We have developed a model for calculating saturated absorption spectra for dipole transitions in multi-level atoms. Using a semiclassical density matrix formalism, we derive a set of coupled differential equations for the internal state of the atom in a standing wave light field. The equations are solved using standard integration techniques. The absorption at each laser detuning is found from an average of the absorption for a number of velocities along the laser field, thermally weighted. The method is relatively efficient computationally yet quantitatively predicts important details of saturated absorption spectra including saturation, crossover resonances, merging of absorption lines at high intensity and optical pumping between hyperfine levels. We have measured saturated absorption and fluorescence spectra of 85 Rb, and compare to our computational results for a 36-level model.

The full-text may be used and/or reproduced, and given to third parties in any format or medium, without prior permission or charge, for personal research or study, educational, or not-for-pro t purposes provided that:• a full bibliographic reference is made to the original source • a link is made to the metadata record in DRO • the full-text is not changed in any way The full-text must not be sold in any format or medium without the formal permission of the copyright holders.Please consult the full DRO policy for further details. The rate of ionization of an atom of helium, argon, or hydrogen exposed to an intense monochromatic laser field and the quasienergy spectrum of their dressed states are studied for values of the Keldysh parameter between 1 and 0.6 and wavelengths between 390 and 1300 nm. The calculations are carried out within the non-Hermitian Floquet theory. Resonances with intermediate excited states significantly affect ionization from the dressed ground state at all the intensities and all the wavelengths considered. The dressed excited states responsible for these structures are large-α 0 states akin to the Kramers-Henneberger states of the high-frequency Floquet theory. Within the single-active-electron approximation, these large-α 0 states become species independent at sufficiently high intensity or sufficiently long wavelength. Apart for the resonance structures arising from multiphoton coupling with excited states, the ab initio Floquet ionization rate is in excellent agreement with the predictions of two different calculations in the strong field approximation, one based on a length-gauge formulation of this approximation and one based on a velocity-gauge formulation. The calculations also confirm the validity of the ω 2 expansion as an alternative to the strong field approximation for taking into account the nonadiabaticity of the ionization process in intense low-frequency laser fields.

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.

A semi-perturbative description of the resonance structures induced by strong laser fields is formulated within the framework of the non-Hermitian Floquet theory. Three cases are examined numerically, namely continuum structures induced by a two-colour embedding field, resonance enhancements in two-colour ionization and Stark-shift-induced resonances in a strong monochromatic field.