The dependence on the ellipticity of the incident laser field of the harmonics generated by atoms of hydrogen in the multiphoton regime is examined for two fundamental wavelengths, 532 and 1064 nm. The single atom response is calculated nonperturbatively using the Floquet method in conjunction with basis sets of complex Sturmian functions. The results are compared with those obtained using lowest order perturbation theory. Nonperturbative effects modify the strength and polarization of the harmonics at the highest intensities investigated, particularly, for the threshold harmonics and in the vicinity of Stark-shift-induced resonances. No anomalous dependence of the rate of harmonic generation on the ellipticity of the fundamental field is found.
Atomic polarization can have a profound effect on the generation of the high-order harmonics during the interaction of the polarized atom with intense field. In this work, the atomic polarization effect is investigated for the specific case of the
21P helium subject to infrared radiation. The strong coupling between the
21P and
21S states of helium is taken into account using a multichannel version of the strong-field approximation of Lewenstein and co-workers [Lewenstein et al., Phys. Rev. A 49, 2117 (1994); Antoine et al., Phys. Rev. A 53, 1725 (1996)]. The analysis reveals a strong dependence of the high-order harmonic generation efficiency on the atomic polarization. While the occurrence of the predicted effect can be explained in terms of the recollision model, its magnitude is deeply affected by the fast population oscillations between the excited bound states. Moreover, the induced bound-bound transitions bring about the interesting double-maximum structure in the ellipticity dependences of the emitted harmonics, both maxima occurring for nonzero field ellipticities. This unusual structure shows up even for the high-energy harmonics generated solely by the bound-continuum mechanism.
The coherent scattering spectrum of a three-level atom driven by an intense laser field with multiphoton resonant excitation is considered. The spectrum corresponding to harmonic generation is investigated both analytically and numerically for different initial atomic states. The analytical calculations are based on the generalized rotating wave approximation, which allows one to explain obtained spectra containing in general both even and odd harmonics. The results applied to the hydrogen atom show that one can achieve efficient generation of moderately high harmonics using multiphoton resonant excitation by appropriate laser pulses.