Superconductive bolometers for submillimeter wavelengths

Abstract

Three types of composite superconductive bolometers are described in which the temperature-sensitive element is a superconducting film at the transition temperature, a superconductor–normal metal–superconductor Josephson junction, or a superconductor–insulator–normal metal quasiparticle tunneling junction. The temperature-sensitive element is evaporated onto a sapphire substrate on the reverse side of which is a bismuth film to absorb the submillimeter radiation. The noise limitations of each type of bolometer are calculated. The fabrication and measured performance of the transition-edge bolometer and the Josephson-junction bolometer are described. The best electrical (noise-equivalent power) NEP obtained with a transition-edge bolometer fabricated on a 4×4×0.005-mm sapphire substrate is (1.7±0.1) ×10−15 W Hz−1/2 at 2 Hz at an operating temperature of 1.27 K. This NEP is within a factor of 2 of the thermal noise limit. The effective absorptivity of the bismuth film is measured to be 0.47±0.05, and the corresponding detectivity D* is calculated to be (1.1±0.1) ×1014 cm W−1 Hz1/2. Suggestions are made for further improvements in sensitivity.