Anisotropy of 2PA, 3PA, and Kerr effect in nonpolar ZnO.

Abstract

Anisotropic optical nonlinearity plays an important role in polarization-dependent optoelectronic devices. Taking the advantage of in-plane crystallographic axis, the polarization dependence of third- and fifth-order nonlinearity in nonpolar ZnO has been investigated by z scan. Here, we established the theory model of anisotropic relation of fifth-order nonlinearity in hexagonal wurtzite crystal. Both two-photon absorption (2PA) and three-photon absorption (3PA) coefficients exhibit anisotropic oscillations with a period of 180°, and the polarization modulation factor and anisotropy coefficient (rp,σ) are measured as (2.10, 1.84) for β2, and (1.66, 1.33) for β3, respectively. This is the first time, to the best of our knowledge, that the anisotropy of the fifth-order optical nonlinear effect has been characterized and confirmed. It is verified that the self-defocusing effect at 500 nm is isotropy, while the self-focusing effect at 800 nm exhibits significant anisotropy. The maximum value of the nonlinear figure of merit is 5.84 along the [0010] crystalline direction, nearly 1.6 times the minimum value, revealing that ZnO exhibits potential for nonlinear applications containing polarization-related all-optical switching components.