Strain-enhanced two-photon absorption in twisted bilayer graphene.

We theoretically investigate the strain-tuned moiré flat bands and two-photon absorption (TPA) in magic-angle (MA) twisted bilayer graphene (TBG). Using a low-energy continuum model for strained TBG and second-order perturbation theory, we demonstrate that uniaxial strain significantly modulates both electronic and optical properties. Strain magnitude controls the flat band separation, while strain direction induces periodic variations in the band structure. The TPA spectra reveal strain-dependent features: intraband transitions dominate at higher strains, enhancing the absorption coefficient by an order of magnitude, while interband transitions exhibit characteristic spectral shifts. Notably, smaller strain direction angles yield larger TPA coefficients with red-shifted peaks. These results provide fundamental insights for developing strain-tunable optoelectronic devices based on moiré materials.