Hajenius M, Barends R, Gao JR, Klapwijk TM, Baselmans JJA, Baryshev A, et al. Local resistivity and the current-voltage characteristics of hot electron bolometer mixers. IEEE Trans Appl Supercond. 2005;15(2):495–8.
Abstract: Hot-electron bolometer devices, used successfully in low noise heterodyne mixing at frequencies up to 2.5 THz, have been analyzed. A distributed temperature numerical model of the NbN bridge, based on a local electron and a phonon temperature, is used to model pumped IV curves and understand the physical conditions during the mixing process. We argue that the mixing is predominantly due to the strongly temperature dependent local resistivity of the NbN. Experimentally we identify the origins of different transition temperatures in a real HEB device, suggesting the importance of the intrinsic resistive transition of the superconducting bridge in the modeling.
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Skalare A, McGrath WR, Echternach PM, Leduc HG, Siddiqi I, Verevkin A, et al. Aluminum hot-electron bolometer mixers at submillimeter wavelengths. IEEE Trans. Appl. Supercond.. 2001;11(1):641–4.
Abstract: Diffusion-cooled aluminum hot-electron bolometer (HEB) mixers are of interest for low-noise high resolution THz-frequency spectroscopy within astrophysics. Al HEB mixers offer operation with an order of magnitude less local oscillator power, higher intermediate frequency bandwidth and potentially lower noise than competing devices made from other materials. We report on mixer experiments at 618 GHz with devices fabricated from films with sheet resistances in the range from about 55 Ω down to about 9 Ω per square. Intermediate frequency bandwidths of up to 3 GHz were measured (1 μm long device), with absorbed local oscillator power levels of 0.5 to 6 nW and mixer conversion up to -21.5 dB. High input coupling efficiency implies that the electrons in the device are able to thermalize before escaping from the device. It was found that the long coherence length complicates mixer operations due to the proximity of the contact pads. Also, saturation at the IF frequency may be a concern for this type of device, and warrants further studies.
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Hajenius M, Yang ZQ, Gao JR, Baselmans JJA, Klapwijk TM, Voronov B, et al. Optimized sensitivity of NbN hot electron bolometer mixers by annealing. IEEE Trans Appl Supercond. 2007;17(2):399–402.
Abstract: We report that the heterodyne sensitivity of superconducting hot-electron bolometers (HEBs) increases by 25-30% after annealing at 85degC in high vacuum. The devices studied are twin-slot antenna coupled mixers with a small area NbN bridge of 1 mum times 0.15 mum, above which there is a SiO 2 passivation layer. The mixer noise temperature, gain, and resistance versus temperature curve of a HEB before and after annealing are compared and analysed. We show that the annealing reduces the intrinsic noise of the mixer by 37% and makes the superconducting transition of the bridge and the contacts sharper. We argue that the reduction ofthe noise is mainly due to the improvement of the transparency of the contact/film interface. The lowest receiver noise temperature of 700 K is measured at a local oscillator frequency of 1.63 THz and at a bath temperature of 4.2 K.
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Trifonov A, Tong C-YE, Grimes P, Lobanov Y, Kaurova N, Blundell R, et al. Development of a silicon membrane-based multipixel hot electron bolometer receiver. IEEE Trans Appl Supercond. 2017;27(4):1–5.
Abstract: We report on the development of a multipixel hot electron bolometer (HEB) receiver fabricated using silicon membrane technology. The receiver comprises a 2 × 2 array of four HEB mixers, fabricated on a single chip. The HEB mixer chip is based on a superconducting NbN thin-film deposited on top of the silicon-on-insulator (SOI) substrate. The thicknesses of the device layer and handling layer of the SOI substrate are 20 and 300 μm, respectively. The thickness of the device layer is chosen such that it corresponds to a quarter-wave in silicon at 1.35 THz. The HEB mixer is integrated with a bow-tie antenna structure, in turn designed for coupling to a circular waveguide, fed by a monolithic drilled smooth-walled horn array.
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Zhang W, Miao W, Li SL, Zhou KM, Shi SC, Gao JR, et al. Measurement of the spectral response of spiral-antenna coupled superconducting hot electron bolometers. IEEE Trans Appl Supercond. 2013;23(3):2300804.
Abstract: Measured spectral response of spiral-antenna coupled superconducting hot electron bolometers (HEBs) often drops dramatically at frequencies that are still within the frequency range of interest (e.g., ~ 5 THz). This is inconsistent with the implied low receiver noise temperatures from the same measurements. To understand this discrepancy, we exhaustively test and calibrate the thermal sources used in Fourier transform spectrometer measurements. We first investigate the absolute emission spectrum of high-pressure Hg arc lamp, then measure the spectral response of two spiral-antenna coupled NbN HEBs with a Martin-Puplett interferometer as spectrometer and 77 K blackbody as broadband signal source. The measured absolute emission spectrum of Hg arc lamp is proportional to frequency, corresponding to an equivalent blackbody temperature of 4000 K at 1 THz, 1500 K at 3 THz, and 800 K at 5 THz, respectively. Measured spectral response of spiral-antenna coupled NbN HEBs, corrected for air absorption, is nearly flat in the frequency range of 0.5-4 THz, consistent with simulated coupling efficiency between HEB and spiral-antenna. These results explain the discrepancy, and prove that spiral-antenna coupled superconducting NbN HEBs work well in a wide frequency range. In addition, this calibration method and these results are broadly applicable to other quasi-optical THz receivers.
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