Superconducting transition edge bolometer and noise in thin films

The development of the composite superconducting transition edge bolometer is reported. The temperature sensitive element is an aluminum strip evaporated onto the sapphire substrate. A bismuth film evaporated on the reverse side of the substrate is used to absorb the submillimeter radiation. The noise limitation of the bolometer is calculated. The fabrication and measured performance are described. The best electrical NEP (noise-equivalent-power) obtained is (1.7 +- 0.1) x 10-15 WHz-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) x 1014 cm W-1 Hz1/2. Suggestions are made for further improvements in sensitivity. The current-dependent noise in thin metal films at the superconducting transition has been further investigated. The measured noise power spectrum of the tin film on sapphire substrate at the superconducting transition is compared with the cosine transforms of the decay curves obtained from step-function and delta-function thermal perturbations. The nature of the noise driving term is found to be a random current flowing inside the sample, in agreement with the uncorrelated thermal fluctuation model. This result is consistent with the case of a freely suspended tin fiber at the superconducting transition, but in contrast to the room temperature measurement where the cosine transform of the step-function response gives the noise power spectrum, in agreement with the correlated fluctuation model