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Lindgren M, Currie M, Zeng W-S, Sobolewski R, Cherednichenko S, Voronov B, et al. Picosecond response of a superconducting hot-electron NbN photodetector. Appl Supercond. 1998;6(7-9):423–8.
Abstract: The ps optical response of ultrathin NbN photodetectors has been studied by electro-optic sampling. The detectors were fabricated by patterning ultrathin (3.5 nm thick) NbN films deposited on sapphire by reactive magnetron sputtering into either a 5×10 μm2 microbridge or 25 1 μm wide, 5 μm long strips connected in parallel. Both structures were placed at the center of a 4 mm long coplanar waveguide covered with Ti/Au. The photoresponse was studied at temperatures ranging from 2.15 K to 10 K, with the samples biased in the resistive (switched) state and illuminated with 100 fs wide laser pulses at 395 nm wavelength. At T=2.15 K, we obtained an approximately 100 ps wide transient, which corresponds to a NbN detector response time of 45 ps. The photoresponse can be attributed to the nonequilibrium electron heating effect, where the incident radiation increases the temperature of the electron subsystem, while the phonons act as the heat sink. The high-speed response of NbN devices makes them an excellent choice for an optoelectronic interface for superconducting digital circuits, as well as mixers for the terahertz regime. The multiple-strip detector showed a linear dependence on input optical power and a responsivity =3.9 V/W.
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Blundell R, Kawamura JH, Tong CE, Papa DC, Hunter TR, Gol’tsman GN, et al. A hot-electron bolometer mixer receiver for the 680-830 GHz frequency range. In: Proc. 6-th Int. Conf. Terahertz Electron. IEEE; 1998. p. 18–20.
Abstract: We describe a heterodyne receiver designed to operate in the partially transparent atmospheric windows centered on 680 and 830 GHz. The receiver incorporates a niobium nitride thin film, cooled to 4.2 K, as the phonon-cooled hot-electron mixer element. The double sideband receiver noise, measured over the frequency range 680-830 GHz, is typically 700-1300 K. The instantaneous output bandwidth of the receiver is 600 MHz. This receiver has recently been used at the SubMillimeter Telescope, jointly operated by the Steward Observatory and the Max Planck Institute for Radioastronomy, for observations of the neutral carbon and CO spectral lines at 810 GHz and at 806 and 691 GHz respectively. Laboratory measurements on a second mixer in the same test receiver have yielded extended high frequency performance to 1 THz.
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Schubert J, Semenov A, Hübers H-W, Gol'tsman G, Schwaab G, Voronov B, et al. Broad-band terahertz NbN hot-electron bolometric mixer. In: Inst. Phys. Conf. Vol 167.; 1999. p. 663–6.
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Il'in KS, Gol'tsman GN, Voronov BM, Sobolewski R. Characterization of the electron energy relaxation process in NbN hot-electron devices. In: Proc. 10th Int. Symp. Space Terahertz Technol.; 1999. p. 390–7.
Abstract: We report on transient measurements of electron energy relaxation in NbN films with 300-fs time resolution. Using an electro-optic sampling technique, we have studied the photoresponse of 3.5-nm-thick NbN films deposited on sapphire substrates and exposed to 100-fs-wide optical pulses. Our experimental data analysis was based on the two-temperature model and has shown that in our films at the superconducting transition 10.5 K the inelastic electron-phonon scattering time was about (111}+-__.2) ps. This response time indicated that the maximum intermediate-frequency band of a NbN hot-electron phonon-cooled mixer should reach (16+41-3) GHz if one eliminates the bolometric phonon-heating effect. We have suggested several ways to increase the effectiveness of phonon cooling to achieve the above intrinsic value of the NbN mixer bandwidth.
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Gerecht E, Musante CF, Jian H, Zhuang Y, Yngvesson KS, Dickinson J, et al. Improved characteristics of NbN HEB mixers integrated with log-periodic antennas. In: Proc. 10th Int. Symp. Space Terahertz Technol.; 1999. p. 200–7.
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