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 µm2 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.
