Generation of High Order Harmonics from Solid Surfaces by Intense Femtosecond Laser Pulses

Generation of optical harmonies of high order using intense ultrashort laser pulses is a topic of great current interest. Extensive recent work has dealt with rare gases1, where high order harmonic generation can be attributed to the nonlinearity of the interaction of intense electromagnetic fields with the gas atoms. On the other hand, a different type of high order harmonic generation was observed more than a decade ago during nanosecond laser-plasma interaction2, 3. In this case harmonic generation was explained4, 5 as being due to the anharmonicity of the electron motion across the steep plasma density gradient which develops during the expansion as a result of counteracting ponderomotive forces.