Charge-imbalance-induced second harmonic generation in twisted graphene

Twist-and-stack engineering provides a programmable degree of freedom for nonlinear optics in two-dimensional materials, yet in a homostructure whose constituents have no second harmonic generation (SHG), how interlayer coupling grants and tunes second-order response remains unclear. Here, we use twisted monolayer-bilayer graphene (t(1 + 2)LG) and combine microscopic SHG spectroscopy with first-principles differential charge-density analysis to establish a unified “permission-and-resonance” mechanism. Interlayer coupling creates an interlayer charge imbalance within the AB-stacked bilayer, breaking inversion symmetry and thereby permitting an in-plane electric-dipole response. At the same time, the twist angle steers van Hove singularities in the band structure to achieve two-photon resonance, which markedly amplifies the susceptibility χ⁽²⁾. Experimentally, at θ = 13.5°, we obtain χ⁽²⁾ = 279.4 pm/V, evidencing a highly efficient second-order response. These results identify SHG as a sensitive probe of interlayer coupling and charge redistribution in homostructure van der Waals systems.