Nonlinear Optical Responses in twisted Bi₂Se₃ Bilayer Systems

The interaction between topologically protected helical surface states with broken inversion symmetries of the topological insulators (TIs) and incident photons has drawn enormous research interest because it brings an excellent opportunity for developing next-generation optoelectronic devices. A challenge, however, has been to understand and control the unique emergence of nonlinear optical responses (NLOR) due to light-matter interactions. To address this challenge, using first-principles calculations and the tight-binding model, we studied the NLOR, more specifically the shift current, in a twisted bilayer and doped bilayer Bi₂Se₃. We constructed a twisted bilayer system at first commensurate twist angle of 21.79° for bilayer systems. Our calculations reveal a noticeable shift current conductivity under both the linear and circularly polarized light. Furthermore, the inclusion of an external electric field and two potential doping effects (Bi_1.94Sb_0.06Se₃ & Bi₂Se_2.96Te_0.04) resulted in a large shift current response under linearly polarized light. The calculated shift current conductivities are consistent with those exhibited by the well-known non-centrosymmetric materials, enabling these investigated 2D systems to be plausible for next-generation 2D photovoltaics.