Anisotropic Third-Order Nonlinearity in Pristine and Lithium Hydride Intercalated Black Phosphorus

Two-dimensional (2D) black phosphorus (BP) displays unique anisotropic properties in its linear optical responses, but its third-order nonlinearity (Kerr effect) has remained relatively unexplored. Here we measured the third-order nonlinear optical responses in exfoliated BP and in lithium hydride intercalated BP (LiH-BP) flakes using polarization-resolved microscopic femto-second Z-scan techniques. Strong optical Kerr nonlinearities were measured for single flakes of BP and LiH-BP, with third-order nonlinear susceptibilities that are larger than those for many 2D materials. The nonlinear coefficients of LiH-BP are higher than those of BP, indicating that lithium hydride intercalation enhances not only the ambient stability of BP but also its nonlinear optical response. Highly anisotropic polarization-dependent Kerr nonlinearities of both BP and LiH-BP flakes were observed. The strong nonlinear and anisotropic optical responses of BP and LiH-BP indicate the great potential of these materials in nonlinear photonics device applications.