Chapter 13_ Forces in confined fields .pdf

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1、Chapter13ForcesinConfinedFieldsAsearlyas1619JohannesKeplersuggestedthatthemechanicaleffectoflightmightberesponsibleforthedeflectionofthetailofcometsenteringoursolarsystem.TheclassicalMaxwelltheoryshowedin1873thattheradiationfieldcarrieswithitmo-mentumandthat”lightpressure”isexertedonillum

2、inatedobjects.In1905Einsteinintroducedtheconceptofthephotonandshowedthatenergytransferbetweenlightandmatteroccursindiscretequanta.Momentumandenergyconservationwasfoundtobeofgreatimportanceinmicroscopicevents.Thediscretemomentumtransferbetweenphotons(X-rays)andotherparticles(electrons)

3、hasbeenexperimentallydemonstratedbyComptonin1925andtherecoilmomentumtransferredfrompho-tonsonatomshasbeenobservedbyFrischin1933[1].Importantstudiesontheactionofphotonsonneutralatomsweremadeinthe1970’sbyLetokhovandotherresearchersintheformerUSSRandinthegroupofAshkinattheBellLaboratorie

4、s,USA.Thelattergroupproposedbendingandfocusingofatomicbeamsandtrappingofatomsinfocusedlaserbeams.LaterworkbyAshkinandco-workersledtothedevelopmentof”opticaltweezers”.Thesedevicesallowtoopticallytrapandmanip-ulatemacroscopicparticlesandlivingcellswithtypicalsizesintherangeof0.1−10micro

5、meters[2,3].Milliwattsoflaserpowerproducepiconewtonsofforce.Duetothehighfieldgradientsofevanescentwaves,strongforcesaretobeexpectedinopticalnear-fields.Theideathatanobjectmightcoolthroughitsinteractionwiththeradiationfieldwassuggestedalreadyin1929byPringsheim[4].However,thefirstproposalto

6、coolatomsincounter-propagatinglaserbeamswasmadebyH¨anschandSchawlowin1975[5].Thisproposalwasthestartingpointforaseriesofexcitingexperimentswhichledtothe1997Nobelprizeinphysics.Themechanicalforceinlasertrappingandcoolingexperimentscanbeunderstoodonasemiclassicalbasiswheretheelec-tromag

7、neticfieldistreatedclassicallyandtheparticlebeingtrappedasaquantizedtwo-levelsystem[6].However,thequantumtheoryofphotonsisusedforthecorrect12CHAPTER13.FORCESINCONFINEDFIELDSinterpretationoftheresults[7].Furthermore,thephotonconceptassertsthattherearequantaofenergyandmomentumtransferbet

8、weent