perturbation:Dipolemoment:Lightwave:Initial:Final:thejointdensityofstatesThisgives:-valentatomsuchasGeorbi-,whichthenevolveintothecon-ductionandvalencebandsofthesemi-conductor4s24p2Selectionrules:Theparityoftheinitialandfinalstatesmustbedifferent.j=-1,0or+1.(Totalangularmomentummustchangebyoneunit)l=1.ms=0.(Spinquantumnumbersneverchange).Electric-dipoletransition:hightransitionrate,shortradiativelifetime(10-9–10-8s)–:smallertransitionratesandlongerradiativelifetime(10-6supwards)–theslowemissionbyelectricdipole-forbiddeniscalled–-