Abstract: We describe two projects aimed at applying the great temperature sensitivity of superconducting thin films when biased at heir critical temperature to high sensitivity radiometry in the 10-50 $mu@m wavelength region. The first is a standards-grade system for installation in the NIST low-background infrared calibration facility. It employs thin-film Nb thermometers, and has an operating temperature of 9 K. This system's absorber is a large aperture Cu cone with a blackened interior, to which a thermometer chip is thermally grounded. The absorber and a thermal isolation platform are each thermally stabilized with separate active feedback loops that sense the resistance of their respective thermometers. Temperature control at the level of 1.4 nK has been achieved with this system. Power sensitivity is presently limited by 1/f noise to approximately 10 pW, approximately a factor of 4 better than the current system based on Ge thermistors. The second system is intended for high-sensitivity imaging arrays, and employs YBCO films on free-standing zirconia membranes. The operating temperature is 89 K for pure YBCO devices and in the range of 70-85 K for Co-doped YBCO devices, depending on doping level. The best measured performance on these devices is an optical noise-equivalent power of 1.1 pW/Hz$+1/2$/, with an associated 3-dB rolloff frequency of 130 Hz for the pure YBCO devices and 0.6 pW/Hz$+1/2$/ and 31 Hz for the Co-doped devices. These values represent the best performance, by approximately a factor of two in NEP and an order-of- magnitude in speed, ever reported for superconducting infrared bolometers operating at liquid-nitrogen temperature. !11
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