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Prober, D. E. (1993). Superconducting terahertz mixer using a transition-edge microbolometer. Appl. Phys. Lett., 62(17), 2119–2121.
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Siddiqi, I., & Prober, D. E. (2004). Nb–Au bilayer hot-electron bolometers for low-noise THz heterodyne detection. Appl. Phys. Lett., 84(8), 1404.
Abstract: The sensitivity of present Nb diffusion-cooled hot-electron bolometer (HEB) mixers is not quantum limited, and can be improved by reducing the superconducting transition temperature TC. Lowering TC reduces thermal fluctuations, resulting in a decrease of the mixer noise temperature TM. However, lower TC mixers have reduced dynamic range and saturate more easily due to background noise. We present 30 GHz microwave measurements on a bilayer HEB system, Nb–Au, in which TC can be tuned with Au layer thickness to obtain the maximum sensitivity for a given noise background. These measurements are intended as a guide for the optimization of THz mixers. Using a Nb–Au mixer with TC = 1.6 K, we obtain TM = 50 K with 2 nW of local oscillator (LO) power. Good mixer performance is observed over a wide range of LO power and bias voltage and such a device should not exhibit saturation in a THz receiver.
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Burke, P. J., Schoelkopf, R. J., Prober, D. E., Skalare, A., Karasik, B. S., Gaidis, M. C., et al. (1998). Spectrum of thermal fluctuation noise in diffusion and phonon cooled hot-electron mixers. Appl. Phys. Lett., 72(12), 1516–1518.
Abstract: A systematic study of the intermediate frequency noise bandwidth of Nb thin-film superconducting hot-electron bolometers is presented. We have measured the spectrum of the output noise as well as the conversion efficiency over a very broad intermediate frequency range (from 0.1 to 7.5 GHz) for devices varying in length from 0.08 μm to 3 μm. Local oscillator and rf signals from 8 to 40 GHz were used. For a device of a given length, the spectrum of the output noise and the conversion efficiency behave similarly for intermediate frequencies less than the gain bandwidth, in accordance with a simple thermal model for both the mixing and thermal fluctuation noise. For higher intermediate frequencies the conversion efficiency decreases; in contrast, the noise decreases but has a second contribution which dominates at higher frequency. The noise bandwidth is larger than the gain bandwidth, and the mixer noise is low, between 120 and 530 K (double side band).
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Prober, D. E. (1993). Superconducting terahertz mixer using a transition-edge microbolometer. Department of Applied Physics and Physics, Yale University, New Haven, Connecticut 06520-2157, , 2119–2121.
Abstract: We present a new device concept for a mixer element for THz frequencies. This uses a superconducting transition-edge microbridge biased at the center of its superconducting transition near 4.2 K. It is fed from an antenna or waveguide structure. Power from a local oscillator and a rf signal produce a temperature and resulting resistance variation at the difference frequency. The new aspect is the use of a very short bridge in which rapid ( < 0.1 ns) outdiffision of hot electrons occurs. This gives large intermediate frequency (if) response. The mixer offers ~4 GHz if bandwidth, z 80 Cl rf resistive impedance, good match to the if amplifier, and requires only l-20 nW of local oscillator power. The upper rf frequency is determined by antenna or waveguide properties. Predicted mixer conversion efficiency is l/8, and predicted double-sideband receiver noise temperatures are 260 and 90 K for transition widths of 0.1 and 0.5 T, respectively.
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Skalare, A., McGrath, W. R., Echternach, P. M., Leduc, H. G., Siddiqi, I., Verevkin, A., et al. (2001). Aluminum hot-electron bolometer mixers at submillimeter wavelengths. IEEE Trans. Appl. Supercond., 11(1), 641–644.
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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