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IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 13, NO. 2, JUNE 2003
Response Time Characterization of NbN Superconducting Single-Photon Detectors Jin Zhang, W. Slysz, A. Verevkin, O. Okunev, G. Chulkova, A. Korneev, A. Lipatov, G. N. Gol’tsman, and Roman Sobolewski
Abstract—We report our time-resolved measurements of NbNbased superconducting single-photon detectors. The structures are meander-type, 10-nm thick, and 200-nm wide stripes and were operated at 4.2 K. We have shown that the NbN devices can count single-photon pulses with below 100-ps time resolution. The response signal pulse width was about 150 ps, and the system jitter was measured to be 35 ps. Index Terms—NbN superconducting films, quantum communications, quantum efficiency, response time, single-photon detector.
I. INTRODUCTION
T
HE NbN superconducting single-photon detectors (SSPDs) are very promising for their ultrafast response time and high quantum efficiency (QE) at radiation wavelengths ranging from ultraviolet to infrared (0.4 m to 3 m) in super[1]–[4]. The typical value of the energy gap conductors is two to three orders of magnitude lower than that is about 2.6 meV at 4.2 K). of semiconductors (for NbN, Thus, an absorbed photon creates a large number of excited quasiparticles in the superconducting stripe [5]. This leads to a resistive hotspot formation in an ultrathin film and extends the detectable range of SSPDs well into the infrared wavelengths. Simultaneously, SSPDs exhibit very low dark counts and low jitter without the need to go to the subKelvin operation environment. 5 10 A/cm .The single-photon counting property of NbN SSPDs has been investigated very extensively, and a model of the hotspot-formation process has been introduced to explain Manuscript received August 5, 2002. This work was supported in part by the AFOSR under Grant F49620-01-0463 and in part by NPTest, San Jose, CA. J. Zhang is with the Materials Science Program and Laboratory for Laser Energetics, University of Rochester, Rochester, NY 14627 USA (e-mail:
[email protected]). W. Slysz is with the Department of Electrical and Computer Engineering and Laboratory for Laser Energetics, University of Rochester, Rochester, NY 14627-0231 USA, and also with the Institute of Electron Technology, PL-02668 Warsaw, Poland. A. Verevkin is with the Department of Electrical and Computer Engineering and Laboratory for Laser Energetics, University of Rochester, Rochester, NY 14627-0231 USA. O. Okunev, G. Chulkova, A. Korneev, and A. Lipatov are with Department of Physics, Moscow State Pedagogical University, Moscow 119435, Russia. G. N. Gol’tsman is with Department of Physics, Moscow State Pedagogical University, Moscow 119435, Russia, and also with the Department of Electrical and Computer Engineering, University of Rochester, Rochester, NY 14627-0231 USA. R. Sobolewski is with the Department of Electrical and Computer Engineering and Laboratory for Laser Energetics, University of Rochester, Rochester, NY 14627-0231 USA, and also with the Institute of Physics, Polish Academy of Sciences, PL-02668 Warsaw, Poland. Digital Object Identifier 10.1109/TASC.2003.813675
Fig. 1.
SEM image of a 4
2 4 m
area SSPD.
2
Fig. 2. I –V curve of a 4 4 m -area SSPD, measured at temperature of 4.2 K. The dashed line represents the switching, 50- -load line.
the photon-counting mechanism observed in our NbN SSPDs [6], [7]. In this paper, we present the results of our time-resolved studies of NbN SSPDs, where we used a special variable optical-delay setup. The Section II reviews very briefly the SSPD fabrication process and presents the experimental setup used in our time-resolved measurements. Section III discusses our experimental results with the emphasis placed on the detector time response and jitter. Finally, our conclusions are in Section IV. II. DEVICE FABRICATION AND EXPERIMENTAL SETUP A. Fabrication of NbN SSPDs The devices used in our experiments were meander-strucnm tured, NbN superconducting stripes with a thickness nm. Fig. 1 shows an SEM image and a nominal width of a 4 4- m detector, demonstrating very high uniformity of
1051-8223/03$17.00 © 2003 IEEE
ZHANG et al.: RESPONSE TIME OF NbN SUPERCONDUCTING DETECTORS
Fig. 3.
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Schematics of our experimental setup with the optical delay stage.
the meander line. The device superconducting transition temperature was about 10.5 K, and the critical current density at 4.2 K was The SSPDs were patterned using electron-beam lithography from NbN films deposited on double-side optically-flat sapphire substrates by reactive dc magnetron sputtering in the Ar N gas mixture. The meander structure was ion milled in Ar atmosphere through a supplementary Ti mask, which was removed in the final stage by wet etching in diluted hydrofluoric acid [8]. characteristics of a Fig. 2 shows a current–voltage – 4 - m SSPD, measured at 4.2 K using a voltage source. The – curve is typical for a very long superconducting stripe. When surpasses the critical current , the detector switches to a resistive plateau. In our experiment, we biased the SSPD (point A in Fig. 2). After absorption of a photon, a at transient resistive barrier is formed across the width of the NbN stripe and the device temporarily switches along the 50- load line into a meta-stable resistive state (point B in Fig. 2), generating an approximately 2-mV voltage signal [4]. B. Experimental Setup A schematic diagram of our experimental setup is shown in Fig. 3 The device was placed on a cold plate inside an optical, liquid-helium cryostat and maintained at 4.2 K. The tested SSPD was wire-bonded to a microstrip line and connected to a bias source and an output circuit via a broadband bias-tee. The photon source was a Ti:sapphire laser with wavelength of 810 nm, repetition rate of 82 MHz, and pulse width of 100 fs. To measure the time-resolved response of our SSPDs, we included an adjustable optical-delay stage in the setup. The input laser beam was split into two beams by a 50/50 beam splitter. One of the beams was delayed by the optical delay stage and then merged back into the original beam by a second beam splitter. The minimum amount of delay is about 100 ps. A bank of adjustable neutral-density filters, placed after the focus lens, was used to attenuate the power of radiation down to a picowatt level. The optical beam diameter was 50 m, much larger than the device size. The SSPD voltage response generated by the incident photons was amplified by a 20-dB-gain, room-temperature
amplifier and then fed to the Tektronix 7404 single-shot digital oscilloscope, synchronously triggered by the Ti:sapphire laser, or counted by the SR400 photon counter. The amplifier and the oscilloscope bandwidths were 0.01 to 12 GHz and 0 to 4 GHz, respectively. The internal risetime of the oscilloscope was 95 ps. III. EXPERIMENTAL RESULTS A. NbN SSPD Time-Resolved Response With the variable-time-delay setup shown in Fig. 3 and the Tektronix 7404 oscilloscope, we have recorded the single-shot transients of our SSPD responses. The results are shown in Fig. 4. All recorded signals [Fig. 4(a)–(e)] had an amplitude of 300 mV, a width of 150 ps, and a risetime of 100 ps, with time characteristics limited, as we mentioned earlier, by our read-out electronics [single pulse is shown in Fig. 4(a)]. When the optical delay was adjusted to be 100 ps [Fig. 4(b)], the observed pulse shape had an increased width and an evidence of the second, time-delayed pulse could be detected (see arrows). Clearly, the SSPD was able to independently record two photons arriving through the separate arms of the delay setup. The detection of two independent photon pulses is clear in Fig. 4(c)–(e), where the delay between the pulses was set to 330 ps, 650 ps, and 1080 ps, respectively. Since the superconducting state had to be recovered between the responses for the detector to be able to respond to the second photon, Fig. 4 demonstrates that the time resolution of our SSPDs is below 100 ps. Correspondingly, they should be able to detect photons with at least 10-Gbit/s counting rate. B. Jitter Measurement Fig. 5 was taken at an incident flux of about 10 photons/pulse , and the oscil(to achieve adequate statistics), with loscope working at the accumulation mode. The resulting trace represents millions of individual responses overlapping one another. Its width is only about 150 ps, including the jitter. We note that the pulses from our SSPD are extremely reproducible and stable. The analysis of the pulse shape performed using the standard histogram method (Tektronix 7404 built-in feature)
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Fig. 4.
IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 13, NO. 2, JUNE 2003
Time-resolved pulse patterns at different optical delays. (a) delay = 0; (b) delay = 100 ps (we can see the signature of the second pulse); (c) delay =
330 ps; (d) delay = 650 ps; (e) delay = 1080 ps.
of reaching 10-GHz counting rates, making them the device-ofchoice for ultrafast, both free-space and fiber-based, quantum communications applications [10], [11]. ACKNOWLEDGMENT The authors thank K. Wilsher and W. Lo for stimulating discussions. REFERENCES Fig. 5. Accumulated photoresponse signal of a NbN SSPD. Jitter measurement of the signal is presented by the histogram at the top part of the figure. The width of the accumulated response is about 150 ps and the system jitter is about 35 ps.
showed (top traces in Fig. 5) that the total system jitter was about 35 ps. This value includes, besides the SSPD, the jitter from our laser 4 ps , as well as from the output circuit and the oscilloscope; thus, the intrinsic SSPD jitter is expected to be much smaller. We stress that the SSPD value of jitter is significantly smaller than any of its semiconductor counterparts [9]. IV. CONCLUSIONS We have studied the time dynamics of the photoresponse of NbN SSPDs. The device operation is based on the hotspot-assisted formation of a resistive barrier across an ultrathin and submicrometer-width superconducting stripe. Using our variable optical delay setup, we demonstrated that our devices can resolve photon trains with pulse separations of less than 100 ps. The measured system jitter of our SSPDs was 35 ps, making them very promising for ultrafast applications. The time-resolution and jitter results show that the NbN SSPDs are capable
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ZHANG et al.: RESPONSE TIME OF NbN SUPERCONDUCTING DETECTORS
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