Engineering Nonlinear Optical Responses via Inversion Symmetry Breaking in Bilayer Bi2Se3

TL;DR

This paper demonstrates how breaking inversion symmetry in bilayer Bi2Se3 through twisting, defects, or electric fields induces strong nonlinear optical responses suitable for advanced optoelectronic applications.

Contribution

It introduces methods to engineer nonlinear optical properties in bilayer Bi2Se3 by symmetry breaking, achieving responses comparable to benchmark 2D materials.

Findings

Twisted bilayer Bi2Se3 exhibits significant nonlinear conductivities in the visible spectrum.

Point defects like selenium vacancies greatly enhance nonlinear responses.

Engineered bilayers show broadband, helicity-dependent current generation.

Abstract

Paucity of naturally occurring noncentrosymmetric materials is stimulating growing interest in engineered two-dimensional systems for nonlinear optical applications. Here, we show that breaking inversion symmetry in centrosymmetric bilayer Bi$_2$Se$_3$ through twisting, point-defect insertion, or the application of an external electric field unlocks rich nonlinear optical responses. In twisted bilayer Bi$_2$Se$_3$ at the first commensurate angle of 21.78$^\circ$, we find peak shift and injection current conductivities of -14 $ nm.\mu AV^{-2}$ and 104 $\times 10^8$ $nm.A V^{-2}s^{-1}$, respectively, which lie in the visible spectrum and enable efficient THz applications. The external electric field and point-defect insertion both transform the bilayer into C$_ {3v}$ symmetry, with the selenium vacancy (V$_{Se}$) achieving peak shift and injection current conductivities of -190 nm.$\mu AV^{-2}$ and -170 $\times 10^8$ $nm.A V^{-2}s^{-1}$. In all three cases, the peak nonlinear optical responses are found to be comparable to those of benchmark 2D materials such as GeS, and the broadband responses, including helicity-dependent current generation, make these engineered bilayers viable candidates for next-generation 2D photovoltaics.