Full Crystallographic Imaging of Hexagonal Boron Nitride Monolayers with Phonon‐Enhanced Sum‐Frequency Microscopy

Abstract Hexagonal boron nitride (hBN) is an important 2D material for van der Waals heterostructures, single photon emitters, and infrared nanophotonics. The optical characterization of mono‐ and few‐layer samples of hBN however, remains a challenge as the material is almost invisible optically. Here, phase‐resolved sum‐frequency microscopy is introduced as a technique for imaging monolayers of hBN grown by chemical vapor deposition (CVD) and visualizing their crystal orientation. Femtosecond mid‐infrared (IR) and visible laser pulses are used for sum‐frequency generation (SFG), which is imaged in a wide‐field optical microscope. The IR laser resonantly excites a phonon of hBN that leads to an ≈800‐fold enhancement of the SFG intensity, making it possible to image large 100 × 100 µm ² sample areas in less than 1 s. Heterodyne detection combined with azimuthal sample rotation further provides full crystallographic information. Combined knowledge of topography and crystal orientation reveals that triangular domains of CVD‐grown monolayer hBN have nitrogen‐terminated zigzag edges. Overall, SFG microscopy is an ultra‐sensitive tool with the potential to image crystal structure, strain, stacking sequences, and twist angles in a wide range of van der Waals structures, where locating and identifying monolayer regions and interfaces with broken inversion symmetry is of paramount importance.