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 we introduce
phase-resolved sum-frequency microscopy as a technique for imaging monolayers
of hBN grown by chemical vapor deposition (CVD) and visualize their crystal
orientation. A combination of femtosecond mid-infrared (IR) and visible laser
pulses is 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 100x100 {\mu}m2 sample areas in less than 1 s. Implementing
heterodyne detection in combination with azimuthal rotation of the sample
further provides full crystallographic information. Through combined knowledge
of topography and crystal orientation, we find that triangular domains of
CVD-grown monolayer hBN have nitrogen-terminated zigzag edges. Overall, SFG
microscopy can be used as an ultra-sensitive tool to image crystal structure,
strain, stacking sequences, and twist angles, and is applicable to the wide
range of van der Waals structures, where location and identification of
monolayer regions and interfaces with broken inversion symmetry is of paramount
importance.