Calculated shift currents of two-dimensional Janus Si and Ge chalcogenides

The unique band structure in quantum spin Hall insulators (QSHIs) is predicted to induce strong nonlinear optical rectification, which is crucial for solar energy conversion and infrared detection. However, the inherent requirement of breaking inversion symmetry for nonlinear optical effects and the scarcity of suitable candidate materials have hindered the exploration of nonlinear optical responses in QSHIs. The synthesis approach involving two-dimensional (2D) Janus materials offers a route to induce inversion symmetry breaking without significantly altering the original band structures. This provides an effective strategy for designing materials with strong nonlinear rectification properties based on QSHIs. Taking advantage of this approach, we designed six Janus structures based on the experimentally synthesized QSHI ${\mathrm{Si}}_{2}{\mathrm{Te}}_{2}$. Although the topological nature of these structure configurations is disrupted, the high joint density of states and the large interband dipole matrix elements are maintained, which actually govern the enhancement of optical response in QSHIs. Remarkably, these noncentrosymmetric Janus materials exhibit large shift currents exceeding 1000 $\textmu{}\text{A}\text{/}{\mathrm{V}}^{2}$ in the infrared spectrum and 240 $\textmu{}\text{A/}{\mathrm{V}}^{2}$ in the visible spectrum. This paper highlights the potential of 2D Janus materials for applications in both infrared detection and energy harvesting.