Controlling Linear and Nonlinear Optical Properties of Boron Nitride with Moiré Superlattices

Van der Waals materials enable the creation of artificial heterostructures with exciting new emergent properties. Recently, the twist angle between adjacent layers has proved to be an important control parameter in these stacks. In this work, we use conventional dry-transfer techniques to fabricate small-twist-angle few-layer hexagonal boron nitride (hBN) heterostructures. We study the resulting moiré superlattice with a variety of linear and nonlinear atomic force microscopy probes. These techniques yield a local variation of the transverse-optical-phonon oscillator strength, near-field second harmonic generation, and the piezoelectric response. These combined demonstrations of hBN moiré physics implicate local strain as an important tuning parameter for the linear and nonlinear optical properties of Van der Waals materials.