The propagation of a high-irradiance laser beam in a plasma whose optical index depends nonlinearly on the light intensity is investigated through both theoretical and numerical analyses. The nonlinear effects examined herein are the relativistic decrease of the plasma frequency and the ponderomotive expelling of the electrons. This paper is devoted to focusing and defocusing effects of a beam assumed to remain cylindrical and for a plasma supposed homogeneous along the propagation direction but radially inhomogeneous with a parabolic density profile. A two-parameter perturbation expansion is used; these two parameters, assumed small with respect to unity, are the ratio of the laser wavelength to the radial electric-field gradient length and the ratio of the plasma frequency to the laser frequency. The laser field is described in the context of a time envelope and spatial paraxial approximations. An analytical expression is provided for the critical beam power beyond which self-focusing appears; it depends strongly on the plasma inhomogeneity and suggests the plasma density tailoring in order to lower this critical power. The beam energy radius evolution is obtained as a function of the propagation distance by numerically solving the paraxial equation given by the two-parameter expansion. The relativistic mass variation is shown to dominate the ponderomotive effect. For strong laser fields, self-focusing saturates due to corrections of fourth order in the electric field involved by both contributions.
We propose a mechanism to explain the nature of the damping of Rabi oscillations with an increasing driving-pulse area in localized semiconductor systems and have suggested a general approach which describes a coherently driven two-level system interacting with a dephasing reservoir. Present calculations show that the non-Markovian character of the reservoir leads to the dependence of the dephasing rate on the driving-field intensity, as observed experimentally. Moreover, we have shown that the damping of Rabi oscillations might occur as a result of different dephasing mechanisms for both stationary and nonstationary effects due to coupling to the environment. Present calculated results are found in quite good agreement with available experimental measurements.
A theoretical study of the effects of a laser field on the electronic and optical properties of GaAs-͑Ga,Al͒As heterostructures is presented by using a Kane model for the GaAs bulk semiconductor and working within an extended dressed-atom approach. For a laser tuned far below any resonances, the effects of the lasersemiconductor interaction correspond to a renormalization of the semiconductor energy gap and conduction/ valence effective masses. This renormalized one-body approach may be used to give a qualitative indication of the laser effects on a variety of optoelectronic phenomena in semiconductor heterostructures for which the effective-mass approximation provides a good physical description. As a test, the exciton Stark shift in quantum wells is calculated and the effects due to the band-structure laser dressing are found to be of the same order of magnitude as those obtained from many-body diagrammatic techniques. We have also analyzed the effects of laser dressing on the shallow-donor peak energies in quantum wells, and found them comparable with those produced by a magnetic field of a few teslas.
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