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Engineering Nonlinear Optical Responses via Inversion Symmetry Breaking in Bilayer Bi2Se3

Vineet Kumar Sharma, Alana Okullo, Barun Ghosh, Arun Bansil, Sugata Chowdhury

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.μAV^{-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.$μAV^{-2}$ 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.

Engineering Nonlinear Optical Responses via Inversion Symmetry Breaking in Bilayer Bi2Se3

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 BiSe through twisting, point-defect insertion, or the application of an external electric field unlocks rich nonlinear optical responses. In twisted bilayer BiSe at the first commensurate angle of 21.78, we find peak shift and injection current conductivities of -14 and 104 , 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 symmetry, with the selenium vacancy (V) achieving peak shift and injection current conductivities of -190 nm. and -170 . 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.

Paper Structure

This paper contains 8 sections, 5 equations, 4 figures, 1 table.

Table of Contents

  1. Introduction
  2. Computational Details
  3. Results and discussion
  4. Electronic structure and nonlinear optical response in twisted bilayer Bi$_2$Se$_3$
  5. External electric field effects on bilayer Bi$_2$Se$_3$
  6. Point-defect-driven nonlinear optical responses
  7. Summary
  8. Acknowledgements

Figures (4)

  • Figure 1: (a) Side view of the twisted bilayer system. (b) Top view of the twisted Moiré pattern with a twist angle of 21.78$^\circ$. (c) 2D Brillouin zone for the twisted Moiré system. (d) Schematic of a non-zero Berry-curvature dipole induced by non-centrosymmetry. (e) Schematic of the generation of photovoltaic currents in a twisted bilayer.
  • Figure 2: (a) Electronic band structure of twisted bilayer Bi$_2$Se$_3$, with the inset showing Rashba-type band splitting at the $\Gamma$-point induced by the broken inversion symmetry. (b) Berry curvature $\Omega_{xy}$, demonstrating a non-zero Berry curvature dipole. (c) Shift-current and (d) injection-current conductivity tensors, reflecting the linear and circular photogalvanic effects, respectively.
  • Figure 3: Electronic structure and nonlinear optical properties of bilayer Bi$_2$Se$_3$ under an external electric field. (a) Band structure at 40 meV/$\AA$, with inset showing Rashba-type band splitting near the $\Gamma$-point induced by broken inversion symmetry. (b) Berry curvature $\Omega_{xy}$ demonstrating a non-zero Berry curvature dipole. (c) Shift current and (d) injection current conductivity at various electric field strengths.
  • Figure 4: Point defects in bilayer Bi$_2$Se$_3$ and nonlinear responses: (a) Se vacancy defect (red circle). (b) Bi-rich antisite defect induced by replacing Se with Bi. (c) Bi-poor antisite defect induced by replacing Bi with Se. (d-f) Band structures corresponding to the defects in panels (a-c). (g) Shift current and (h) injection current conductivities.