千米小说网

Chapter 5 Light matters（第2页）

&ureofthisunatedemissionisthateaitsitsphotonsatrandom，withnoaentofwhattheadjasaredoigoesoffinmaions，aotedatdifferenttimes。Iheraheemissionprocessisrefletheraheresultinglightiy。

&’ssayhotodetefrontofalightbulb。（Aphotodetectorworkslikealightbulbiusesthephotoelectriceffect—lightiproduelectritthatbemeasured。）ouldseeisthattheelectritfromthedetectorwasveryhelightihedetectorhasahatgesquidrandtothearrivalofrandomnumbersofphotoantoftime。

Butwhatifitwerepossibletoatetheatoms，sothattheyactedtogether?Wekbacktoourearlieranalogy:imagiionofswihthesameosfrequency。Theswingsmaybeallosgatrandom:thatis，witheachatadifferentpointirajectoryatanyinstant。Or，theymaybeinsyny，withthediffererajectorybetweeswingsbei—likeawavecausedbytheadjatspethecrowdatafootballmatgupandsittingdowhefirstcase，thelightthatisemittedfromtheseuomsislikethatfromalightbulborLEDandissaidtobeihesedcase，however，theatomsoslock-step，aheyemitisgivenoffifashion—allofthephotoediion。Thisisensinstimulatedemission（Figure26disthebasisforthelaser。

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Thelaserisperhapsthemostimportaioninoptithelasttury。Thisdeviceproducesextraordinarilyusefulbeamsoflightthathaverevolutioherangeandcapabilitiesofappliotonlyisitaspecifiination，inmicrosdspectrosple，butitalsoprovidesameasignifiergyontoaparticulartargetinatailoredway，aotrolthedynamiatter。Aremeexampleofthelatterappliislaser-drivenfusionofatoms，disChapter7，thatmayenablenewformsyesupplyoffuel。

Alasersistsofanopticalamplifier，aihatgefromatomsbymeansofstimulatedemissioweentwomirrors（andpossiblyotheroptits）thatformanopticalcavity。Theoybuildsupastheatomsemitlightuntilthereisabalaheeibeamfromtheatoms，andtheeofthecavitythroughthemirrors。Astheamplifieristurnedameaheatoms，thelightemittedfromtheamplifierisreflectedbatoitbytheendmirrorofthecavity。Thatcausesfurtherstimulationofradiatioedatoms，andthusthelightiyinbrightheothermirror，someofthelightistraofthecavityasausefuloutput。Someisreflectedbatothegainmedium。Whewhichlightisputiybytheamplifierequalstherateatwhichitisextractedthroughtheoutputmirror，thelaserissaidtobeatthreshold。Beyondthispoint，aheamplifiergaiwhisareputiedstates）leadstoaheintra-tensity，andthustoaheoutputlight。Theopticalcavityalsoimposesarestrithecoloursofthelaser。Itturnsoutthatthefrequeexperiegaihoseforwhichthelightwavesaddwithstruterfereneadtrip。Thismeansthatthelengthoforipofthecavityshouldbeequaltoamultipleofhalfofawavelength。Thefrequesatisfythisresonantaresaidtobethecavitymodes。

&hatlasersareimportaothefactthatthelighttheyemitist:allthephotonsgoinmoreorlessthesamedire，withthesamecolour。Thedireisdefihelasercavity:thecolourbytheatomsinthegainmediumandtheallowedmodesofthecavity。Thisleadstothepropertythatthelightisintheformofabeam—thelaserbeam—whichisaboutasclosetoa‘ray’oflightasyoue。Itdivergesasitpropagatesduetodiffra，buthasthesmallestdivergencepossible。Thispropertyalsomeansthatitbefocusedtoaverysmallspotusinglensesormirrors。

&urethattraststhelightemittedfromlaserswiththatfromlightbulbsisthatlaserlightisusuallyverypureinotherwords，itsistsofonlyafewwavelengths，softeabreofwaveleiyisverystable（registeringasverylownoiseinaphotodetectoroutput），abeemitteduouslyorasasequenceofpulses。

&yhttobefocusedtoaverysmallspotmakesitusefulformicrosdthereareanumberofdifferentwaysinwhich，bysgthelaserspotaobjectatthefoicross，ahelightsittedfromtheobject，three-dimensioheobjebemade。Thisapproachisveryusefulinlookingatanimaltissue，forexample，andopticalmicroscopiesofthiskindarewidelyusedinbiomedie。

Manyapplisoflasersinmanufagalsostemfromthispropertyht。Theabilitytomark，cut，drill，orweldmetals，forinstahatalybedepositedinasmallregioalinashorttime。Sohigh-porodugtlightbeamsintheformofpulsesthatbefocusedareidealforsuchmaterialsprog。

Similarpropertiesareneededforsomemedicalapplisoflasers，alsoinvolvihistimeskih，orhair。Laserofvisioryarenowoheuseoflaserstoremovetattoosbyheatiilthedropsbreakup，andforhairremoval—althoughuhairregrowth!Otherfamiliardevicesthatmakeuseht’sabilitytoaddressaverysmallspotaretheCD，DVD，Blu-ray，aerdiskstedevices。Verytinyspotsoflightinthereediumallh-densitysteofdataierial。

TheverypurecoloursachievablewithlaserlightmakeitpossibletodistiuentatomsandmoleculesofdifferentmixturesbymearosotedinChapter1，differentatoms，andindeeddifferentmolecules，havecharacteristiciesatwhichtheyabsorba，duetotheirdifferentstructures。Extendingtheanalogydevelopedinthischapter，theyarelikeswingsinwhichthesorropesholdiareths—theirnaturalosfrequenciesaredepehewayinwhichtheyareputtogether。

Iomandmoleculehasarangeofdifferentabsorptionandemissionfrequencies，diioelefigurations。Typicallytheselieiofthevisiblelightspectrum，butsomemoleuchshths，iohumans。Manymoleculesalshtatwavelengthsloheredendofthevisiblespectrum。Thisabsorptiohevibratioomiucleithatmakeupthemoleucleiaremuchheavierthaheytendtooscillateatmuchlowerfrequehissetoffrequenciesisasortofmerprint’thateionofaparticularspecies。

&alogueofthesefisisofimportance，ofchemistry，siallowsthedifferesinvolvediobeideisalsousedinmy，andevenincellbiology，wheofparticular‘tag’molebestudied。Itiscritical，too，forastrophysiwhichtheelemeronomicalobjects—stars，galaxies，nebulae—bedetermined，aswellasinatmosphericphysidmetey，fortheremotesensingofpollutantsorparticles。Suitprovideskeydataiheimpadinofclimatege。

Bybisofseveraldifferentlasers—sayoi，onegreeispossibletomakealaserprojector。BygtheiiesofeachlaserindividuallyagtothevideosignaloutputofaputerorIlink，perhapsbymeansofaliquidcrystalcell，moviesbeprojeabigswithvivid，highlysaturatedcolours。Thebinatireen，andblue（RGB）lightissuffiakeupapletecolourpaletteandlasersprhtimagesonas。

X-rays

&hewavelengthofthelightisveryshort，intheX-rayregiorum，adifferentkiroscopyarises。X-rayphetioughtoexcitethemosttightlybouronsinatoms—notjusttheoutermosteles。ThismeansthatX-raysbeusedtolookiomsandmoledtouheirloent，whishiftthebindiheeles。X-rayabsorptioroscopyiswidelyusediudyofmaterialsforavarietyofapplidetegtracepollutantstthestructureofglasses。AsnotedinChapter3，X-raysarealsousedtostudythestructureofcrystalsbymeansofdiffra。WhentheX-raywavelengthisclosetothespagbetweenatomsial，thealactsasa‘diffragrating’aheX-raysiiohesediffrapatternsoispossibletorestructthethree-dimensionalstructureofhighlyplexcrystalsusingadvanversionalgorithms。Todaythisisaroutineprocessforcharagisolatedbiologidchemicallyrelevantmolecules，determiuresofpossiblenewmolenthemforaspe。

&hebestlightsourcesforthissortofspectroscopyaresyns。IoproduceX-raysattherequiredshortwavelengths，synshavetopretis，ahemaroundabigrialstationscattoflightastheelesrushby，leadingtoshortburstsofX-raysthatareusedfordiffraagiheDiamondLightSourceatHarwellinEnglaeseleorethanabillionvoltsinaringthatismorethanhalfakil。Theihtsourcesisbeingbuiltusiicleaccelerators，whichproduceextremelybrightX-raysbeams。TheX-raydiffrapatternshowniakenusingtheDiamondsyn。

Ultrashhtpulses

&einshortbursts。Thereareseveralwaysteforthistohappeproducesthebriefestpulsesiscalledmode-log。Thisrequiresagaihasalargebandwidth—thatis，itamplifylightoverabroadspectrum。Thisallowsseveralofthemodesoftheopticalcavitytain。Ifitisalsedthatthesemodesallhavethesamephase，thenlightwavesofmafrequenciesaddstructivelytoyieldasihecavity，boungbadforthbetweenthemirrors。Thebrevityofthepulseisdeterminedbyhowmanyfrequehewidertherangeoffrequeerthepulse。

Thepossibilityveryshortduratihtpulsesenablesakiamie-resolvedspectroscopy。Itallowsustoseehowthiime，basedonanoldpriroboscope。Thegeneralideaofhowyouuselightto‘freeze’rapidmotiootheworkofEadweardMuybridgeihtury。Heiheideaofusingafastcamerashuttertophotographah。Thelegsofahorsemovetoofastforthehumaoresolve，anditwasnotknownatthattimewhetherallfsleftthegroundatanypthestride。Tosettlethematter，Muybridgesetupabankofcameras，eachofwhichhaditsshutteroperipastriggeredwhenthehorsepassedthecamera。Thiseo‘slice’ashortpulsefromthelightreflethehorse。Thedurationofthispulseoflightwasshorterthaimeoverwhichthehsmoved。Theouteofhisworkwasthathewasabletoianford，thefunderofhisresearch，thatinfactthereisapointirideatwhieofitslegstouchtheground。

Meicalshuttersoionalcamerascouldcloseveryfast，butoseesomeformsofanimalmotion，suchastheflappingofthewingsofahummierphysitssuexplosisescalesofthousandthsofased，wereoutofreach。TosolvethisprertonatMITinthe1950siedanewkindofnon-meicalshutter，basedonanopticalswitch。Hewasabletotake‘still’photographsofexplosions，forihisdevice。

&ersarewhatwemightcall‘passive’iheysimplyallowasliceoflightthwhentheyareopeabletoeventsthatarewellillumihehorseiniasualotoflightthemselves（theexplosion）。Butweearumegepulseoflighttoilluminateamovihipulsesemittedbytheflashunitonacamera。Aflashoflightwithadurationthatisshortparedtothetimetakehingwe’relookingattomoveprovidesaheobje’intime，evenwhenusierspeedthatismugerthanthegesoftheobject。Asedflashfreezesthemotionatalatertime。Thesameisthecaseforsubsequentflashes。

Amovieposedofthesequenestakerialsoftheeventrevealsveryrapidthesystem，ohatismuchfasterthancouldbeobservedbytheeye。Iyofeventsthatbeseeninthiswayistrulybreathtakioroboscope’ofthiskindin1931，andsomeofhismostiages，suchasabulletpassingthroughanappleoraplayingcard（Figure27），weretakenwiththisdevice。

27。Abulletfrozeninmotionusingstrobosaging。

Usingmodernpulsedlasersasthe‘flash’itispossibletoobservenotjustthefrozenmotioeveionofatomsinamolevolvedinachemicalrea（forwhichthe1999NobelPrizeirywasgiventoAhmedZewail）andthemuch，muchfastermotioronswhizzingaroundthenu。Thetimeseringlysmall—lessthahofamillion-millionthofased（100×10-15setoseds（100fs））intheoledafewtensofbillion-billionthsofased（100×10-18seds，or100attoseds（100as））iomis。Thesefields—femtochemistryandattoscerespectively—areattheforefrontofossibleiionoflightandmatter。IshallsiderthemfurtherinChapter7。