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Infrared Upconversion for Astronomy

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Infrared Upconversion for Astronomy Infrared UpconversionUpconversion for Astronomy Robert W.W. Boyd Department of PhysicsPhysics University of California Berkeley, California 94720 Abstract The fieldfield ofof infraredinfrared upconversionUpconversion forfor astronomy astronomy is is reviewed. reviewed. The basic theory of upconversionUpconversion is presented, along with a brief historical summary of upconversionUpconversion techniques.techniques. SeveralSeveral investi-investi­ gators have employed upconverters in astronomical studies, but have met withwith onlyonly modestmodest success.success. UpconversionUpconversion willwill becomebecome IR a useful detection method for astronomy only if substantial butbut perhaps forseeable improvementsimprovements can be realized. Introduction A novel solutionsolution toto the problem ofof infraredinfrared detectiondetection isis affordedafforded by thethe processprocess ofof upconversion. Upconversion. TheThe upconversionUpconversion processprocess isis shown schematicallyschematically inin FigureFigure 1.1. InfraredInfrared radiationradiation of frequency Figure 2.2. ImagingImaging property ofof the the upconversion Upconversion process. process. ConservationConservation of photonphoton momentummomentum (hk)(hk) requiresrequires thatthat the the sum sum frequencyfrequency photon be emitted inin aa unique direction. rections of propagation. In fact,fact, toto aa goodgood approximation,approximation, the Nonlinear crystal angle of propagation ofof thethe sumsum frequencyfrequency radiationradiation isis relatedrelated to the angleangle of propagation of thethe infraredinfrared radiationradiation byby Figure 1.1. Schematic description of thethe upconversion Upconversion process.process. InfraredInfrared vIR 05°IR (1) radiation of frequencyfrequency vIR^|R isis mixedmixed with with an an intense intense laser laser beambeam ofof fre-fre­ PL quency vLv\_ inin aa nonlinearnonlinear crystal,crystal, producingproducing aa signalsignal at the sum frequency PS."s- where OS6$ andand #JRBIR are defined inin FigureFigure 2.2. Thus, if the optical sys-sys­ tem which collects the infraredinfrared radiationradiation isis designeddesigned soso as toto map vIRv^ is mixed withwith anan intenseintense visible visible oror near near-infrared -infrared laserlaser beambeam each point in the fieldfield ofof viewview into aa planeplane wavewave at the crystalcrystal of frequencyfrequency vLv^ in a nonlinear crystal.crystal, The nonlinearity causescauses a with aa directiondirection ofof propagationpropagation BIR, #JR, Eq.Eq. (1) predicts that the sum signal toto be generated at thethe sumsum frequency,frequency, vs,^§, whichwhich is in the frequency response will be aa perfectperfect imageimage ofof thethe infraredinfrared field,field, visible region;region; thus the infrared signalsignal is converted to thethe visible,visible, demagnified byby thethe factor vIR^IR/^L. /vL where sensitive, lowlow noisenoise detectors areare readilyreadily available.available. The This article will reviewreview thethe variousvarious instrumentsinstruments for astronomy conversion efficiencyefficiency ofof infraredinfrared photonsphotons to visible photonsphotons at which have been constructed usingusing thethe upconversionUpconversion process.process. De-De­ the sumsum frequencyfrequency cancan approachapproach 100100%. %.11 Furthermore,Furthermore, the up- spite the potentialpotential ofof thethe technique, technique, only only modest modest results results havehave conversion processprocess isis inherentlyinherently noise free in the sense that ener-ener­ been achieved toto date. OneOne seriousserious problem has been thatthat the gy conservation prohibits the generationgeneration ofof aa responseresponse atat thethe sumsum quantum conversion efficiency hashas been quite low forfor upcon-upcon- frequency in thethe absenceabsence ofof aa signalsignal atat thethe infraredinfrared frequency.frequency. verters employing continuouscontinuous wave wave lasers.lasers. AnAn additional problem A related use of the upconversionUpconversion processprocess isis toto convert convert images images is thatthat manymany workersworkers inin thethe fieldfield havehave beenbeen limitedlimited byby noisenoise from the infraredinfrared spectralspectral regionregion toto thethe visible. visible. AsAs illustrated in sources largerlarger thanthan expected. As a result,result, thethe techniquetechnique hashas Figure 2, thethe upconversionUpconversion process,process, inin conservingconserving photon mo-mo­ yielded only marginal resultsresults inin terms of astronomical detection. mentum (hic),(hk), insures that aa oneone-to-one -to -one correspondencecorrespondence exists be-be­ However, sincesince the the problemsproblems toto datedate do not appear to be ones ofof tween infrared directions of propagationpropagation andand sumsum frequencyfrequency di-di- principle, it is hoped that upconversionUpconversion maymay becomebecome aa useful de-de­ tection technique at somesome future time.time. Work partially supported byby NASANASA GrantsGrants NGL NGL 05- 05-003-272 003 -272 and NGRNCR 05-05-003-452. 003 -452. Upconversion Theory IR-105IR -105 receivedreceived June June 8,8, 1977;1977; revisedrevised JuneJune 30, 1977.1977. Some of thethe theoreticaltheoretical aspects aspects ofof the the upconversion Upconversion processprocess willwill NovemberNovember-December -December 1977 1977 / Vol./ Vol. 16 16 No. No. 6 6 / IOPTICAL OPTICAL ENGINEERINGENGINEERING // 563 Downloaded From: http://opticalengineering.spiedigitallibrary.org/ on 02/27/2015 Terms of Use: http://spiedl.org/terms ROBERT W. BOYD be considered in this section. Only those fundamentals needed to where OkAk is the propagation vector mismatchmismatch understand the descriptiondescription ofof various various upconversionup conversion systemssystems dis-dis­ cussed later will be provided. The excellent treatments ofof Hulme2Hulme2 AkOlc = = llt IkSs -£ - kLL -lc -kIRIIR l , (6) and of MidwinterMidwinter and Zernike3Zernike3 are recommendedrecommended forfor thosethose de-de­ I?£ isis thethe length of thethe nonlinearnonlinear crystal,crystal, cc isis thethe velocityvelocity ofof light;light; siring additionaladditional information. nIR,nIR> nL,nL' andanc* nSnS areare the^e indicesmdices of°f refractionrefraction forfor the the three three waves; waves; When transparenttransparent mattermatter is subjected to intenseintense electromag-electromag­ AJRAIR andand XsX§ are the vacuum wavelengths ofof thethe sum andand infraredinfrared netic radiation, the response of the matter (measured(measured by the po-po­ fields, ILIL isis the laser power per unit area,area, andand deftdeff isis thethe effec-effec­ larization, oror dipoledipole momentmoment perper unitunit volume) volume) ceases ceases toto bebe linear-linear­ tive value of dykdijk obtained by carryingcarrying outout thethe summationsummation indi-indi­ ly dependentdependent onon thethe incidentincident fieldfield amplitudeamplitude andand displays displays non-non­ cated in Eq.Eq. (2}I.(2), For For thethe casecase ofof perfectperfect phasematching thethe term linear effects.effects. TheThe lowestlowest orderorder nonlinearitynonlinearity gives gives riserise toto thethe sec- sec­ in squaresquare brackets in Eq. (5) isis equal to one.one. We note that thethe ond order polarization, aadar^d it is tinstis nonlinearity that isis exploited system quantum efficiency is proportional to ddeff2eff2 , toto thethe laserlaser in mostmost upconverters.upconverters. IfIf EL and EIREj^ areare thethe electricelectric fieldfield ampli-ampli­ power per unitunit area,area, andand toto the the square square ofof the the lengthlength ofof thethe crys-crys­ tudes associatedassociated with the laserlaser beam and infraredinfrared field,field, respec-respec­ tal. This lastlast resultresult followsfollows fromfrom thethe phasematching phasematching conditioncondition tively, thethe nononlinearity linearity willwill induceinduce aa secondsecond orderorder polarizationpolarization which allows the individualindividual dipoles within the crystalcrystal to radiate of amplitude P withinwithin thethe AI nonlinear onlinear crystalcrystal at the sumsum andand dif-dif­ coherently. ference frequencies, where PP isis given by The infrared bandpassbandpass ofof anan upconverterupconverter isis limitedlimited to thethe ex-ex­ -JR tent thatthat thethe termterm inin square square bracketsbrackets inin Eq.Eq. (5)(5) fallsfalls toto zerozero asas Pi =22dkijk E.EIk. (2) OkAk increasesincreases as aa resultresult ofof the the infrared infrared frequency frequency being being varied varied from itsits "centralcentral value. The actualactual valuevalue ofof the the bandpass bandpass will will de-de­ dijkis-is the nonlinear coefficient of the medium,medium, and thethe indicesindices pend upon thethe detailsdetails ofof the the dispersion dispersion of of the the refractive refractive indicesindices i, j,1, andand k refer to cartesian components. InIn practice,practice, dijkdyj, cancan al-al­ for any particular crystal,crystal butbut aa goodgood rulerule ofof thumbthumb isis thatthat thethe most never bebe calculated,calculated, andand isis regardedregarded asas anan empiricalempirical con-con­ infrared bandpass inin cmcm ï is numerically equal to thethe inverseinverse of stant. the lengthlength of thethe crystalcrystal inin cm,cm, andand itft isis generallygenerally truetrue thatthat thethe Not all optically transparent materials allow this lowest order bandpass is inversely proportionalproportional to the lengthlength
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