Abnormal Surface Nonlinear Optical Responses in Topological Materials

TL;DR

This paper develops a Green's function framework to study surface nonlinear optical responses in topological materials, revealing distinct and enhanced surface effects compared to bulk responses, with implications for probing surface states.

Contribution

The paper introduces a generic Green's function approach to analyze surface nonlinear optical responses in topological materials, including many-body and high-order effects, highlighting unique surface behaviors.

Findings

Surface responses can differ markedly from bulk under illumination.

Surface currents can flow oppositely to bulk currents.

Surface spin currents can be significantly stronger than bulk.

Abstract

Nonlinear optical (NLO) responses of topological materials are under active research in recent years. Yet by far, most studies focused on the bulk properties, whereas the surface effects and the difference between surface and bulk responses have not been systematically studied. Here we develop a generic Green's function framework to investigate the surface NLO properties of topological materials. The Green's function framework can naturally incorporate many-body effects and can be easily extended to high-order NLO responses. Using Td-WTe2 as an example, we reveal that the surface can behave disparately from the bulk under light illumination. Remarkably, the shift and circular currents on the surface can flow in opposite directions to those in the bulk interior. Moreover, the light-induced spin current on the surface can be orders of magnitude stronger than its bulk counterpart. We also study the responses under inhomogeneous field and higher-order NLO effect, which are all distinct on the surface. These anomalous surface NLO responses suggest that light can be a valuable tool for probing the surface states of topological materials. On the other hand, the surface effects shall be prudently considered when investigating the optical properties of topological materials, especially if the material is of nanoscale and/or the light penetration depth is small.