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&omsaiclesbehaveindividually，‘doihiheyemitwhentherearemanyofthemisasortofunatedsetofwaves.EvenaverysmallLED，withasizeofmuchlessthanamillimetre，savastoms，sothisisaonsituation.
&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.SomeisreflectedbatothegainmediuWhewhichlightisputiybytheamplifierequalstherateatwhichitisextractedthroughtheoutputmirror，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.ManymoleculesalshtatwavelengthsloheredendofthevisiblespectruThisabsorptiohevibratioomiucleithatmakeupthemoleucleiaremuchheavierthaheytendtooscillateatmuchlowerfrequehissetoffrequenciesisasortofmerprint’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，itamplifylightoverabroadspectruThisallowsseveralofthemodesoftheopticalcavitytain.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.
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