HIGH-ORDER OPTICAL HARMONIC-GENERATION FROM SOLID-SURFACES

During the interaction of an intense ultrashort laser pulse with solid targets, a thin layer of surface plasma is generated in which the den sity drops to the vacuum level in a distance much shorter than the wav elength. This sharp plasma-vacuum boundary performs an oscillatory mot ion in response to the electromagnetic forces of the intense laser lig ht. It is shown that the generation of reflected harmonics can be inte rpreted as a phase modulation experienced by the light upon reflection from the oscillating boundary. The modulation side-bands of the refle cted frequency spectrum correspond to odd and even harmonics of the la ser frequency. Retardation effects lead to a strong anharmonicity for high velocities of the plasma-vacuum boundary. As a result, harmonic g eneration is strongly enhanced in the relativistic regime of laser int ensities.