Superconducting juncton bolometers

A composite bolometer was developed in which the temperature-sensing element was a superconducting junction, the radiation absorber was a thin film of bismuth, and the support substrate was a piece of sapphire. Two types of junction were used: the superconductor-normal metal-superconductor (SNS) junction and the superconductor-insulator-normal metal (SIN) junction. The performance of these bolometers was measured and compared with that calculated from fundamental noise limits. Near-ideal performance was achieved with the SNS bolometers, and the best electrical NEP (noise equivalent power) observed was 5 x 10-15 W/ √Hz. The sensitivity of the SIN bolometers was limited by amplifier noise, but use of the best available amplifier combined with an impedance-matching device at cryogenic temperatures might provide performance comparable with that of the SNS bolometers. The absorption of infrared radiation by bismuth films of various thicknesses on sapphire substrates was measured over the frequency range 3 to 300 cm-1 using a Fourier-transform Michelson interferometer. The results were found to be in good agreement with theoretical calculations. Fabry-Perot fringes appeared in the absorption spectra due to interference between reflections from the two faces of the sapphire substrate. The maximum average total absorption observed for the sapphire-bismuth composites was 58 percent. (auth)