Nonlinear Optical Imaging of Few-Layer Hexagonal Boron Nitride

Few-layer hexagonal boron nitride (hBN) has attracted considerable attention for its nanophotonic and 2D dielectric applications. Because TEM and STM are limited to very small length scales and by demanding sample requirements, a wide-area nondestructive probe with high throughput is required to characterize crystalline domains and defects in wafer-scale hBN [1]. In this study, we report on polarimetric second-harmonic generation (SHG) imaging that uses a ratiometric detection of two orthogonally polarized SHG fields. The method revealed grain boundaries of polycrystalline CVD-grown hBN films on the length scale of hundreds of micron with crystallographic orientation and quality. A significant variation in the width of orientational distribution was observed for single-crystal samples from various sources, which implied varying crystallinity. Confirming a positive correlation between the widths of orientational distribution and Raman peak, we propose the nonlinear optical analysis can be a new structural probe for synthesized hBN films. Since the properties of hBN films are closely linked to their structure, the reported method will contribute to the controlled synthesis of hBN with specific material properties [2].