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Radiation trapping and light shift in DFM in Na vapor

Wulfhard K. Lange, M. Schiffer, E. Cruse, G. Ankerhold

Published May 8, 1994• 0 citations
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Abstract

The influence of radiation trapping on degenerate four-wave-mixing (DFWM) processes has recently been discussed theoretically.1 Its role in the nonlinear behavior of sodium atoms in a buffer-gas atmosphere has also been examined.2 We used an experimental scheme similar to that reported in Ref. 3 to study its influence on DFWM in dense sodium vapor (1013 -1014 atoms cm −3).

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Physical Review A•Sep 1, 2001•4 citations
The influence of radiation trapping on degenerate four-wave-mixing (DFWM) processes has recently been discussed theoretically.1 Its role in the nonlinear behavior of sodium atoms in a buffer-gas atmosphere has also been examined.2 We used an experimental scheme similar to that reported in Ref. 3 to study its influence on DFWM in dense sodium vapor (1013 -1014 atoms cm −3).
Degenerate four-wave mixing in dense sodium vapor under conditions of strong pressure-induced Zeeman pumping is investigated. Thermal diffusion and its interplay with the Zeeman light shift are found to be responsible for the generation of a grating mechanism that is not sensitive to thermal washout. The effect can be observed experimentally after eliminating the influence of radiation trapping by the use of nitrogen as a buffer gas. Under these conditions large nonlinearities are produced which also influence the propagation of the signal wave via collision-aided circular birefringence.
Degenerate four-wave mixing (DFWM) in excited atoms is investigated as a probe of the excited-state density produced by collision-assisted transitions. Sodium atoms are excited to the 3P state by absorption of light in the extreme blue wing of the D lines in the presence of He, Ne and Ar perturbers. The rare-gas pressure dependence of the DFWM signals, resonantly enhanced by the 3P-4D transition, is studied to determine the regime where the signals give an unambiguous measure of the excited-state (3P) density. The observed behaviour is compared with a simple model of DFWM in absorbing media. Good qualitative agreement is obtained and the results illustrate the role of absorption, saturation and finite laser bandwidth. The technique allows small excited-state densities ( approximately 10 9 cm -3 ) to be detected with good temporal and spatial resolution.
We have investigated degenerate four-wave mixing (DFWM) in a strongly absorbing molecular gas. A theory taking into account saturable absorption of the interacting beams is used to explain the dependence of the DFWM reflectivity on laser intensity and on the molecular number density. These parameters have a great influence on the observed spectra, owing to linear absorption that suppresses the DFWM signal in the line center of the molecular transitions. The great sensitivity of the spectra to the experimental conditions limits the applicability of DFWM for spectroscopic studies in absorbing media.
We present experiments of a laser with variable pulse energy and beam width saturating sodium vapor near the D1{D}_{1}D1​-line transition. At high optical depth and in a buffer gas atmosphere of argon, the effect of nonlinear radiation trapping is shown to drastically change the spatial distribution and temporal evolution of the excited-state population and of the fluorescence light trapped by the nonlinear medium. Besides observing the transient behavior of the fluorescence light, a pump-probe beam experiment enables us to gain a tomographic view inside the atomic vapor. Nitrogen as a buffer gas is used to compare with the situation where radiation trapping effects are negligible. Finally, numerical simulations based upon equations of radiative transfer for a two-level atomic system elucidate the role of nonlinear radiation trapping and support given interpretations of the experiments.