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Physical Review A

Experiments on nonlinear radiation trapping in a saturable atomic vapor

Tobias Scholz, Joachim Welzel, Wulfhard K. Lange

Published Sep 1, 2001• 4 citations
Publisher

Abstract

We present experiments of a laser with variable pulse energy and beam width saturating sodium vapor near the ${D}_{1}$-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.

DOI
10.1103/physreva.64.043409

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We have measured the time-dependent fluorescence of the sodium DDD lines following pulsed excitation of one DDD line, in the presence of radiation trapping with optical depths k0L{k}_{0}Lk0​L of \ensuremath{\sim}10 to 2000. When collisional coupling of the 3P123{P}_{\frac{1}{2}}3P21​​ and 3P323{P}_{\frac{3}{2}}3P23​​ levels and different radiative-escape probabilities for each DDD line are taken into account, we obtain excellent agreement with Holstein's theory for the effective radiative decay rates in the Doppler region (${k}_{0}L\ensuremath{\sim}10\ensuremath{-}300$) and in the redistributed Lorentzian region (k0L>1000{k}_{0}L>1000k0​L>1000). For k0L{k}_{0}Lk0​L between these two regions, we observe an abrupt transition between the two limiting formulas. The buildup rate of the sensitized fluorescence signal also yields a $3{P}_{\frac{3}{2}}\ensuremath{\rightarrow}3{P}_{\frac{1}{2}}$ excitation transfer cross section in agreement with our previously reported cw measurement. Additionally, we have measured the dependence of the 3P3P3P fluorescence on laser power and beam diameter, and we explain the observed approach to saturation. We suggest that the previously reported "anomalous" approaches to saturation may be explained in terms of the laser beam burning through the optically thick vapor. Laser-beam spatial intensity variations and self-focusing also contribute to fluorescence signals that deviate from the usual single-atom saturation behavior.
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