Charge-imbalance-induced second harmonic generation in twisted graphene

doi:10.1088/1674-1056/ae3557

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 $\chi^{(2)}$. Experimentally, at $\theta =13.5^\circ $, we obtain $\chi^{(2)}=279.4$ 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.

Keywords: second harmonic generation charge imbalance twisted graphene

Received: 08 December 2025
Revised: 26 December 2025
Accepted manuscript online: 08 January 2026

PACS:	42.65.Ky	(Frequency conversion; harmonic generation, including higher-order harmonic generation)
	78.67.-n	(Optical properties of low-dimensional, mesoscopic, and nanoscale materials and structures)

Fund: Project supported by the National Natural Science Foundation of China (Grant Nos. 12574339, 12574457, and 12174207) and Tianjin Science and Technology Project (Grant No. 24ZXZSSS00120).

Corresponding Authors: Zhibo Liu
E-mail: liuzb@nankai.edu.cn

Cite this article:

Ronghui Luo(罗荣辉), Shiyang Liu(刘诗洋), Xiao Dong(董校), Jianguo Tian(田建国), and Zhibo Liu(刘智波) Charge-imbalance-induced second harmonic generation in twisted graphene 2026 Chin. Phys. B 35 034201

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Charge-imbalance-induced second harmonic generation in twisted graphene

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