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Current Assisted Hot-Spot DetectionRegimej > j Chν2rj < j CDetection criteriarm2w≥1−IIA.Semenov et. al,Physica C 351, 349 (2001),European J. Appl. Phys. (2003)Crm=hν1ζ∆ 2 DτdthN0∆⎛ 1⎜4⎝πN0⎞⎟∆ ⎠1/3⎛2ln⎜1+π⎝hν∆ Dτth1d N0∆⎞⎟⎠The detection mechanismfeatures:- tunable spectral cutoffr ∝ hν/mT C- energy resolution19.20.160.14 16.8Spot radiusSpot radius, nm14.4 0.1212 0.10.08 4.80.02 2.40 0I = 0.9 I Cλ = 30 µmSimulations for5 nm thick Ti filmat 0.5 K0 2 4 6 8Time, psInstitut für Weltraumsemsorik und Planetenerkundung 4

Multiphonon processw 2w 1Pho to ncreatedhot-spottimeProbability of absorbing m photns from thepulse containing N photonsP(m)=eNNm!m∝mNm!for small NInstitut für Weltraumsemsorik und Planetenerkundung 5

Experiment - CW regimeNb single-photon detectorExperimental Setup & Detector LayoutBiasBias TeeLight SourceSpectrometerDetectorInstitut für Weltraumsemsorik und Planetenerkundung 6

Experimental datasingle & multiphoton regimeGöl‘tsman et all, APL 2001, MSPU in co-operation with theUniversity of Rochester, USATime delay of the resistive state formation hasBeen recently demonstrated that confirms thecurrent assisted nature of the detection mechanism.Uni. Rochester & MSPU, PRB 2003Institut für Weltraumsemsorik und Planetenerkundung 7

Experimental dataBeyond the spectral cut-off ?There is no plateau in the current dependencesof the detection efficiency, instead it continuouslyDrops when current decreases. The slope dependson the photon energyVerevkin et all, APL 2002Institut für Weltraumsemsorik und Planetenerkundung 9

What about another material?We used NbN films deposited at low N 2pressure and at a high deposition rate.Nitrogen deficiency resulted in lowertransition temperature, smaller criticalcurrent and reduced density of electronicstates.Institut für Weltraumsemsorik und Planetenerkundung 10

Spectral cut-off1E-31E-4Q.E.1E-51E-60,64 I c0,77 I c0,89 I c1E-71E-81E-9200 400 600 800 1000 1200 1400 1600 1800 2000 2200 2400 2600λ (nm)The hot-spot model predicts constant efficiencybefore the cut-off and vanishing efficiency atlarger wavelengths. Indeed, thereappears the plateau and the clearly seen roll-offwavelength that depends on the bias currentInstitut für Weltraumsemsorik und Planetenerkundung 11

Signal counts vs Bias Current1.00E+001.00E-01Bias Current (I/IC)1.00E-021.00E-031.00E-041.00E-051.00E-061.00E-070.4 0.5 0.6 0.7 0.8 0.9 1 1.1Count Rate (norm)Current dependences of signal counts showalmost “ideal” behaviour. However, the modelpredicts abrupt drop of the efficiency beyondthe roll-offInstitut für Weltraumsemsorik und Planetenerkundung 12

Energy resolution3x10 5800 nm - 1.6 eVnumber of pulses (s -1 )2x10 51x10 50-10 0 10 20 30 40ε (eV)File: UNTITLEDWindow: Graph28The HWFM resolution of 12 eV was found forunoptimized readout.Institut für Weltraumsemsorik und Planetenerkundung 13

Experimental dataDark countsDark count rate, sec -11000000001000000010000001000001000010001001010.10.011.8 K2.1 K2.5 K2.8 K3.0 K0 0.5 1 1.5 2 2.5 3Temperature, K2⎡ 2∆Fξ d ⎛ ⎞⎤⎢ ⎜ ⎛ I ⎞n = A⎟⎥⎢ ⎜⎜⎟Rexp − 1−k⎟BT⎥⎣ ⎝ ⎝ IC⎠ ⎠⎦Dark counts appear due to order-parameterfluctuations. Relevant volume has the sizecomparable to the coherence length.Institut für Weltraumsemsorik und Planetenerkundung 14

Edge effectA> BABHot-spotdiameter2.5SD221.510 1 1.5 2 2.5 3Position of the absorption siteInstitut für Weltraumsemsorik und Planetenerkundung 15

Nonuniformities as possible reasonfor current dependence of QEw 1w 2w 3w 410.10.01QE0.0010.00010.000010.60 0.70 0.80 0.90 1.00Current, rel.unitsInstitut für Weltraumsemsorik und Planetenerkundung 16

Morphologe of 3-nm thick NbN filmInstitut für Weltraumsemsorik und Planetenerkundung 17

Applications- Debugging of transistors in the PC processors –University of Rochester & MSPU- Spectroscopy of interstellar dust clouds atwavelengths around 70 mm.Background limited performance down to 4 Ktelescope temperature- X-ray spectroscopy1 ppm energy resolution- Solar blind applications where UV photons aredetected in the presence of bright visible photonsources.Tune the cut-off wavelength to ultraviolet- Another foreseen applications – tracingdynamics of single molecules, Quantumcryptography.Institut für Weltraumsemsorik und Planetenerkundung 18

Background limited quantum detectorfor astronomyComputed Noise Equivalent Power for the Tidirect quantum detectorNEP (W Hz -1/2 )1.0E-1210 -121.0E-1410 -141.0E-1610 -161.0E-1810 -181 - η = 0.1I/I C = 0.9λ 0 = 119 µm2 - η = 1I/I C = 0.9λ 0 = 119 µm11.0E-2010 -20233 - η = 0.1I/I C = 0.85λ 0 = 71 µm1.0E-2210 -221 10 100Background temperature (K)Expected energy resolution at 40 µm – 1/20that is sufficient for spectroscopy of interstellardust cloudsInstitut für Weltraumsemsorik und Planetenerkundung 19

X-Ray Photon CounterResponse duration of the Ta X-ray photoncounter as function of the photon energyResponse duration (ns)2.521.51Resistance, Ω21.510.50∆t0 1 2 3Time, ns1230.501 - d = 70 nm; I/I C = 0.942 - d = 70 nm; I/I C = 0.903 - d = 90 nm; I/I C = 0.900 2 4 6 8Photon energy (keV)Pixel 2.5 X 2.5 µm 2Film thickness 70 nmProjected energy resolution - ≈ 10 -4 for 6 keV photonsInstitut für Weltraumsemsorik und Planetenerkundung 20

CONCLUSIONThe mechanism of current-assistedhot-spot single photon detection in acurrent biased superconducting filmhas been proposedPractical NbN SuperconductingQuantum Detectors have beendemonstrated, although the mechanismis not yet fully proved.Variation of the material parametersallowed us to observe key features ofthe envisioned detection mechanism.The technology seems to be feasiblefor further applications. To prove thatstudy of SQD based on anothermaterials is necessary.Special thanks to:Gregory Gol‘tsmanAlexander KorneevOleg OkunevInstitut für Weltraumsemsorik und Planetenerkundung 21

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