Frunzio, L.

Li, L.

Mukherjee, A.

Prober, D. E.

Savu, V.

Schoelkopf, R. J.

Wilson, C. M.

Infoscience - École polytechnique fédérale de Lausanne

ARTICLE IN PRESSNuclear Instruments and Methods in Physics Research A 520 (2004) 237–239*Corresp203-432-428E-mail a0168-9002/$doi:10.1016Diffusion-engineered single-photon spectrometer forUV/visible detectionV. Savu, L. Li, A. Mukherjee, C.M. Wilson, L. Frunzio, D.E. Prober*,R.J. SchoelkopfDepartaments of Applied Physics and Physics, Yale University, P.O. Box 208284, New Haven, CT 06520, USAAbstractWe present predictions for a diffusion-engineered, single-photon spectrometer in the UV–visible range using asuperconducting tunnel junction. Quasiparticles are created by photoexcitation, with charge Q0. After tunnelingthrough the junction, the quasiparticles can either backtunnel or diffuse away. With conﬁnement by a higher gap or bynarrow leads the quasiparticles in the counterelectrode dwell next to the junction and backtunnel, increasing thecollected charge to Q ¼ pQ0; p>1. For very narrow leads the dwell time is inversely proportional to the lead width, upto the recombination time of Al, 1ms at 0.2K. The new aspect of our work is the use of narrow leads to control thecharge gain p, while minimizing self-heating. This charge gain will improve the energy resolution compared to the casep ¼ 1; where the electronic noise is dominant, and compared to much larger charge gain, pE50, where large self-heatingresulted with extra noise.r 2003 Elsevier B.V. All rights reserved.PACS: 74.50.+r; 85.25.Am.; 85.25.CpKeywords: Photon detector; Tunnel junction; Backtunnelling1. IntroductionDetectors using superconducting tunnel junc-tions (STJs) have been employed for detection ofsingle photons from X-ray to IR energies. Thereare important applications in astronomy andbiology for UV through IR detection, but thesmall size of the photon-induced (initial) charge, afew thousand electrons, limits the energy resolu-tion that can be achieved with present ampliﬁers.onding author. Tel.: +1-203-432-4280; fax: +1-3.ddress: daniel.prober@yale.edu (D.E. Prober).- see front matter r 2003 Elsevier B.V. All rights reserve/j.nima.2003.11.332To enhance the charge above that initially created,Q0; one employs backtunneling which causescharge recycling. The collected charge is thenQ ¼ pQ0: Backtunneling can be achieved byconﬁnement of the quasiparticles near the tunnelbarrier by a higher gap superconductor (Ta [1,2] orNb [3] conﬁnes quasiparticles in Al), or by slowingdown the diffusion away of the quasiparticles. It isthis latter approach that we introduce in this work,for use with lateral structure devices. Suchengineered outdiffusion can be used in otherapplications where non-equilibrium quasiparticlesare detected, including kinetic inductance detec-tors [4].d.ARTICLE IN PRESSAlAelectrodelwFig. 2. Device Schematic.V. Savu et al. / Nuclear Instruments and Methods in Physics Research A 520 (2004) 237–2392382. Engineered diffusionWe consider the lateral STJ geometry, Fig. 1. Inthis geometry the area of the junction can beoptimized separately from that of the absorber.For example, it allows one to use a long absorberstrip with two attached Al tunnel junctions toachieve one-dimensional imaging. Other groupsuse a stacked geometry, with either Ta [2] or Nb [3]as the higher gap superconductor/absorber, toconﬁne quasiparticles in Al.Our detector uses a Ta absorber (DTa=0.7meV); Qo/eE4000 quasiparticles are created bya 4.89 eV photon [1]. These are trapped intothe lower-gap Al trap electrode, DAl=0.18meV.For an electrode area Aelectrodes=20 mm2(=Atrapelectrode+Acounterelectrode) there areE600 thermal quasiparticles in equilibrium at0.21K. The expected signal-to-noise (FWHM)for the case of no backtunneling ðp ¼ 1Þ for a4.89 eV photon is the energy resolving power, E/DE=50, including the thermal quasiparticle cur-rent and intrinsic creation statistics. However, theampliﬁer and junction (quiescent) white noise willlimit resolution to be a factor of E5 worse [1].With backtunneling, the signal increases linearlywith p, and the noise scales as p1/2, for noisesources that remain the same. If p ¼ 20; animprovement of the energy resolution is expected,to that limited by the noise of backtunnelling, E/DE=25. For many applications this would qualifythe device.For very large p (E50) there is the heating dueto the current. This increases the number ofsteady-state quasiparticles and gives extra noise[1]. Heat removal is poor due to Andreev reﬂectionby the higher gap superconductor. Also, quasi-particles can multiply at energy E>3DAl, and forpure Al, DTa>3DAl, so after repeating tunnelingsome quasiparticles do multiply. For intermediate                     counter-electrode AlTa AlleadtrapFig. 1. Device geometry.values of p, say 3–20, heating does not appear tolead to signiﬁcant extra noise [2,3].We propose a method to achieve intermediatebacktunneling (p=5–20) in our lateral geometry.We use a geometric constriction to restrict out-diffusion, a thin Al lead which connects to a largepad (Fig. 2). The idea arose from experiments withX-ray devices, where the backtunneling was due tonarrowed leads [5]. The signature was an extendedcurrent pulse, with a decay time constant of order200 ms.3. Diffusion simulationWe modeled quasiparticle diffusion away fromthe tunnel barrier for the geometry of Fig. 1 in thefollowing limits: (1) the tunnel time is muchsmaller than the time for outdiffusion from theelectrodes; each electrode has the same volume; (2)the internal diffusion time within the electrodes isshort compared to the outdiffusion time; thismeans the electrodes can be treated as one volume;(3) the volume of the Al lead is not much largerthan that of the electrode, and the length of theAl lead is such that the diffusion time along thislead, l2/D, is shorter than the outdiffusion timefrom the electrodes; (4) tinelasticottunnelotoutdiffusiono trecombination. Under these fairly rea-sonable assumptions we ﬁnd:toutdiffusion ¼ Aeff ðl=wÞ=D ð1Þand p=(toutdiffusion/ttunnel). The effective area of theelectrodes is Aeff=(Aelectrodes+lw/2). The term lw/2accounts for the approximate half ﬁlling of thevolume of the lead by quasiparticles during theoutdiffusion time. If (lw) is small, the effective areais just that of the electrodes, and also the signal ismaximized. For example, consider a device withl=200mm, w=0.05mm, Aelectrodes=20mm2, andD=20cm2/s. In that case, toutdiffusion=50ms.ARTICLE IN PRESSFig. 3. Concentration proﬁle in time.V. Savu et al. / Nuclear Instruments and Methods in Physics Research A 520 (2004) 237–239 239With ttunnel=2.5ms, p=20. This requires accuratelithography, which we have achieved. Much thicker,cleaner Al ﬁlms (d=160nm) have [6] DE60 cm2/s.Eq. 1 was discerned from simulations done inMATLAB of 2D diffusion, using ‘pdetool’. Thesewere originally done for our X-ray devices. Thesimulations presented above in Fig. 3 are forsomewhat different device parameters. We plotthe concentration proﬁle in the electrode-lead planeat two times; t=2ms, shortly after the quasiparticlesare ‘placed’ in the electrodes; and t=20ms, at whichtime the diffusion down the lead has occurred andhas reached the steady-state linear distribution forEq. (1). The decay of concentration in the electro-des is evident while the linear distribution is beingestablished. The correction for ﬁnite lead volumegiven in Eq. (1) is found from our simulations to beaccurate to 5% up to lw=Aelectrodes.Backtunneling occurs during the entire time thatthe density of quasiparticles is ﬁnite in the regionof the junction, if the voltage bias is large [7]. STJdevices constructed with a lead width comparableto the width of the counter-electrode show only aslight enhancement of the charge, due to a smallbacktunneling effect.AcknowledgementsThis work was supported by NASA NAG5-11425. We thank A. Szymkowiak and M. Devoretfor useful discussions.References[1] C.M. Wilson, L. Frunzio, D.E. Prober, Phys. Rev. Lett. 87(2001) 067004.[2] P. Verhoeve, et al., J. Appl. Phys. 92 (2002) 6072; also I.Jerjen et al., Nucl. Instr. and Meth. A (2004), theseProceedings.[3] S. Friedrich, et al., IEEE Trans. Appl. Supercond. 9 (1999)3330.[4] B. Mazin, et al., Multiplexable kinetic inductancedetectors, In: F.S. Porter, D. McCammon, M. Galeazzi,C.K. Stahle (Eds.), Proceedings of the Ninth InternationalWorkshop on Low Temperature Detectors—2001, AIPConference Proceedings, Vol. 605, AIP, New York, 2001,p. 309.[5] L. Li, L. Frunzio, C.M. Wilson, D.E. Prober, J. Appl. Phys.76 (2003) 1137.[6] S. Friedrich, Ph.D. Thesis, Yale (1997);A. Mukherjee, Senior Thesis, Yale 2002, unpublished.[7] K. Segall, et al., Appl. Phys. Lett. 76 (2000) 3998.

Diffusion-engineered single-photon spectrometer for UV/visible detection

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Diffusion-engineered single-photon spectrometer for UV/visible detection

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