Charge-imbalance-induced second harmonic generation in twisted graphene

【摘要】 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.更多还原

【Abstract】 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.更多还原