Infrared detection with high-Tc bolometers and response of Nb tunnel junctions to picosecond voltage pulses

Oxide superconductors with high critical temperature Tc make sensitive thermometers for several types of infrared bolometers. The authors built composite bolometers with YBa2Cu3O7-δ thermometers on sapphire substrates which have higher sensitivity than competing thermal detectors which operate at temperatures above 77 K. A 1 x 1 mm bolometer with gold black serving as the radiation absorber has useful sensitivity for wavelengths 20--100 μm. A 3 x 3 mm bolometer with a bismuth film as the absorber operates from 20--100 μm. High-Tc bolometers which are fabricated with micromachining techniques on membranes of Si or Si3N4 have potential application to large-format arrays which are used for infrared imaging. A nonisothermal high-Tc bolometer can be fabricated on a membrane of yttria-stabilized zirconia (YSZ) which is in thermal contact with the heat sink along the perimeter of the membrane. A thermal analysis indicates that the YSZ membrane bolometer can have improved sensitivity compared to the sapphire bolometer for spectrometer applications. The quasiparticle tunneling current in a superconductor-insulator-superconductor (SIS) junction is highly nonlinear in the applied voltage. The authors have made the first measurement of the linear response of the quasiparticle current in a Nb/AlOx/Nb junction over a broad bandwidth from 75--200 GHz. Nonlinear measurements made with these pulses may provide information about the quasiparticle lifetime. Preliminary data from such measurements are presented