Bolometric and nonbolometric infrared photoresponses in ultrathin superconducting NbN films

The photoresponse of 10 nm thick superconducting NbN meander lines is measured using an amplitude modulated infrared semiconductor laser operating at a wavelength of 1300 nm. The response time of the film is found to be less than 1 ns with a measured responsivity of up to 1500 V/W of absorbed power at 100 kHz. Thermal properties of the film are extracted from current-voltage characteristics using a self-heating hot spot model. At temperatures well below the superconducting transition, the magnitude of the photoresponse is found to be an order of magnitude too large to be purely bolometric, even when electron heating and effects due to intergranular weak links are taken into account. The photoresponse is seen to be bolometric near T-c. Other contributions to photoresponse are discussed, including kinetic inductance and the photofluxonic effect. (C) 1996 American Institute of Physics.