Multiphoton absorption and dispersive nonlinear refraction of ZnO in VIS-NIR bands

Zinc oxide (ZnO), as one of the third-generation semiconductor materials, has attracted extensive research interest, but its nonlinear optical properties in near-infrared (NIR) band are rarely involved. Herein, the beam propagation diffraction model has been extended to handle up to the 9th-order optical nonlinearity, and still has excellent performance of fitting experimental data. Multiphoton absorption effects were experimentally investigated by z-scan measurement in visible (VIS)-NIR bands. In an N-photon absorption (N-PA) spectral regime, the respective N-PA effect dominates nonlinear absorption, while all other orders are negligible under the experimental laser intensities. In VIS-NIR bands, only the third-order nonlinear refraction is significant for Kerr nonlinear refractive index, while the higher orders can be ignored. The nonlinear figure of merit is low due to the strong 2PA effect in VIS band, but increases by an order of magnitude, up to a maximum value of 5.8, in the spectral region where the 2PA effect diminishes. These results indicate that ZnO is a valuable candidate for nonlinear photonics applications in a broad NIR band accommodating telecom wavelength.