Observation of polarization and thickness dependent third-harmonic generation in multilayer black phosphorus

We experimentally investigate the power, polarization, and thickness dependence of third-harmonic generation in multilayer black phosphorus (BP) flakes. We observe that the third-harmonic response radiated from BP is highly dependent on the polarization of the excitation owing to the anisotropic nature of BP, as well as being very sensitive to the flake thickness owing to the interference effects between surface and bulk nonlinear contributions. Compared to the linear optical microscopic examination, the demonstrated third-harmonic generation can be used for nonlinear optical identification of the crystal orientation and thickness of multilayer BP with higher contrast and fewer restrictions on substrate materials.