Giant and Actively Tunable Second-Harmonic Generation in Bilayer Graphene.

Graphene, a prototypical 2D material celebrated for exceptional electronic and optical properties, is constrained by its centrosymmetric lattice, which intrinsically suppresses second-order nonlinear responses such as second-harmonic generation (SHG). Leveraging the symmetry tunability of van der Waals materials, we address this limitation via electrostatic control of the Hamiltonian. By employing an ion-gel top gate to exert a strong out-of-plane displacement field, we effectively break the inversion symmetry of bilayer graphene and unlock its latent χ(2) nonlinearity. We demonstrate a giant, actively tunable SHG response with a nonlinear susceptibility χ(2) ∼ 300 pm/V in the communication band, exceeding that of monolayer MoS2 by over an order of magnitude, whereas the signal remains undetectable under conventional SiO2 back-gating. Polarization-resolved measurements confirm a symmetry reduction to the C3v point group, while thickness-dependent studies establish a generalized framework for nonlinear optics in centrosymmetric stacks. Our work positions electrostatic symmetry control as a general approach for activating and controlling nonlinear responses in 2D materials, with potential applications in actively tunable integrated photonics and quantum light sources.