Forbidden Second-Harmonic Generation in Pristine Centrosymmetric Bilayers

Optical spectroscopy based on second-order nonlinearity is a critical technique for characterizing two-dimensional (2D) crystals, and it also finds numerous applications in bioimaging and quantum optics. It has been generally believed that second-harmonic generation (SHG) in crystals with inversion centers (centrosymmetric crystals), such as graphene and other bilayer 2D crystals, is negligible without externally breaking the symmetry via strong surface effects. Here we show that with a new ultra-sensitive detection technique, we could circumvent the symmetry-imposed constraint and observe robust SHG in pristine centrosymmetric crystals, even without any symmetry-breaking field. With the exceptional sensitivity, we directly observe polarization-resolved SHG in bilayer hexagonal boron nitride (h-BN), bilayer WSe2, and remarkably, Bernal-stacked bilayer graphene, allowing us to unambiguously identify the crystallographic orientation in all these crystals via SHG for the first time. We also demonstrate that the new technique can be used to non-invasively detect uniaxial strain and geometric phase in these centrosymmetric crystals. Our new technique expands the capability of nonlinear optical spectroscopy to encompass a large class of centrosymmetric materials that could never be measured before, and can be used for quantum sensing of moiré materials and epitaxial films.

Speaker: Yuan Cao, UC Berkeley