Observation of terahertz second harmonic generation from Dirac surface states in the topological insulator Bi$_2$Se$_3$

TL;DR

This paper reports the first observation of efficient terahertz second harmonic generation from the Dirac surface states of topological insulator Bi$_2$Se$_3$, revealing intrinsic nonlinear optical responses linked to topological surface states.

Contribution

It demonstrates intrinsic terahertz second harmonic generation in Bi$_2$Se$_3$ surface states, highlighting a new nonlinear optical property of topological insulators.

Findings

High conversion efficiency (~0.005%) of terahertz SHG observed

SHG is absent in topologically trivial Bi$_2$Se$_3$

Nonlinear response is thickness-independent and linked to Dirac fermions

Abstract

We report the observation of second harmonic generation with high conversion efficiency $\sim 0.005\%$ in the terahertz regime from thin films of the topological insulator Bi$_2$Se$_3$ that exhibit the linear photogalvanic effect, measured via time-domain terahertz spectroscopy and terahertz emission, respectively. As neither phenomena is observable from topologically trivial In-doped Bi$_2$Se$_3$, and since no enhancement is observed when subject to band bending, the efficient thickness-independent nonliear responses are attributable to the Dirac fermions of topological surface states of Bi$_2$Se$_3$. This observation of intrinsic terahertz second harmonic generation in an equilibrium system unlocks the full suite of both even and odd harmonic orders in the terahertz regime and opens new pathways to probing quantum geometry via intraband nonlinear processes. We hope our work will motivate the theoretical development of a full treatment of second harmonic generation for probing the quantum geometry in various inversion-breaking topological and twisted materials.