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Nonlinear Optics

The role of radiation trapping in nonlinear optical processes in atomic vapors

Wulfhard K. Lange, G. Ankerhold, M. Schiffer, D. Mutschall, Tobias Scholz

Published Jan 1, 1992• 0 citations
Publisher

Abstract

In this work we report on experimental and theoretical investigations of novel features of radiation transport in gases and discuss their significance for nonlinear optical processes in atomic media. Radiation diffusion is found to alter many nonlinear processes based on intensity dependent changes of level populations, to influence the timescales involved in experiments and to have drastic effects on the spatial distribution of extrinsic nonlinearities.

DOI
10.1364/nlo.1992.tud14

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In this work we report on experimental and theoretical investigations of novel features of radiation transport in gases and discuss their significance for nonlinear optical processes in atomic media. Radiation diffusion is found to alter many nonlinear processes based on intensity dependent changes of level populations, to influence the timescales involved in experiments and to have drastic effects on the spatial distribution of extrinsic nonlinearities.
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Effects of radiation transport in the interaction of a laser beam with sodium vapor in an argon atmosphere are reported. Investigations of the temporal and spatial evolution of ground-state orientation, which is produced by optical pumping with a circularly polarized near-resonant laser beam, reveal a strong deviation from the expected pure diffusive behavior. Transverse profiles are narrowed considerably with increasing sodium density as well as with increasing pump intensity. The temporal decay of orientation is also drastically altered. A simplified nonlinear and nonlocal theoretical model of radiation trapping gives excellent qualitative agreement with the observations and allows an intuitive interpretation.
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We describe the influence of radiation trapping on the spatial distribution of the excited-state population in sodium vapor, interacting with a linearly polarized Gaussian laser beam. Counterintuitively the excitation volume experiences a strong spatial shrinking at high optical depth. The interpretation of this nonlocal effect is based on the numerical solution of the stationary Holstein equation, which takes into account the full frequency dependence of the reabsorption probability within the process of diffusion of radiation. The calculations performed are in good qualitative agreement with the spatial effects observed experimentally. \textcopyright{} 1996 The American Physical Society.