High Harmonic Generation Efficiency

Abstract

In the last few years the availability of very intense ultrashort laser pulses has created a new experimental domain for the study of laser atom interactions. Among many new phenomena, the generation of high order harmonics has opened up the possibility of producing coherent radiation in the extreme ultraviolet and (soft) X-ray regions. By harmonic generation we mean the conversion of the fundamental laser wavelength to high odd harmonic wavelengths when an atom is irradiated by a strong laser field. A complete description of the process should include both the response of the single atom to the laser radiation as well as the collective response of an ensemble of atoms. The harmonic conversion efficiencies obtained in experiments are dependent, to a large extent, on collective effects. Since there will be significant ionization of the medium, the refractive index will be modified and this in turn changes the phase matching between the harmonics and the fundamental. Nevertheless it is the strongly nonlinear response of the single atom that produces the large conversion efficiency from the fundamental to shorter wavelengths.