D. Vonderlinde et K. Rzazewski, HIGH-ORDER OPTICAL HARMONIC-GENERATION FROM SOLID-SURFACES, Applied physics. B, Lasers and optics, 63(5), 1996, pp. 499-506
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.