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Optical/UV single-photon imaging spectrometers using superconducting tunnel junctions

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* Corresponding author.Nuclear Instruments and Methods in Physics Research A 444 (2000) 449}452Optical/UV single-photon imaging spectrometersusing superconducting tunnel junctionsC.M. Wilson*, K. Segall, L. Frunzio,L.Li, D.E. Prober, D. Schiminovich,B. Mazin, C. Martin, R. VasquezYale University, New Haven, CT 06520, USACalifornia Institute of Technology, Pasadena, CA, USANASA Jet Propulsion Laboratory, Pasadena, CA, USAAbstractWe present preliminary test results of optical/UV single-photon imaging spectrometers using superconducting tunneljunctions. Our devices utilize a lateral trapping geometry. Photons are absorbed in a Ta thin "lm, creating excessquasiparticles. Quasiparticles di!use and are trapped by Al/AlOV/Al tunnel junctions located on the sides of theabsorber. The Ta/Al interface does not overlap the junction area. Imaging devices have tunnel junctions on two oppositesides of the absorber. Position information is obtained from the fraction of the total charge collected by each junction. Wehave fabricated high-quality junctions with a ratio of subgap resistance to normal state resistance greater than 100 000at 0.22 K. We have measured the single-photon response of our devices. For photon energies between 2 and 5 eV, wemeasure an energy resolution between 1 and 1.6 eV. We can estimate the number of pixels the device can resolve fromthe energy resolution. We "nd that these early devices have as many as 4 pixels per strip.  2000 Published by ElsevierScience B.V. All rights reserved.PACS: 07.60.Rd; 42.79.Pw; 85.25!j; 85.60.GzKeywords: Superconductivity; Spectrometer; Junction; Imaging; Detector1. IntroductionThere has been great interest in the concept ofsingle-photon spectroscopy in recent years. Twocompeting technologies are superconducting tun-nel junctions (STJ) and transition edge sensors(TES) [1,2]. These detectors can operate at energiesranging from the optical to the X-ray. In the op-tical/UV region, much attention has been focusedon imaging spectrometers. The general approachpursued to date is large format arrays of single pixeldetectors [3].We propose to develop STJ detectors with intrin-sic imaging, meaning that the detectors have manymore pixels than read out channels. We do thisusing STJ detectors with lateral trapping. Thiswork is an extension of successful X-ray work,where we have made detectors with a resolution of26 eV referred to a 6 keV X-ray [4,5].0168-9002/00/$ - see front matter  2000 Published by Elsevier Science B.V. All rights reserved.PII: S 0 1 6 8 - 9 0 0 2 ( 9 9 ) 0 1 4 2 1 - 7 SECTION IX.Fig. 1. Schematic of an imaging STJ detector using lateraltrapping and backtunnelling. Not shown is an insulating SiOlayer between the trap and wiring.2. Operating principleFig. 1 shows a schematic drawing of an imagingSTJ detector. Many physical processes are involvedin the operation of these detectors. First, an incidentphoton is absorbed in the central Ta "lm breakingCooper pairs and creating quasiparticles. Thequasiparticles di!use until they reach the Al. In theAl, they can scatter inelastically, losing energy untilthey reach the Al gap. Once the quasiparticles scat-ter below the gap of Ta, they are `trappeda in the Alelectrode. The quasiparticles then tunnel and areread out as an excess subgap current. The currentpulsesarethenintegratedtoobtainachargefromeach junction, Qand Q.WehavetherelationQ#QJE*2,where Eis the photon energy and *2is theenergy gap of Ta.The fraction of charge collected in each junctiontells us the location of the absorption event. If thephoton is absorbed in the center, then the chargedivides equally. If the photon is absorbed at oneedge of the absorber, then most of the charge iscollected by the closest junction. In the limit of noabsorber loss and perfect trapping¸*¸5(2E*E,where ¸ is the length of the absorber, *¸ is theuncertainty in the position and *E is the uncertain-ty in energy [6]. *¸ is the e!ective size of a pixel.Another important process in our detectors isbacktunneling. Considering one half of Fig. 1, wehave a lateral Ta/Al/AlOV/Al/Ta tunnel junction.We inject excited quasiparticles into the Al from theTa absorber. The high-Ta gap then con"nes excitedquasiparticles near the tunnel barrier. Quasipar-ticles can then circulate, tunneling and backtunnel-ing. Because both tunneling and backtunnelingtransfer a charge in the forward direction, wemeasure an integrated charge many times greaterthen the number of quasiparticles. This e!ect givesthe junctions charge gain.We have designed our devices to maximize back-tunneling. We have interrupted the Al wiring withTa plugs. The absorber and the plugs con"ne thequasiparticles near the junction.3. Experimental conditionsAll devices have been fabricated at Yale ina high-vacuum deposition system with in situ ionbeam cleaning. We start with an oxidized Si sub-strate. The Ta absorber and plugs are then sput-tered at 7503C. Next, a Nb ground contact issputtered. The Al trilayer is then evaporated in onevacuum cycle. An SiO insulating layer is evapor-ated and "nally Al wiring is evaporated. An in situion beam cleaning is performed before each metaldeposition to ensure good metallic contact. Alllayers are patterned photolithographically usingeither wet etching or lift-o!.Measurements are made in a two stageHedewar. The base temperature is 220 mK.To measure the photo-response of our junctions,we use a room temperature JFET current ampli"er.We use an Amptek A250 ampli"er with a 2SK146input transistor. Extra circuitry is added thatallows the A250/2SK146 to be DC coupled to thejunctions [7]. The ampli"er thus provides an activevoltage bias for the junction.We illuminate the detectors using a small Hglamp calibration source. A band-pass "lter is usedto select one photon energy at a time. We bringlight into the dewar using an optical "ber. The "beris UV grade fused silica. The "ber is Al coated toenhance UV transmission up to energies of 6 eV.The "ltered light passes through a "ber splitter thatdivides the light equally between two "bers. Oneof these "bers is fed into the dewar. The other "beris fed into a photomultiplier tube which simulta-neously measures the intensity.450 C.M. Wilson et al. / Nuclear Instruments and Methods in Physics Research A 444 (2000) 449}452Fig. 2. Subgap I}< curves of two junctions. Both measure-ments are made at ¹"220 mK. The junction parametersare: (a) area"400

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