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Soviet Journal of Quantum Electronics

Theory of amplification and generation of light in activated solutions in the presence of intermolecular relaxation

Nikolay G Bakhshiev, O. P. Girin

Published Sep 30, 1975• 0 citations
Publisher

Abstract

An analysis is made of the intermolecular relaxation processes in activated solutions, which transform a two-level system of vibronic states of the activator molecules to a four-level system. It is shown that this alters considerably the main relationships in the theory of amplification and generation of light in activated solutions. More rigorous relationships obtained in the new model predict changes (in the course of pumping) not only of the intensity but generally also of the position and form of the absorption and luminescence spectra of such solutions, which should help in the interpretation of the known and new stimulated emission phenomena.

DOI
10.1070/qe1975v005n09abeh011942

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An analysis is made of the intermolecular relaxation processes in activated solutions, which transform a two-level system of vibronic states of the activator molecules to a four-level system. It is shown that this alters considerably the main relationships in the theory of amplification and generation of light in activated solutions. More rigorous relationships obtained in the new model predict changes (in the course of pumping) not only of the intensity but generally also of the position and form of the absorption and luminescence spectra of such solutions, which should help in the interpretation of the known and new stimulated emission phenomena.
A theoretical study is made of the influence of changes in the relaxation of a three-level system in a resonance field on the dispersion, absorption, and difference between the populations of levels activated by noncoherent pumping. This change in the relaxation is due to a tendency of a thermal reservoir to create an additional difference between the populations of quasienergy states that appear under the influence of the resonance field. It is shown that as a consequence there is a considerable change in the nature of the interaction with the radiation field in laser crystals with three-level active centers when the pump power is close to the threshold. The lasing conditions are found subject to such changes. It is shown that the threshold pump power can be reduced in this case to a value at which the difference between the populations of the active levels vanishes and lasing appears because of an inversion of the populations in a system of quasienergy levels. Under transient conditions the lasing frequency near the threshold should then depend on the pump
An analysis is made of the dynamics of the electronic spectra of complex molecules associated with the nuclear motion, and of its manifestation on exposure of a molecule to high-power optical radiation. A model is proposed for the inhomogeneous broadening dynamics. This describes the random walk of the spectral frequency in the form of a Gauss-Markov random process and allows for the relaxation of the spectra which accompanies an electronic transition. Simple readily interpreted relationships, which describe the influence of nuclear relaxation on the stimulated emission frequency, are obtained.
A study has been made of the effect of relaxation times on the four-wave mixing (FWM) signal response in frequency space for a two-level system where intramolecular coupling is explicitly taken into account within a vibronic model. The model is a generalization of the conventional two-level approach in that it includes a simplified description of the molecular structure and the explicit consideration of permanent dipole moments. Significant changes in both the intensity and shape of the peaks of the FWM spectrum are observed, due mainly to changes in the ratio between the longitudinal and transversal relaxation times, T 1/T 2.
Using a numerical solution of a system of equations for matrix elements of file-level model molecule density operator time dependencies of population on its states at different values of two irradiation pulses parameters (the pump pulse and the dump (or probe) pulse) and constants, determining rates of induced radiative transitions of the molecule, and radiative and nonradiative decays of the molecular states are calculated. The values of these constants were taken, in particular, close to these of the corresponding constants of molecules with intramolecular hydrogen bond as the 3-hydroxyflavone (3-HF) molecule. Values of irradiation pulses parameters are obtained, at which after enol-keto photoisomerization, induced by pump pulse, dump pulse induces molecular transition from excited electronic state in keto-form to ground state in this (ldquotautomericrdquo) form. This transition is accompanied by stimulated emission with a frequency of ldquotautomericrdquo fluorescence. Examples of periodic change of intensity of the stimulated emission are given, the frequency of oscillations of fluorescence intensity being proportional to dump pulse intensity.
An asymmetry effect in the amplification and efficiency of an optically dense amplifier is predicted theoretically and confirmed experimentally for opposite and concurrent collinear signal and pump paths. The effect is due to a difference in the losses as a result of the two-stage absorption of pump radiation in a system of luminescing centers described by a model of three energy levels. Calculations are made of various characteristics of this type of amplifier using the quasisteady-state approximation.
A nonperturbative treatment of the dynamical behavior of a vibronic system in a resonant, coherent, driving light field is developed. The potential energy surfaces for the vibrational motion in the electronic ground and excited state are assumed to be displaced and distorted relative to one another. A system of three integrodifferential equations is derived from the infinite set of density-matrix equations. Both approximate analytical and numerical solutions are presented for the intensity and the intensity-correlation function of the scattered light. The results are valid for arbitrary ratios of the Rabi frequency to the relaxation rates. A system with fast vibrational relaxation behaves as a two-level system. For systems with slow vibrational relaxation, the applicability of perturbation theory requires pump fields that are weaker than those of a two-level system. The Rabi oscillations occurring in a strong pump field are damped less than those of a two-level system, and the intensity substantially decreases. The long-time behavior (determined by the vibrational relaxation time) is characterized by the increase of the intensity toward its stationary value.