Transformation of electromagnetic radiation at moving inhomogeneities of a medium

The possibility of a significant up-conversion of the electromagnetic-radiation frequency due to its Doppler shift by ultrarelativistic nonlinear-medium inhomogeneities induced by high-intensity counterpropagating radiation pulses is analyzed. It is shown that the efficiency of parametric redistribution increases resonantly as the velocity of the inhomogeneity approaches the phase velocity of the high-frequency radiation.