Abnormal nonlinear optical responses on the surface of topological materials

Abstract

The nonlinear optical (NLO) responses of topological materials are under active research. Most previous works studied the surface and bulk NLO responses separately. Here we develop a generic Green’s function framework to investigate the surface and bulk NLO responses together. We reveal that the topological surface can behave disparately from the bulk under light illumination. Remarkably, the photocurrents on the surface can flow in opposite directions to those in the bulk interior, and the light-induced spin current on the surface can be orders of magnitude stronger than its bulk counterpart on a per-volume basis. We also study the responses under inhomogeneous field and higher-order NLO effect, which are all distinct on the surface. These anomalous surface responses suggest that light can be a valuable tool for probing the surface states of topological materials. Besides, the surface effects should be prudently considered when investigating the optical properties of topological materials.