Roles of resonances and recollisions in strong-field atomic phenomena. II. High-order harmonic generation

The question of the enhancements found in simulations and in an experiment for high-order harmonic generation (HHG) emission spectra at intensities for which there was a multiphoton resonance with an excited state was addressed. Several simulations, based on the resolution of the TDSE for different model potentials, indicated that in addition to the harmonics at frequencies close to the resonant one, there was a notable enhancement of a large number of other harmonics located in the plateau. The results confirmed that multiphoton resonances could enhance harmonic emission rates at intensities ≈10¹⁴W cm^-2 for a Ti:sapphire laser, i.e., at the border between the multiphoton and the tunnelling regime.