格格党

手机浏览器扫描二维码访问

Chapter 7 Lighting the frontiers（第1页）

Chapter7Lightiiers

banner"

>

&slonghistoryaspossiblytheoldestuousbranaturalphilosophyaisattheforefrontofreseardappli.Itisubiquitous:asat，imaging，andunis，asrovidinglore，disdillustratenewfualeffects.

Lighteratesattheextremesofwhatisknowntobepossibleagtophysics，suchasextremesoftemperaturearemesofpressureadourally，exceptperhapsiofstars.Andlightbeusedtoobservearolreallyfastevents，suchasthemovemerooms.

Further，lightexhibitstrauresassociatedwiththequantumwevenineverydayeofthetuitiveaspectsofthefitfulworldofrauhestable，solidworldofournormalexperiehischapter，Iwillexploresomeofthefroowhidacrosswhich，lightistlytakiioiersispossiblebecauseofthegreatteologicalstridesthathavebeesources，opticalsystems，aors，whiableexquisitetrolovertheshapeay，inbothspadtime，alightbeamtake.

Lightmeics

Lightexertforobjects.Thisallows‘remotetrol’ofbitsshapedlightbeams.Lightbeusedtomovematteraround，andbringitintotactwithotherobjeaernalfigurationofatomsandmolecules，ftheminto，forexample，simplechemicalreawaysthatallowboththestudyaionofunusualmaterialproperties.That’saraordinarilypowerfulfeatureinmanyareasofreseardstudy.

&ofmeicalffromlighthasitsiumcarriedbyea.Forinstaoedfromamirror，thatmirrorexperiencesaforcethatprovidestheexertiooredirectthephoton，justaswaterfromafirehosehittingawallexertsaforthewallbyvirtueofitsboungoff.

Similarly，hotoeditgesdiredthis，too，requiresaforce.Thusthephotosafortherefragelement.Ifalightbeamisionaglassbead，raysthatareaothebeadwillexperiedire.Aphtherayinthelowerhalfofthebeadisdirectedupagatesthroughtheglasssurfaces.Thebeadthereforeexperiencesafortheoppositedireofthephotonintheforwarddire（thedireitwasmovieringthebead）isreduced，thereisalsoaheforwarddire.Thestrengthofthisfordsonhohotoedpersed.Alightbeamthatismoreiapositioreofthebeamthanonitsperipherywilltherefthebeadtowardsthehigher-iypartofthebeam.

Thiseffebeusedtomakeafocusedlightbeamintoan‘opticaltweezer’，whichisabletoholdontoamiaaroubeamissteered.Opticaltweezersfindapplis，forexample，inbiology，byenablingtrolofthepositioofindividualstrandsofDerizationoftiors.SpecifiA，proteins，andiolebestutothesebeadsandthereforebehandledusingopticaltweezers.Theirpositiorolledreuchlessthahoflight.Thisallowsextremelysmallforeasured—suchashappenswhenbiologicalcellsadheretosurfacesorothercells—aswellastoholdtheplacesastheyareoperatedonusingotherlasers（so-calledcellularsurgery）.Opticaltweezersbihvariousothertestmethods，suchaslightsgfromaerosols，orspectroscopy，tocharacterizeparticlesthatmaybepollutamosphere.

&o-meicalforalsobeusedtoapletelyionofsmallobjeparticular，itisnowpossibletobuildtinymeitilevers，illustratedinFigure31，andtobothobserveandotionusinglight.Lightforbeusedtocoolorheattheossofthetilever—likerunningdupariitistatepossible，whereonlyquantumflusofthemotiondisturbthepletestasisofthelever.Lightforalsobeusedtouchsmallerobjedthisrevealseveraumstatesofmatter.

31.Ailevertrolledusinglightforces.Thedisymirrorsabout30μmier.

Ultra-cold

What’sthecoldestthingyou’veexperienced?ColderthanwinterinOxford（approximately2°Ottawa（-20°C），ortheSouthPole（-50°C）?Orperhapstheeffectsofliquid-200°C.Thesearelycold，butbyhecoldestthingspossible.Itturnsoutthatthereisalowerlimitfortemperature:-273°C，or0K（Kelvin），belowwhiotpossibletogsfurther.Thisisthetemperatureatwhigsareasstillasthey’regoihjusttheeffemeicstocausealittlejigglingaboutofatomsandmolecules.

It’snotactuallypossibletobuildamaetogettoabsolutezerobutitispossibletogetverygan‘erator’.Infact，yougetaketheatomsalmoststopmoving.Whatthismeansisthattheirsizegetsbigger.（Quaellsusthatyou’tsimultaneouslyspecifythepreciselodspeedofaheatomispletelystopped，thenitmustbeextendedoverallspace.）Thereforeallatomsihathasbeeedoccupythesameregiohisgivesrisetenewphenomena.

Aneratorworksbyusio‘aginealaserbeamshiningonanatomthatismoviht，say.Thelasershiherighttoleft，sothatastreamofphotonshitstheatom.Thelaseristuobeabsorbedbyatomsthataremovingataparticularveloow，wheomabsorbsaphotonfromthelaserbeam，itgetsakithephoton，andthusslowsdown.（Morespecifically，themomentumofthephotonistraheatom.Siisiediretotheinitialmomentumoftheatom，itredu，andthusthespeedoftheatom.）Theatommustre-emitthephotoime，anditwillgetakitheoppositediretothatinwhichitemitsthephotoioninwhichitre-emitsthephotonisrandom—itaall.

Ifyoulookatenoughoftheseabsorptievents，thenyouwillfindthat，althoughthelightisalwaysabsorbedfromoheininglaserbeam）itisemitteduniformlyiioioheceofthisisthateasmovingiiohelaserbeamgrindstoahalt，ahrandommatemperaturethatisproportionaltohowlongitholdsobeforere-emittingit.

Thereareseveralrefihisapproach，eachofwhichuseslighttoolecules）toeveures，andforwhichlightactslikea‘viscousfluid’inwhiovesloweraiseveouselighttotrapatomsusingopticaltweezersoheyareslowenough.Thisallolioresophisticatedoptigteiques，bywhichitispossibletogettotemperaturesofabillionthofadegreeaboveabsolutezero.

&osomeresidual‘jigglihathappezerotemperature，arisingfromquantummeics.Therahisjiggliofasthespatialexteself.Thatis，agtoquaheatomisn’tjustwanderingaroundinarandomfashionoverasmallregioheritisactuallypresentacrossallthatregioomstrappedatsuperaturesthesizeionmaybeseveralthousandthsofametre.That’saremarkablylargeatom，givearoomiucleusislessthahofabillionthofametre.What’seveseveralatomsoccupythisregiohesametime.

That’sceptuallyverytuitive.Weoftenthinkofatomsasbeilebilliardballs，thatbepackedclosetogether，asinasolidmaterial，butwhitheirindividualdistinbyvirtueoftheirlosidethematerial.That’snotthecasefortheseverys.TheyareeacheverywhereatonewstateofmatteridentifiedbyEiheIyendraNathBoseasurprisingly，aBose–Einsteie.

Thisverystraehassomeremarkableproperties.Foriisasuperfluid，ithoutviscosity.Further，itispossibletosplittheeomihalfandrebioshowquantumiweearts，esseingthatabigobjetainingmanyatomsandofapalpablyvisiblesize）exhibitsquantumcharacter，attributabletotheuyofwhetheranatomisiofthecloudortheother.Ohiomogbothseparatepohesametime.

&hesesbetrappediisalsopossibletocreatespatialstructuresouthtbeamsthatbeusedtomaoms.Foriwolightbeamscidetheyformaterer3）inwhichthereareregionsofhighandlowiy.sliketosettleiherions（youadjustwhiebygaparticularwavelengthofthelight）.Astheiyofthelightbeamsisturheatomsfallie’-likeopticaltrapsthatappearitern，asshowninFigure32a.Andtheydosoieresting>

32.strappedinanopticallattice:a.afewhuomspercell（several10sμK），b.individualatomsateachsite（afewnK）.