Time-reversedLasingandControlofAbsorptionina
Two-channelCoherentPerfectAbsorber
WenjieWan,Y.D.Chong,LiGe,HeesoNoh,A.D.Stone,HuiCao
DepartmentofAppliedPhysics,YaleUniversity,NewHaven,Connecticut06520,USA
Inthetime-reversedprocesstolaseremission,incidentcoherentopticalfieldsareperfectlyabsorbedwithina
resonatorwhichcontainsalossmedium,insteadofagainmedium.Theincidentfieldsandfrequencymustcoincide
withthoseofthecorrespondinglaserwithgain.Wedemonstratethiseffectforasiliconcavityinthecaseoftwo
counter-propagatingincidentfields,showingthatabsorptioncanbeenhancedbytwoordersofmagnitude.Aunique
featureofthetwo-channelcaseisalsodemonstrated,thatbyvaryingtherelativephaseoftheincidentfields,
absorptioncanbesuppressedsubstantiallyaswell.Thusthedevice,a“coherentperfectabsorber”,functionsasan
absorptiveinterferometer,withpotentialpracticalapplicationsinintegratedoptics.
RecentlyChonget.al.[1]haveexploredtheoreticallyanexacttime-reversalsymmetrypropertyofoptical
systems:thetime-reversedanalogoflaseremission.Inthelasingprocess,acavitywithgainproducesoutgoing
opticalfieldswithadefinitefrequencyandphaserelationship,withoutbeingilluminatedbycoherentincoming
fields.Chonget.al.showedthat,duetothetime-reversalsymmetryoftheseequations,thesamecavitywiththegain
mediumreplacedbyanequivalentabsorbingmediumwillperfectlyabsorbthesamefrequencyoflightifitis
illuminatedwithincomingwaveswiththesamefieldpattern.Thisnewdevicewastermeda“coherentperfect
absorber”(CPA).
ThesimplestpossibleCPAisasingleportreflectorthatusesacavityresonancetoenhancematerialabsorption.
Severalexistingdevicesoperateonthisprinciple,includingasymmetricFabry-Perotreflectors[2],resonant
cavity-enhancedphotodectors[3]andcritically-coupledresonators[4].However,somekeypropertiesoftheCPA
canonlybeobservedinasystemwithmultipleports,andhencenon-trivialscatteringeigenvectors.Forexample,if
theCPAcavityisilluminatedwithcoherentfieldpatternsnotcorrespondingtothetime-reversedlasingmode,thatit
ispossibletodecreasetheabsorptionwellbelowthevalueforincoherentillumination.ThesepropertiesofCPAs
pointtoanewmethodforcontrollingabsorptionthroughcoherentillumination.Inthecurrentworkwepresentthe
firstexperimentalsystemwhichdemonstratesboththeenhancementandreductionofabsorptioninthismanner.
(f)
Wavelength(nm)
Fig.1:Phasemodulationofbeamabsorption.(a)Theoreticalplotofnormalizedtotaloutputintensities,asafunctionofwavelengthλ,forparity-
even(blue)andparity-odd(red)scatteringeigenmodes.(b-d)Theoreticaloutputintensitiesatthreerepresentativeλ,astherelativephaseofthe
inputbeamsisvaried,showingintensitiesemittedtotheright(magenta)andleft(green)sidesoftheslab,andthetotalintensity(black).Λ
correspondingto(b)-(d)aremarkedbyverticallinesin(a);(b)istheCPAresonance.(e-g)Experimentalresultsatλapproximately
correspondingto(b)-(d);solidlinesarefitstothedata,nottheorycurves.(f)Modulationdepth—theratioofmaximumtominimumoutput
intensityobtainablebyvaryingtherelativeinputphase,M=max(Iout)=min(Iout),asafunctionofwavelength.Thewavelengthspacingof
adjacentM-peaksisaround1.27nm,closelymatchingthefreespectralrangeofthewafer.Betweenthesemaxima,Mgoesnearlytounity,
correspondingtothe“phase-insensitivepoints”.
References
[1]Y.D.Chong,LiGe,HuiCao,andA.D.Stone,“CoherentPerfectAbsorbers:Time-ReversedLasers”Phys,Rev.Lett.105,053901(2010).
[2]J.Heffernan,et,al,“Alloptical,highcontrastabsorptivemodulationinanasymmetricFabry–Perotetalon,”Appl.Phys.Lett.,58,2877(1991).
[3]K.Kishino,et,al,“Resonantcavityenhancedphotodetectors,"IEEEJ.Quant.Elect.27,2025-2034(1991).
[4]M.Cai,O.Painter,andK.J.Vahala,“ObservationofCriticalCouplinginaFiberTapertoaSilica-MicrosphereWhispering-GalleryMode
System",Phys.Rev.Lett.85,74(2000). |
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