We report the observation of second harmonic generation with high conversion efficiency ∼0.005% in the terahertz regime from thin films of the topological insulator Bi2Se3 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 Bi2Se3, 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 Bi2Se3. 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.
@article{arxiv.2301.05271,
title = {Observation of terahertz second harmonic generation from Dirac surface states in the topological insulator Bi$_2$Se$_3$},
author = {Jonathan Stensberg and Xingyue Han and Zhuoliang Ni and Xiong Yao and Xiaoyu Yuan and Debarghya Mallick and Akshat Gandhi and Seongshik Oh and Liang Wu},
journal= {arXiv preprint arXiv:2301.05271},
year = {2024}
}