Cover story: Ren J, Lin H, Zheng XR, Lei WW, Liu D et al. Giant and light modifiable third-order optical nonlinearity in a free-standing h-BN film. Opto-Electron Sci 1, 210013 (2022)

Hexagonal boron nitride (h-BN) is a two-dimensional (2D) layered wide-bandgap insulating material with structure similar to that of graphene. In addition to the characteristics of ordinary 2D materials, such as planar 2D structure, atomic level flatness, no dangling bonds, etc., h-BN also has excellent mechanical, chemical and thermal stability, so it has broad applications in ultraviolet lasers/detectors, near-field optics/imaging, protective coating materials, dielectric layers, tunneling layers and nonlinear optical devices.

Recently, Prof. Baohua Jia's group from RMIT University, Australia, has developed a method for laser-tunable third-order optical nonlinearity of functionalized h-BN free-standing film. Low-repetition-rate femtosecond laser was used to directly fabricate micro-patterns on the h-BN film, creating an ultra-fine low damage method to manufacture arbitrary h-BN structures. Femtosecond laser direct writing is found to be an effective way to achieve high-precision processing on h-BN films. The changes of the chemical bond hybridization of h-BN film before and after laser processing are studied. The carrier transport characteristics and dramatic increase of its nonlinear refractive index from 0.0143 cm²/GW to 0.1638 cm²/GW have been observed. The third-order nonlinear magnetic susceptibility was found to increase by 20 times, which promotes the applications of h-BN in the fields of nonlinear optics such as optical switches, wavelength converters and signal repeaters, and realizes the control and operation of light at the micro/nano scale.