Abstract

We have performed measurements of the anisotropy of the nonlinear refractive index n₂ of crystalline solids using a variation of our recently developed Z-scan technique.¹ This method gives a sensitive, absolutely calibrated measurement of n₂ using a single beam. In the Z-scan technique the transmittance of a sample with an aperture placed in the far field is measured as a function of the position of the sample with an aperture placed in the far field is measured as a function of the position of the sample with respect to the Gaussian beam waist. This results in a peak signal followed by a valley for n₂<0 and the opposite for n₂ <0 as the sample is moved from before to after focus. If the aperture is removed the Z-scan is sensitive only to nonlinear absorption. For the measurement of the anisotropy of n2 we modify this technique by placing the sample at a position of peak (valley) signal and rotate a wave plate, either a half-wave plate to rotate the linearly polarized light or a quarter-wave plate to change from linear to circular polarization. By placing samples at the beam waist with the aperture fully open, this method yields the anisotropy of the nonlinear absorption coefficient. We demonstrate this technique on materials such as KTP where we haveseen n₂ vary by a factor of 1.6 as the linear polarization is rotated . Thus, n₂ of flux grown, z-cut KTP at 1.06µm varies from0.62 × 10⁻¹²esuto 1.0 × 10⁻¹² esu. In x-cut KTP, we have observed the unusual effect of a reversal in sign of the nonlinear refraction from self-sign change is correlated with the generation of second harmonic light, with the maximum defocusing occurring at the polarization producing the maximum second harmonic.

© 1990 Optical Society of America