Enhanced optical detection in a Josephson junction

Shifting of the energy gap of a Nb Josephson junction under direct optical illumination was measured. The response is linear with optical input power over more than five orders of magnitude and is nearly independent of temperature from 4 to 8 K. The rise time of this signal is faster than the 2- mu s rise time of the chopped light signal. These direct signals are 500 to 1500 times larger than those obtained when the same optical power is focused elsewhere on the substrate. This enhanced direct response is interpreted as resulting from thermal isolation of the Josephson junction from the substrate due to thermal boundary resistance. >