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Acta Physica Polonica A

Saturation Processes in Nonlinear Media Modeled by Four-Level Configuration with Coincident Absorption and Emission Bands

Jihad S. Addasi

Published Jul 1, 2012• 2 citations
Publisher

Abstract

The four-level model is used to study the nonlinear media with coincident absorption and emission bands in both principal (S0S1) and excited (T1T2) channels.The excitation of molecules occurred by two light elds: radiations with intensity I12 at frequency ω0 tuned at the centre of absorption band of principal singletsinglet channel and radiations with intensity I34 at frequency ω tuned at the centre of absorption band of excited triplettriplet channel.The monotonic dependence of phase response of media in both principal and excited channels and the saturation processes are studied theoretically and numerically for four-level model.

DOI
10.12693/aphyspola.122.156

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The four-level model is used to study the nonlinear media with coincident absorption and emission bands in both principal (S0S1) and excited (T1T2) channels.The excitation of molecules occurred by two light elds: radiations with intensity I12 at frequency ω0 tuned at the centre of absorption band of principal singletsinglet channel and radiations with intensity I34 at frequency ω tuned at the centre of absorption band of excited triplettriplet channel.The monotonic dependence of phase response of media in both principal and excited channels and the saturation processes are studied theoretically and numerically for four-level model.
Saturation of complex molecules affects the nonlinear processes occurring in the medium. The medium can be described by two-, three-, and four-level configurations. The principal singlet-singlet and excited triplet-triplet channels of a four-level, N-type, configuration were used to study a nonlinear medium, which was excited by radiation with two frequencies. Under weak population of energy levels in the triplet-triplet channel (a small ratio of the total probability of spontaneous and irradiate transitions for both channels p23/p31), the principal (singlet-singlet) channel is similar to a two-level configuration. At the same time, the effective population of the energy levels in the triplet-triplet channel (the ratio p23/p31 is large and the radiation intensity in the principal channel is high) makes the excited (triplet-triplet) channel coincide with the two-level configuration. The saturation intensity of radiation in principal (excited) channel, as effective two-level configuration, and nonlinear processes in this channel can be controlled by external radiation acting on excited (principal) channel. Finally, the average excitation of molecules into the energy levels in the triplet-triplet channel (ratio p23/p31 ≈ 1) makes the four-level configuration similar to the three-level configuration with effective level as a result of combination between second singlet level S2 and first triplet level T1.
Two-photon transitions have provided the basis for numerous useful nonlinear interactions with radiation. In this paper we calculate the absorption and coupling coefficients for one-, two-, and three-wave mixing in two-photon homogeneously broadened media. The resulting field equations form the basis for understanding saturation spectroscopy, phase conjugation,1 and single-mode instabilities2 in lasers and optical bistability in two-photon media. We assume that the wave frequencies are not resonant with any intermediate state j and that the probe and conjugate waves cannot saturate the two-photon transition. This allows us to use the two-photon two-level model and to transfer much of our knowledge of the one-photon two-level systems to the two-photon case. Two major differences occur between the two models. First, dynamic Stark shifts of the active levels play an important role in generating asymmetric spectra and nonzero dispersion (≃ω s T1) at line center. Second, high pump intensities do not bleach the absorption coefficient at the Rabi sidebands.
Saturation effects in gaseous media in the presence of a high-intensity resonant laser field and in the field of incoherent radiation caused by the intrinsic spontaneous emission are considered. A simple nonlinear set of kinetic equations is proposed for describing the saturation effect in the case of optically thin media and nondegenerate levels. The Bennett structure and the band of homogeneous saturation, whose parameters are interdependent, exist in the velocity distribution. The regularities in the behavior of the band and the Bennett structure under variation in the transition parameters and the intensity of laser radiation are elucidated.
Abstract The physical processes forming the saturated absorption resonance spectra at a number of degenerate atomic transitions at their interaction with coherent optical fields and incoherent ield of spontaneous radiation of excited atoms are studied numerically. The features of behavior of nonlinear resonance spectra depending on the degree of degeneration and relaxation parametersof level transitions, character of polarizations and intensities, as well as direction of propagation of optical fields are analyzed.