Disentangling High Harmonic Generation from Surface and Bulk States of a Topological Insulator

TL;DR

This study demonstrates how to distinguish and control the contributions of surface and bulk states to high harmonic generation in a topological insulator by tuning film thickness and applying terahertz fields.

Contribution

It introduces methods to isolate surface state responses from bulk in HHG, revealing the influence of topological properties like Berry curvature.

Findings

Ultrathin films enhance surface state HHG

Thicker films favor bulk HHG

Terahertz fields disentangle surface and bulk responses

Abstract

The discovery of topological phases has introduced a new dimension to materials science. Three-dimensional (3D) topological insulators (TIs) are a remarkable class of matter that is insulating in the bulk while hosting conductive topological surface states (TSSs) with unique charge and spin properties. High-order harmonic generation (HHG) has emerged as a powerful tool to probe condensed matter systems by providing insights into their electronic structure and dynamic behavior. Here, we investigate HHG in the prototype 3D-TI Bi$_2$Se$_3$. We demonstrate that the contributions of bulk and surface states to the harmonic emission can be controlled by tuning the thickness of thin film samples. An ultrathin (6 nm) film substantially enhances HHG from the surface states, while the bulk states dominate HHG in a thicker (50 nm) film. By applying a quasi-static terahertz perturbing field, we disentangle the bulk and surface responses and reveal the significant impact of the surface states' shift vector and Berry curvature on HHG. Our study provides effective methods for isolating the optical responses of TSSs from those of the bulk, which opens the door to resolving an ongoing debate regarding whether it is possible to reliably extract topological signatures in HHG.