8A2 - The self-focusing of light of different polarizations

In an effort to verify the hypothesis that molecular reorientation (ac Kerr effect) is responsible for the self-focusing of light beams in certain liquids, we have measured and compared the thresholds and other characteristics of self-focusing for circularly and linearly polarized beams incident on these liquids. We show that for plane waves propagating in a homogeneous, isotropic, ensemble of molecules having anisotropic polarizability tensors, the nonlinear index should be four times as great for linearly as for circularly polarized waves. The hope that this difference in indexes would be reflected in a four-fold increase in the threshold power for self-trapping when circularly instead of linearly polarized light is incident was not realized. In practice, the increase was always found to be much less. However, in every case studied, the trapped light from a beam, circularly polarized to better than 1 part in 200, was markedly, if not completely, depolarized as soon as self-trapping could be detected. We show qualitatively that this polarization instability should exist for all but linearly polarized light and for a variety of nonlinear mechanisms. However, in the absence of even an approximate quantitative theory of the self-trapping of light that is not linearly polarized, the comparative measurements of thresholds cannot be said either to verify or disprove the hypothesis of molecular reorientation.