Z-scan measurements of the anisotropy of nonlinear refraction and absorption in crystals.

The techniques most often used to determine components of the third-order susceptibility, x(3), include degenerate four-wave mixing, 1 nearly degenerate threeand four-wave mixing, 2 ellipse rotation, 3 optical third-harmonic generation,4 photoacoustics,5 beam-distortion methods,6 8 and nonlinear transmission.9 Some of these have been adapted to measure the anisotropy of x0 • 8 It is shown that D..Tpv = plD..ol = pkiAnlL, where k is the propagation vector and L is the length of the sample. The constant p depends on the size of the aperture in the far field and equals 0.36 for an aperture allowing 40% linear transmittance. Placing the sample at the Z position corresponding to the peak, a half-wave plate situated in front of the sample is rotated, and the transmittance through the aperture is measured as a function of the incident linear polarization direction. This is repeated with the sample positioned at the valley. By subtracting these two sets of data, we obtain l:l T pv as a function of incident electric-field polarization, from which we can infer the polarization dependence of n2 • Here we define n2 by the change in refractive index, D..n = n 21. When the aperture is removed and all the transmitted energy is collected, the experiment is sensitive only to changes in transmittance due to the presence of nonlinear absorption. When the sample is placed