Effect of microwave radiation on tunneling between superconductors

The tunneling current between two dirty superconductors irradiated with microwaves is calculated to second order in the microwave field. It is shown that the microwave field modifies the two factors in the tunneling current: the density of states and the quasiparticle distribution function. The main change in the density of states is brought about by the enhancement of the order parameter caused by the field under appropriate conditions. The structure observed in the tunneling current at voltages eV=, where is the radiation frequency, is shown to arise mainly from the change in the quasiparticle distribution function.