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 Hz^1/2. Suggestions are made for further improvements in sensitivity.