Abstract

We report a simple extension of the Z-scan technique that permits a spectral line-shape measurement of the real and the imaginary parts of $n_2$. In this technique the sample is placed at the peak position of the usual Z-scan curve while the laser frequency is scanned. We employed this method to investigate the nonlinear susceptibility of the $R$ lines of ruby and alexandrite, using a cw dye laser. This susceptibility can be explained by the resonant interaction and by a nonresonant contribution that is due to the difference in polarizability between ${\mathrm{Cr}}^{3+}$ excited and ground states. For ruby, the nonresonant contribution to the technique is 1 order of magnitude larger than the resonant contribution. However, for alexandrite both contributions are comparable, and their interference leads to a shift between $n_2{}^{\prime }$ and $n_2{}^{″}$ spectra that is not observed in ruby.

© 2002 Optical Society of America

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