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Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE

Femtosecond pulse propagation in a medium of two-level atoms

Anatoli V. Andreev, Alexander B. Kozlov

Published Mar 5, 1999• 2 citations
Publisher

Abstract

The interaction of femtosecond pulse with an extended medium of the two-level atoms or molecules is considered. It is shown that in the ultrahigh intensity layer field the atomic response is extremely nonmonochromatic due to the nonlinearity appropriate to the two-level system. The theoretical analyses shows that the two-level system tends to the generation of harmonics of the driving field. The results of the computer simulations show that in the ultrahigh fields the spectrum of the medium response is unusually broad in comparison with that for the fields of moderate intensity.

DOI
10.1117/12.341487

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The interaction of femtosecond pulse with an extended medium of the two-level atoms or molecules is considered. It is shown that in the ultrahigh intensity layer field the atomic response is extremely nonmonochromatic due to the nonlinearity appropriate to the two-level system. The theoretical analyses shows that the two-level system tends to the generation of harmonics of the driving field. The results of the computer simulations show that in the ultrahigh fields the spectrum of the medium response is unusually broad in comparison with that for the fields of moderate intensity.
We present numerical solutions of the propagation of harmonics in a dense nonlinear medium, constituted by two-level atoms. The propagation of the harmonics is analysed taking into account the nonlinear atomic response to all the harmonics generated in the sample. We compare our results with the case in which only generation from the fundamental is considered, finding that the profile of the harmonics inside the medium changes drastically from one situation to the other. These results indicate that the role of the phase mismatch could be much less severe than previously expected.
The spontaneous emission of an atomic system in the field of a high-intensity femtosecond laser pulse is considered within the framework of a consistent quantum-mechanical approach based on an examination of the interaction of a quantum system with a set of quantized field modes in a vacuum state. Both even and odd harmonics of the driving field are shown to appear in the atomic emission spectrum, and the mechanisms of their generation are elucidated. A comparison with the semiclassical theory of laser harmonic generation is made. The semiclassical approach in describing the spontaneous emission in strong laser fields, especially under conditions of significant depletion of the ground (initial) state in a laser field, is shown to be limited.
The theory of the interaction of a centrosymmetric atom with a superstrong spatially inhomogeneous laser field is developed. This theory employs the two-level approximation to describe the dynamics of spatially nonlocal interactions related to variations in the populations of the levels. We propose a model that includes spatially nonlocal interactions, such as magnetic-dipole and quadrupole interactions and interactions due to the gradient of the ponderomotive potential of the field. We consider the interaction of a homogeneous medium of centrosymmetric atoms with a superstrong laser field, which is represented as a superposition of two plane-wave ultrashort pulses propagating at some angle with respect to each other. Perturbation theory, which is valid for the fields of moderate intensity, is developed for the atomic response. The results of numerical simulations are compared with the predictions of this perturbation theory. The specific features of the nonlinear-optical response of an atom in a superstrong field are investigated. The angular distribution of the second- and third-harmonic emission is calculated in the constant-field approximation for different polarisations of the incident field.
With the development of laser technology, the femtosecond laser has been more and more extensively used both on scientific research and practical application. The interaction of femtosecond laser with two-level atoms in optical materials was discussed. The evolution of femtosecond pulse in resonant atomic medium with long distance, and the influence of the distribution of the laser field amplitude, which were Gaussian, hyperbolic secant, and asymmetric hyperbolic secant, on the two-level atomic transition probability is presented. Also the dynamic analysis of temporal evolution of atomic transition in a two-level system under the radiation of the frequency-chirped field in optical materials is analyzed.