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Soviet Journal of Quantum Electronics

Superluminescence and stimulated emission from unstable photoisomers of polymethine dyes

A. M. Bonch-Bruevich, Калитеевская Елена Николаевна, T. K. Razumova

Published May 31, 1978• 0 citations
Publisher

Abstract

The criteria governing the superluminescence and stimulated emission from unstable photoisomers of polymethine dyes are studied for the first ime. It is shown that this is an important factor in stimulated emission from polymethines under conditions of anti-Stokes pumping of initial stable molecule solutions. Numerical calculations are made of the gain spectra of polymethine solutions containing photoisomers pumped by radiation of varying spectral composition. Conclusions are drawn on the influence of photoisomerization on the stimulated emission characteristics of these media under different pumping conditions.

DOI
10.1070/qe1978v008n05abeh010203

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The criteria governing the superluminescence and stimulated emission from unstable photoisomers of polymethine dyes are studied for the first ime. It is shown that this is an important factor in stimulated emission from polymethines under conditions of anti-Stokes pumping of initial stable molecule solutions. Numerical calculations are made of the gain spectra of polymethine solutions containing photoisomers pumped by radiation of varying spectral composition. Conclusions are drawn on the influence of photoisomerization on the stimulated emission characteristics of these media under different pumping conditions.
A report is given of a study of the amplitude-spectral asymmetry of the stimulated emission of superfluorescence from dye solutions pumped longitudinally by nanosecond laser pulses. This amplitude asymmetry is manifested by the difference between the powers of the forward and backward waves, and it is a function of the rate of pumping and of the optical density of the active medium. The asymmetry of stimulated emission from a ring laser is manifested by the appearance of a wave with a traveling-wave ratio of ~90 in the absence of any optical isolators. The spectral asymmetry of a ring laser is manifested by a difference between the spectra of strong and weak waves traveling in opposite directions. A quantitative analysis is made of the effects based on the solution of the transfer equations for radiation traveling in a nonlinearly absorbing active medium.
A calculation is made of the stimulated emission threshold of organic dye solutions as a function of the diffusion coefficient, intensity of induced singlet–singlet or triplet–triplet absorption at the stimulated emission wavelength, steepness of the leading edge of pump pulses, and other parameters. It is shown that, if the dipole moment of a fluorescence transition is perpendicular to the dipole moments of associated induced singlet–singlet or triplet–triplet absorption transitions, the laser threshold of high–viscosity solutions is considerably lower than that of low-viscosity systems. The maximum value of the ratio of the cross sections of the singlet-singlet absorption and stimulated emission, at which a given molecule ceases to emit stimulated radiation, increases from 1 to 3 when the rotational diffusion coefficient decreases from ∞ to 0.
The features of fluorescence emission in a dye-doped dense multiple scattered medium under pulsed laser pumping are considered in terms of confined excitation in small zones associated with laser speckles occurring in a pumped medium. The results of numerical modeling of the fluorescence emission kinetics are compared to the experimental data obtained using the rhodamine 6G-doped layers of the densely packed TiO2 (anatase) particles pumped at 532 nm by 10 ns laser pulses. The intensity of pump radiation during the action of laser pulses was varied from 1·105 W/cm2 to 5·107 W/cm2. In the recovery of the ratios of stimulated to a spontaneous emission, the spectra of the stimulated component were fitted using the spectral function derived by R. Dicke. In the framework of the considered concept, saturation of the ratio of the stimulated to a spontaneous emission and linear growth of an integrated fluorescence output with a practically unchangeable half-width of the emission spectra at high pump intensities are interpreted.
Amplified spontaneous emission of laser media having a spherical or disk shape is numerically analyzed. Some properties are discussed and a comparison with linear systems is made. Analytical approximations, for the case of high pumping rates and high efficiency, are obtained. Energy level schemes of dye systems are used in the analysis and in the numerical calculations. A coupled set of radiative transfer and rate equations is solved under quasi-steady-state conditions. The model takes into account the fluorescence spectrum and gain saturation effects with radially symmetric distributions of the active dye molecular densities. As a comparison, the numerical examples also include an H2O system which is relevant in astrophysics.
Experimental data on the amplification of polarized light in optical quantum amplifiers utilizing laserpumped dyes show that, in general, the polarization of the amplified light differs from that of the light being amplified. A theory is put forward whereby this effect is related to the anisotropy of the stimulated emission characteristic of complex organic molecules and is interpreted as a difference between the polarizations of the stimulated emission and stimulating radiation. The complex polarizability tensor is used as the main optical characteristic of complex molecules. The possibility of using the difference between the polarizations of the stimulated emission and stimulating radiation to produce light-amplifying polarizers and optical switches is indicated.