Giant and light modifiable third-order optical nonlinearity in a free-standing h-BN film

Recently, hexagonal boron nitride (h-BN) has become a promising nanophotonic platform for on-chip information devices due to the practicability in generating optically stable, ultra-bright quantum emitters. For an integrated information-processing chip, high optical nonlinearity is indispensable for various fundamental functionalities, such as all-optical modulation, high order harmonic generation, optical switching and so on. Here we study the third-order optical nonlinearity of free-standing h-BN thin films, which is an ideal platform for on-chip integration and device formation without the need of transfer. The films were synthesized by a solution-based method with abundant functional groups enabling high third-order optical nonlinearity. Unlike the highly inert pristine h-BN films synthesized by conventional methods, the free-standing h-BN films could be locally oxidized upon tailored femtosecond laser irradiation, which further enhances the third-order nonlinearity, especially the nonlinear refraction index, by more than 20 times. The combination of the free-standing h-BN films with laser activation and patterning capability establishes a new promising platform for high performance on-chip photonic devices with modifiable optical performance.