PERTURBATION-THEORY STUDY OF HIGH-HARMONIC GENERATION

We have computed nonlinear susceptibilities for harmonic generation in hydrogenic ions, as described by lowestand next-lowest-order perturbation theory, up to fairly high orders of nonlinearity (∼30). These results provide a framework for determining the quantitative predictions and the range of validity of perturbation theory. Our results indicate that, for laser intensities as low as 10ll W/cm2, the next-lowest-order contribution to the susceptibility becomes comparable with that of the lowest order. Caution must thus be exercised in applying concepts derived from perturbation theory to the interpretation of current experimental data. © 1990 Optical Society of America.