Optical detection by suppression of the gap voltage in niobium junctions

The application of niobium Josephson junctions as detectors in infrared focal plane arrays is being investigated. The dependence of the optical response of these junctions on several variables including temperature, frequency of light chopping, and light intensity has been measured. In addition the authors have investigated the effect of different substrate materials, namely, single-crystal silicon and amorphous fused quartz, and the effect of two different cryogenic environments, namely, vacuum and superfluid helium. For measurements in vacuum, cooling is provided by contact between the substrate and a cold surface. Immersion in superfluid helium is expected to reduce heating effects considerably owing to its extremely high thermal conductivity, thus isolating nonequilibrium effects. The NEP (noise equivalent powers) and D* (detectivity) of the detectors is limited by noise in the room-temperature electronics. For a typical niobium junction of area 100 μm² on a fused quartz (SiO₂) substrate in vacuum at 4.2 K the responsivity is 3000 V/W, NEP is 6×10^-13 W, and D* is 2×10⁹ cm-Hz^1/2/W for a 1-Hz bandwidth at a wavelength of 1 μm