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Optics and Spectroscopy

Stimulated processes in a mixture of rubidium vapor and buffer gas under two-photon excitation

M. E. Movsesyan, T. O. Ovakimyan, Svetlana Shmavonyan

Published Sep 1, 1986• 5 citations
OpenAlex

Abstract

The article gives the results of a study of the effect of He buffer gas on nonlinear processes during two-photon excitation of rubidium vapor. In addition to four-photon parametric luminescence and a stimulated four-photon parametric process, lines of two-photon decay and stimulated electronic Raman scattering, not previously observed in rubidium vapor, have been obtained. The dependences of these processes on the pressure of the buffer gas and on the frequency of the exciting radiation are investigated and discussed.

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The article gives the results of a study of the effect of He buffer gas on nonlinear processes during two-photon excitation of rubidium vapor. In addition to four-photon parametric luminescence and a stimulated four-photon parametric process, lines of two-photon decay and stimulated electronic Raman scattering, not previously observed in rubidium vapor, have been obtained. The dependences of these processes on the pressure of the buffer gas and on the frequency of the exciting radiation are investigated and discussed.
Investigations are made of hyper-Raman scattering and four-photon parametric processes in two-photon excitation of metal vapor. The laws of propagation of infrared and ultraviolet radiation generated in a metal vapor medium by these processes are found for the steady-state interaction. The nonlinear polarization of the medium is obtained in terms of elements of the atomic density matrix allowing for saturation by the pump field and for the influence of collisions. The spectral and tuning characteristics of the converted radiation are studied and a comparison is made with experimental data. A dip observed in the infrared radiation spectrum for hyper-Raman scattering and its broadening are explained. A detailed discussion is given of the role of collisions that substantially narrow the tuning range of the infrared radiation in hyper-Raman scattering.
An enhanced two-photon emission has been observed between the 6S and the 4S levels of potassium. The priming field is the stimulated electronic Raman emission at 2720 cm-1due to scattering of ruby laser radiation from the4P_{3/2}state in potassium. The intense emission at the complementary frequency ω2is close to the 5P → 4S resonance lines. As a result, the coupling coefficient has a resonance character that is favorable for the enhancement. The observed results are compared with a simple analysis. The relation of the enhanced emission with two-quantum absorption, stimulated Raman effect, and parametric four-photon processes is discussed.
It is shown that by proper use of the excitation spectra, the emission spectra of the transition and the far-field patterns it is possible to distinguish between competing processes involved in the two-photon excitation to the 6S state of potassium, namely: (a) resonant fluorescence emission associated with the cascading spontaneous emission (ASE) and (b) the four-wave mixing parametric process. The effect of the buffer gas on the intensities of the spectra of the radiations emitted is discussed. Conical emission and spatial instabilities of the excitation radiation and the emission were also observed for high excitation and high vapour pressure of potassium.
The nonlinear response of sodium vapor with a laser tuned near the 4D two-photon resonance is investigated over a large range of sodium concentrations and pump intensities with emphasis on the influence of coherent cancellation effects on the induced polarizations and the primary mechanism for the generation of the new frequencies. Over a wide range of circumstances, the response of the medium is dominated by parametric four-wave mixing as compared to amplified spontaneous emission or stimulated electronic hyper-Raman scattering. Secondary four-wave-mixing fields are generated by stimulated Raman scattering of one parametric four-wave-mixing field and a coupling of these fields with the driving laser field. A sharp onset of saturation of the parametric four-wave-mixing signals with sodium number density and a transition from a quadratic to a linear power dependence is observed. Both effects are explained in terms of the two-photon cancellation effect.
Stimulated and enhanced two-photon emissions from excited states in potassium have been observed. This emission is primed by stimulated electronic Raman radiation generated by scattering of a giant pulse ruby radiation. Four-photon parametric processes also take place. The two processes can be distinguished by tuning the laser frequency. Most of the emitted radiations are plane-polarized. A pumping model is proposed explaining the required population inversions and the polarization properties of the radiations.