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Quantum Electronics

Anomalous components of the scattering under the conditions of a developed instability of laser radiation in a resonant medium

Alexei G. Leonov, A. A. Panteleev, Andrei N Starostin, Dmitriy I. Chekhov

Published Feb 28, 1994• 1 citation
Publisher

Abstract

Experimental and theoretical investigations were made of the characteristics of an 'anomalous' component of the scattering of high-intensity laser radiation in a resonant medium (sodium vapour) observed earlier in the frequency range ω L > ω D2 . The appearance of this component had been attributed by other authors to parametric downconversion. Contrary to this interpretation, the component in question was observed also at frequencies of ω L < ω D1 . An analysis showed that the generation of this component and its properties can be explained fully by a four-wave interaction in a gas of three-level atoms.

DOI
10.1070/qe1994v024n02abeh000042

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Experimental and theoretical investigations were made of the characteristics of an 'anomalous' component of the scattering of high-intensity laser radiation in a resonant medium (sodium vapour) observed earlier in the frequency range ω L > ω D2 . The appearance of this component had been attributed by other authors to parametric downconversion. Contrary to this interpretation, the component in question was observed also at frequencies of ω L < ω D1 . An analysis showed that the generation of this component and its properties can be explained fully by a four-wave interaction in a gas of three-level atoms.
An analysis was made of four-photon interaction processes under conditions of two-photon resonance excitation of the 3 D sodium level by two-frequency radiation from a single-pulse picosecond YAG:Nd laser with frequency doubling and a KDP optical parametric oscillator. The spatial and frequency spectra of the four-photon parametric scattering (FPPS) were recorded and studied experimentally at various sodium vapor densities (10 15 –10 17 cm −3 ) and also using collinear and noncollinear excitation. It was found that the observed conical structure of the FPPS radiation may be interpreted by an analysis of whether the frequency and spatial phase-matching conditions were satisfied. Dependences of the intensity of the FPPS radiation on the intensity of the exciting radiation, the sodium vapor density, and the detuning of the exciting radiation frequency from the two-photon resonance were obtained.
Numerical modelling is reported of the propagation of laser radiation in a resonant three-level medium. The transient nature and the spatial inhomogeneity of the investigated processes are taken into account. It is shown that the radiation is scattered into narrow cones and that the number and vertex angles of these cones depend on the experimental conditions. The spatiotemporal spectrum of the radiation field emerging from the scattering medium and the dependence of this spectrum on the laser radiation frequency are compared with the results of experiments carried out under similar conditions. The calculated and experimental results are shown to agree under various scattering conditions when the spectral distributions of the scattered-radiation field are qualitatively different.
An analysis is made of the propagation of a short coherent optical pulse in a three-level resonant medium with an essentially nonequidistant spectrum as a function of the degree of preexcitation of the medium. It is found that in addition to self-induced transparency and resonance scattering, stimulated Raman scattering (Stokes or anti-Stokes) should be observed under certain spatial phase-matching conditions. The area theorem is formulated for all radiation components. An analysis is made of the case of propagation of a resonant electromagnetic pulse of frequency ω in a system of three-level atoms having an equidistant spectrum. Under conditions of preexcitation of the medium at the frequency 2ω by longitudinal acoustic pulses, a scattered transverse acoustic wave at the frequency ω may be observed.
The propagation of a nearly resonant strong laser in Na vapor results in a number of non-linear optical processes such as parametric four-wave mixing (FWM), three-photon scattering (TPS), and Raman scattering.1 The Raman and TPS processes involve the emission of radiation at specific and unique frequencies determined by the detuning of the laser from the atomic resonance, δ, and the Rabi frequency, Ω. Parametric FWM, however, need only satisfy the condition that 2ωL = ω3+ ω4, and can occur over a broad range of frequencies. The frequency range is restricted only by the available gain for the FWM process. The gain depends strongly on the possibility of phase matching at the given frequencies involved in the interaction, as well as on nearly all of the experimental parameters such as relaxation rates (collisional and otherwise), Rabi frequencies, laser detuning, the length of the Na interaction region and the Na vapor pressure.
A linear response of a resonant medium to a weak electromagnetic field is studied in the case when a medium is a gas of three-level atoms subjected to a strong saturating pulse. A noncoherent intense pulse and a coherent 2π pulse (of the kind encountered in the theory of self-induced transparency) are considered. The expressions obtained are used to study the problem of the spectrum of the reflection of light by a pulse moving in a medium. An analysis is made of the possibility of using the Doppler effect, which occurs when light is reflected by a moving pulse, in an investigation of the characteristics of pulse propagation, determination of its parameters, and study of the nonlinear characteristics of the medium itself. Possibilities of experimental observation of the predicted effects are discussed, particularly the possibility of conversion of an electromagnetic radiation spectrum.