Nonlinear refraction and optical limiting in [quotation mark]thick[quotation mark] media

We experimentally and theoretically investigate optical beam propagation in nonlinear refractive materials having a thickness greater than the depth of focus of the input beam (i.e., internal self-action). A simple model based on the "constant shape approximation" is adequate for analyzing the propagation of laser beams within such media under most conditions. In a tight focus geometry, we find that the position of the sample with respect to the focal plane, <i>z</i>, is an important parameter in the fluence limiting characteristics of the output. The behavior with <i>z</i> allows us to perform a "thick sample Z-scan" from which we can determine the sign and magnitude of the nonlinear refraction index. In CS₂, we have used this method to independently measure the negative thermally induced index change and the positive Kerr nonlinearity with nanosecond and picosecond CO₂ laser pulses, respectively. We have experimentally examined the limiting characteristics of thick CS₂ samples that qualitatively agree with our analysis for both positive and negative nonlinear refraction. This analysis is useful in optimizing the limiting behavior of devices based on self-action.

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