The 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 ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ through twisting, point-defect insertion, or the application of an external electric field unlocks rich nonlinear optical responses. In twisted bilayer ${\mathrm{Bi}}_2{\mathrm{Se}}_3$ at the first commensurate angle of $21.{78}^{\circ }$, we find peak shift- and injection-current conductivities of $-14\mathrm{nm}\mu {\mathrm{A}\mathrm{V}}^{-2}$ and $104\times {10}^8\mathrm{nm}{\mathrm{A}\mathrm{V}}^{-2}{\mathrm{s}}^{-1}$, 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 ($V_{\text{Se}}$) achieving peak shift- and injection-current conductivities of $-190\text{nm}\mu {\mathrm{A}\mathrm{V}}^{-2}$ and $-170\times {10}^8\mathrm{nm}{\mathrm{A}\mathrm{V}}^{-2}{\mathrm{s}}^{-1}$. In all three cases, the peak nonlinear optical responses are found to be comparable to those of benchmark two-dimensional (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.

Close

PrevNext

View figure in article

• Electronic structure
• Photocurrent
• First-principles calculations