Far-infrared photoresponse of quasi-two-dimensional granular NbN/BN films

The photoresistive response to pulsed far-infrared radiation has been measured for a thin quasi-two-dimensional film of granular NbN/BN near the resistive onset of the film. The photoresponse dependence on temperature and light intensity is observed to be a strong function of whether the energy of the far-infrared photons is less or greater than the superconducting gap energy of bulk NbN. The results are consistent with a mechanism of direct coupling to intergrain Josephson currents that dominates for below-gap light and depairing processes (vortex-antivortex or Cooper pairs) for above-gap light.