Nonlinear optical response of granular Y-Ba-Cu-O films

The nonlinear optical response of a highly granular film of YBa2.1Cu3.4O_7-x has been interpreted on the basis of the two-dimensional characteristics of the superconducting transition in this film. The behavior of the current-voltage characteristics is consistent with a dissipation mechanism which is dominated by vortex motion as in a Kosterlitz-Thouless transition. The optical response at temperatures near the transition to zero film resistance cannot be interpreted satisfactorily in terms of a realistic thermal model. We believe that nonequilibrium effects contribute to the photoresponse, particularly at low bias currents and low temperatures.