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Soviet Journal of Quantum Electronics

Interaction of a light pulse with a multilevel molecular system

Maria V. Bogdanova, Tamara M. Il'inova, R. V. Khokhlov

Published Sep 30, 1976• 1 citation
Publisher

Abstract

The results are given of a theoretical investigation of the interaction of a laser pulse with a molecular medium. Allowance for the vibrational relaxation processes is made within the diffusion approximation framework which makes it possible to describe relaxation as the diffusion of particles in continuous electron energy bands. A simultaneous solution is obtained of the diffusion equations for the medium and the equations for the radiation field in the special cases of slow and fast vibrational relaxation. It is shown that the fast relaxation, which prevents gain saturation, increases the signal energy compared with the case of slow relaxation in an amplifying medium and reduces this energy compared with such relaxation in an absorbing medium. Expressions are obtained for the functions which describe the distributions of the band particles established by the investigated signal. The analysis made shows that the diffusion model can be applied to the interaction between laser pulses and molecular media.

DOI
10.1070/qe1976v006n09abeh011831

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The results are given of a theoretical investigation of the interaction between two ultrashort pulses J1 and J2 of frequencies ω31 and ω21 with a three-level amplifying medium. It is assumed that the signal J1 of frequency ω31 is a steady-state π pulse and the signal J2 is weak so that its influence on the level populations can be ignored. The case when the signal J2 has a steady-state amplitude profile and an exponential gain is considered. The analysis is made ignoring the group delay effects and also allowing for the mismatch of the group velocities of the signals. The results obtained can be used in studies of the amplification of ultrashort pulses in molecular media such as organic dyes or CO2 molecules.
The Maxwell–Schrödinger equations describing the ultrashort laser pulse propagation in multilevel molecular media are shown to be analytically solvable and to reduce to nonlinear multisine-Gordon equations for certain models of the field–molecule system. Special cases of level schemes are shown to lead to soliton propagation. A numerical study is presented in order to examine the effect of nonresonant (virtual) transitions on pulse propagation and amplification in a three-level molecular system as a function of intensity and detuning.
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Rate equations for the density matrix are used to describe classically the orientational states of the angular momenta of levels and are solved for arbitrary intensities and polarizations. Allowances are made for collisions which alter the velocity, orientation, and magnitude of the angular momentum, and also the quantum number of a molecular vibrational state. Expressions are derived for the polarization of the medium in the case of homogeneously and inhomogeneously broadened lines. An analysis is made of the polarization dynamics of high-intensity radiation (deformation and rotation of the polarization ellipse) during its propagation in amplifying (absorbing) media.