Lewenstein Model Research Articles

Effect of orbital symmetry on high-order harmonic generation from molecules

It was found experimentally with increasing the laser ellipticity that the intensity of the 45th order harmonic from ${\mathrm{O}}_{2}$ decreases slower than that from ${\mathrm{N}}_{2}.$ The difference is attributed to the fact that the recombination probability is suppressed for ${\mathrm{O}}_{2}$ but enhanced for ${\mathrm{N}}_{2}$ in a linearly polarized field. Simulation results obtained by extending the Lewenstein model to molecules agreed qualitatively with the experimental discoveries.

Harmonic generation by atoms in circularly polarized two-color laser fields with coplanar polarizations and commensurate frequencies

The generation of harmonics by atoms or ions in a two-color, coplanar field configuration with commensurate frequencies is investigated through both an analytical calculation based on the Lewenstein model and the numerical ab initio solution of the time-dependent Schr\"odinger equation of a two-dimensional model ion. Through the analytical model, selection rules for the harmonic orders in this field configuration, a generalized cutoff for the harmonic spectra, and an integral expression for the harmonic dipole strength are provided. The numerical results are employed to test the predictions of the analytical model. The scaling of the cutoff as a function of both one of the laser intensities and frequency ratio $\ensuremath{\eta}$ as well as entire spectra for different $\ensuremath{\eta}$ and laser intensities are presented and analyzed. The theoretical cutoff is found to be an upper limit for the numerical results. Other discrepancies between analytical model and numerical results are clarified by taking into account the probabilities of the absorption processes involved.

Role of long quantum orbits in high-order harmonic generation

Single-atom high-order harmonic generation is considered in the strong-field approximation, as formulated in the Lewenstein model, and analyzed in terms of quantum orbits. Orbits are classified according to the solutions of the saddle-point equations. The results of a numerical integration are compared with the saddle-point approximation and the uniform approximation. Approximate analytical solutions for long orbits are presented. The formalism developed is used to analyze the enhancement of high-order harmonic generation near channel closings. The enhancements exactly at the channel closings are extremely narrow and built up by the constructive interference of a very large number of quantum orbits. Additional broader enhancements occur slightly below channel closings. They are generated by the interplay of a medium number of orbits.

Quantum-mechanical model for ultrahigh-order harmonic generation in the moderately relativistic regime

A quantum theory of ultrahigh-order harmonic generation is presented that generalizes the Lewenstein model into the relativistic regime. Various effects appear in the harmonic spectrum: a multiplateau structure in the weakly relativistic case; for stronger fields, the interference pattern familiar from the nonrelativistic case disappears, the height of the plateau decreases towards high harmonic orders, and the cutoff is lower compared to the nonrelativistic case. All of these features are explained in terms of relativistic quantum trajectories.