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To implement the detector-based radiometric scale in the new Medium Background Infrared (MBIR) facility at the National Institute of Standards and Technology (NIST), we have developed an electrical substitution cavity radiometer that can be operated just above liquid nitrogen temperature. This MBIR Active Cavity Radiometer (ACR) utilizes a temperature-controlled receiver cone and an independently temperature-controlled heat sink. Being a thermal-type detector, low noise and drift of the radiometer signal depends mainly on low-noise temperature control of the receiver and heat sink. Using high critical-temperature (T c) superconducting thin film temperature sensors in the active control loops, we have achieved closed-loop temperature controllability of better than 10 µK at 89 K for a receiver having an open-loop thermal time constant of about 75 seconds. For a flux level of 1 µW to 10 µW, the rms noise floor over a measurement cycle time is below 20 nW. This is the lowest noise level yet reported for a liquid nitrogen cooled electrical-substitution radiometer, and it is the first demonstration of the use of high-T c superconductors in such a radiometer. Potential uses for this ACR in the MBIR facility include absolute measurement of the broadband radiance of large-area 300 K cryogenic blackbody sources, and absolute measurement of the spectral radiance of laser-illuminated integrating spheres for improved relative spectral responsivity measurements of infrared transfer standard radiometers.

We report measurements of the optical responses of a La2/3Sr1/3MnO3 (LSMO) sample at a wavelength of 533 nm in the 300 -400 K range. The 200 nm thick film was grown by pulsed laser deposition on (100) SrTiO3 substrate and showed remarkably low noise. At 335 K the temperature coefficient of the resistance of a 100 µm wide 300 µm long LSMO line was 0.017 K -1 and the normalized Hooge parameter was 9×10 -30 m 3 , which is among the lowest reported values. We then measured an optical sensitivity at I = 5 mA of 10.4 V.W -1 and corresponding noise equivalent power (NEP) values of 8.1×10 -10 W.Hz -1/2 and 3.3×10 -10 W.Hz -1/2 at 30 Hz and above 1 kHz, respectively. Simple considerations on bias current conditions and thermal conductance G are finally given for further sensitivity improvements using LSMO films.The performances were indeed demonstrated on bulk substrates with G of 10 -3 W.K -1 . One could expect a NEP reduction by three orders of magnitude if a membrane-type geometry was used, which makes this LSMO device competitive against commercially available uncooled bolometers.Submitted to Appl. Phys. Lett. 1The remarkable electronic and magnetic properties of rare-earth manganite oxides have raised lot of interests for applications [1]. In addition to the widely studied magnetoresistive sensors and spin electronics, uncooled bolometers using the large resistance change at the metal-to-insulator transition are among the most promising. Indeed uncooled technologies open new opportunities for infrared detection for both military and commercial applications [2]. Actual materials used for the fabrication of uncooled resistive bolometers are amorphous semiconductors, polycrystalline SiGe, semiconducting YBa2Cu3O7 (YBCO), VO2 and VOx. Around 300 K they present typical maximum temperature coefficient of the resistance (TCR), defined as the relative resistance derivative (1/R)×(dR/dT), of 0.02 to 0.06 K -1 [3, 4], -0.007 K -1 [5], -0.029 K -1 [6], -0.017 K -1 [7, 8] and -0.033 K -1 [9], respectively.Rajeswari et al. [10] showed that the optical response of La0.67Ca0.33MnO3 was of thermal nature, but the maximum of the resistance derivative (corresponding to TCR = 0.079 K -1 ) was obtained at 260 K. Goyal et al. [11] mentioned the high TCR values of several compositions of manganites. The highest values (0.2 K -1 ) were obtained in Nd0.7Sr0.3MnO3 well below room temperature (235 K). Only La0.7Sr0.3MnO3 and La0.7Ba0.3MnO3 that showed TCR of 0.02 K -1 and 0.04 K -1 , respectively, were suitable for uncooled operation. Other manganite compositions such as La2/3(Ca,Pb)1/3MnO3 [12] and (La0.6Pr0.4)0.67Ca0.33MnO3 [13] even showed TCR as high as 0.1 K -1 at 300 K. However no noise data are available for the first material and preliminary measurements showing a high low frequency noise level are reported for the second one. In comparison TCR of La0.5Ba0.5MnO3 and La0.5Sr0.5MnO3 were lower (about 0.015-0.02 K -1 ) but with a much lower noise level at 30 Hz, which makes them potentially more suitable for uncooled bolometer fabricati...