Two-dimensional materials (2DM) and their heterostructures offer tunable electrical and optical properties, primarily modifiable through electrostatic gating and twisting. While electrostatic gating is a well-established method for manipulating 2DM, achieving real-time control over interfacial properties remains a frontier in exploring 2DM physics and advanced quantum device technology. Current methods, often reliant on scanning microscopes, are limited in their application scope, lacking the accessibility and scalability of electrostatic gating at the device level. In the first half of this seminar, I will introduce an on-chip platform for 2DM with in situ adjustable interfacial properties, employing a microelectromechanical system (MEMS). This platform comprises compact, precise, and versatile devices capable of voltage-controlled manipulation of 2DM, including approaching, twisting, and pressurizing actions. We demonstrate this technology by creating synthetic topological singularities in the nonlinear optical susceptibility of twisted hexagonal boron nitride (h-BN).
In the second half of this seminar, I will talk about our recent progress in observing symmetry-forbidden second-harmonic generation in almost any 2D crystals, which is extremely useful for deterministic twistronics. 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. However, 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. We also demonstrate that the new technique can be used to non-invasively detect uniaxial strain and geometric phase in these centrosymmetric crystals.