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Soviet Journal of Quantum Electronics

Estimate of the stimulated emission cross section of a four-level active medium from the kinetics of decay of its superradiance

Aleksander A. Makovetskii, R. P. Tishchenko

Published Sep 30, 1983• 0 citations
Publisher

Abstract

An analysis is made of the problem of kinetics of the superradiance decay in the case of nonlinear quenching of excited centers in four-level active media. A method is proposed for estimating the initial axial gain and of the stimulated emission cross sections of active media: it is based on a simultaneous analysis of the normalized superradiant flux and integrated population of the active medium. The proposed method is not sensitive to nonlinear quenching. A qualitative analysis is made of the decay of excitation in neodymium pentaphosphate crystals after pulsed excitation with laser radiation of the 0.538 μ wavelength and stimulated emission cross sections are estimated for the 4 F 3/2 → 4 I 11/2 transition in Nd 3+ resulting in lasing at 1.051 μ and at some other wavelengths of this transition.

DOI
10.1070/qe1983v013n09abeh004616

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An analysis is made of the problem of kinetics of the superradiance decay in the case of nonlinear quenching of excited centers in four-level active media. A method is proposed for estimating the initial axial gain and of the stimulated emission cross sections of active media: it is based on a simultaneous analysis of the normalized superradiant flux and integrated population of the active medium. The proposed method is not sensitive to nonlinear quenching. A qualitative analysis is made of the decay of excitation in neodymium pentaphosphate crystals after pulsed excitation with laser radiation of the 0.538 μ wavelength and stimulated emission cross sections are estimated for the 4 F 3/2 → 4 I 11/2 transition in Nd 3+ resulting in lasing at 1.051 μ and at some other wavelengths of this transition.
An analysis is made of a one-dimensional problem involving superradiance of four-level active media. The rate equations for the populations and the transport equations for the superradiance fluxes are used to derive differential equations for the description of the behavior of the integrated population and the observed superradiance flux in the active medium excited by the pump flux. A detailed analysis is made of the superradiance decay. Analytic dependences are obtained to describe the decay of the normalized superradiance flux. The gain of the active medium at the onset of decay occurs as the parameter in these dependences and they are used as the basis of a technique for obtaining upper and lower estimates of this parameter.
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.
A study was made of various cases of a population inversion between the 4I11/2 and 4I13/2 levels in Y3Al5O12:Er3+ crystals. It is found that when nonradiative transitions from the upper levels to the active levels by the mechanism of multiphonon energy transfer are more probable, laser action occurs due to transitions between the Stark components of the 4I11/2 and 4I13/2 levels with the highest oscillator strengths. If nonradiative transitions due to ion-ion interaction are more probable, a red shift is observed in the lasing spectra.
An orderly account is given of the processes of coherent propagation, amplification, and generation of pulses in polyatomic resonant media. The characteristic features of the propagation of solitons in resonant media of finite length are considered. Comparisons are made of the exponential and lethargic amplification of pulses in resonant media. It is shown that an allowance for the reflection of the field from the boundaries of an active medium gives rise to a qualitatively different dependence of the parameters of the pulses on the reflection coefficients under superradiance and lasing conditions, which makes it possible to introduce a criterion to distinguish lasing from superradiance. An analysis is made of the characteristics of collective superradiance, which is a new case of coherent generation of pulses. Experimental and theoretical investigations of rf superradiance emitted by systems of nuclear spins are reviewed.