Disentangling high harmonic generation from surface and bulk of Bi₂Se₃

Topological insulators (TIs) are a compelling class of quantum materials because of their insulating bulk and conductive surface properties. In recent years, high harmonic generation (HHG) has proven to be an increasingly useful tool to probe electronic and dynamic behavior in condensed phase matter. However, HHG becomes complicated in TIs as it is difficult to separate surface and bulk emission. This study focuses on Bi₂Se₃, which is a prototypical TI, providing an opportunity to distinguish HHG from surface and bulk states by use of thin films and a mid-infrared (MIR) and terahertz (THz) two-color probe. To isolate surface and bulk effects, the MIR-driven HHG for varying thin film thicknesses was measured to maximize even harmonic yield which originates from the surface due to intrinsic symmetry breaking. Additionally, by employing a quasi-static THz perturbing field, the harmonic yield in two crystal orientations was measured and subtracted separating the surface response from that of the bulk.