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Semenov, A., Engel, A., Il'in, K., Gol'tsman, G., Siegel, M., & Hübers, H. - W. (2003). Ultimate performance of a superconducting quantum detector. Eur. Phys. J. Appl. Phys., 21(3), 171–178.
Abstract: We analyze the ultimate performance of a superconducting quantum detector in order to meet requirements for applications in near-infrared astronomy and X-ray spectroscopy. The detector exploits a combined detection mechanism, in which avalanche quasiparticle multiplication and the supercurrent jointly produce a voltage response to a single absorbed photon via successive formation of a photon-induced and a current-induced normal hotspot in a narrow superconducting strip. The response time of the detector should increase with the photon energy providing energy resolution. Depending on the superconducting material and operation conditions, the cut-off wavelength for the single-photon detection regime varies from infrared waves to visible light. We simulated the performance of the background-limited infrared direct detector and X-ray photon counter utilizing the above mechanism. Low dark count rate and intrinsic low-frequency cut-off allow for realizing a background limited noise equivalent power of 10−20 W Hz−1/2 for a far-infrared direct detector exposed to 4-K background radiation. At low temperatures, the intrinsic response time of the counter is rather determined by diffusion of nonequilibrium electrons than by the rate of energy transfer to phonons. Therefore, thermal fluctuations do not hamper energy resolution of the X-ray photon counter that should be better than 10−3 for 6-keV photons. Comparison of new data obtained with a Nb based detector and previously reported results on NbN quantum detectors support our estimates of ultimate detector performance.
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Il'in, K., Siegel, M., Semenov, A., Engel, A., Hübers, H. - W., Hollmann, E., et al. (2004). Thickness dependence of superconducting properties of ultrathin Nb and NbN films. In AKF-Frühjahrstagung.
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Il'in, K. S., Currie, M., Lindgren, M., Milostnaya, I. I., Verevkin, A. A., Gol'tsman, G. N., et al. (1999). Quantum efficiency and time-domain response of superconducting NbN hot-electron photodetectors. IEEE Trans. Appl. Supercond., 9(2), 3338–3341.
Abstract: We report our studies on the response of ultrathin superconducting NbN hot-electron photodetectors. We have measured the photoresponse of few-nm-thick, micron-size structures, which consisted of single and multiple microbridges, to radiation from the continuous-wave semiconductor laser and the femtosecond Ti:sapphire laser with the wavelength of 790 nm and 400 nm, respectively. The maximum responsivity was observed near the film's superconducting transition with the device optimally current-biased in the resistive state. The responsivity of the detector, normalized to its illuminated area and the coupling factor, was 220 A/W(3/spl times/10/sup 4/ V/W), which corresponded to a quantum efficiency of 340. The responsivity was wavelength independent from the far infrared to the ultraviolet range, and was at least two orders of magnitude higher than comparable semiconductor optical detectors. The time constant of the photoresponse signal was 45 ps, when was measured at 2.15 K in the resistive (switched) state using a cryogenic electro-optical sampling technique with subpicosecond resolution. The obtained results agree very well with our calculations performed using a two-temperature model of the electron heating in thin superconducting films.
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Il'in, K. S., Lindgren, M., Currie, M. A., Semenov, D., Gol'tsman, G. N., Sobolewski, R., et al. (2000). Picosecond hot-electron energy relaxation in NbN superconducting photodetectors. Appl. Phys. Lett., 76(19), 2752–2754.
Abstract: We report time-resolved characterization of superconducting NbN hot-electron photodetectors using an electro-optic sampling method. Our samples were patterned into micron-size microbridges from 3.5-nm-thick NbN films deposited on sapphire substrates. The devices were illuminated with 100 fs optical pulses, and the photoresponse was measured in the ambient temperature range between 2.15 and 10.6 K (superconducting temperature transition TC). The experimental data agreed very well with the nonequilibrium hot-electron, two-temperature model. The quasiparticle thermalization time was ambient temperature independent and was measured to be 6.5 ps. The inelastic electron–phonon scattering time Ï„e–ph tended to decrease with the temperature increase, although its change remained within the experimental error, while the phonon escape time Ï„es decreased almost by a factor of two when the sample was put in direct contact with superfluid helium. Specifically, Ï„e–ph and Ï„es, fitted by the two-temperature model, were equal to 11.6 and 21 ps at 2.15 K, and 10(±2) and 38 ps at 10.5 K, respectively. The obtained value of Ï„e–ph shows that the maximum intermediate frequency bandwidth of NbN hot-electron phonon-cooled mixers operating at TC can reach 16(+4/–3) GHz if one eliminates the bolometric phonon-heating effect.
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Cherednichenko, S., Yagoubov, P., Il'In, K., Gol'tsman, G., & Gershenzon, E. (1997). Large bandwidth of NbN phonon-cooled hot-electron bolometer mixers on sapphire substrates. In Proc. 8th Int. Symp. Space Terahertz Technol. (pp. 245–257).
Abstract: The bandwidth of NbN phonon-cooled hot electron bolometer mixers has been systematically investigated with respect to the film thickness and film quality variation. The films, 2.5 to 10 mm thick, were fabricated on sapphire substrates using DC reactive magnetron sputtering. All devices consisted of several parallel strips, each 1 1.1 wide and 211 long, placed between Ti-Au contact pads. To measure the gain bandwidth we used two identical BWOs operating in the 120-140 GHz frequency range, one functioning as a local oscillator and the other as a signal source. The majority of the measurements were made at an ambient temperature of 4.5 K with optimal LO and DC bias. The maximum 3 dB bandwidth (about 4 GHz) was achieved for the devices made of films which were 2.5-3.5 nm thick, had a high critical temperature, and high critical current density. A theoretical analysis of bandwidth for these mixers based on the two-temperature model gives a good description of the experimental results if one assumes that the electron temperature is equal to the critical temperature.
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