Reversing the sign of the effective nonlinear coefficient of a nearly isotropic Kerr photonic crystal via the local-field effect

The sign of the Kerr nonlinear coefficient, which determines the nonlinear propagation characteristics of high-power lasers, is usually only sensitive to the operating frequency. Here, we report a method to reverse the sign according to the polarization of the input beam. The sign reversal is realized in the first band of a two-dimensional Kerr photonic crystal (PhC) by using local-field effects. By appropriately adjusting the unit cell, we find that the effective nonlinear coefficient of the PhC is negative for the TM mode while positive for the TE mode. Using the FDTD method, polarization-dependent spectral broadening and narrowing, self-focusing and defocusing are demonstrated in the nearly isotropic PhC.