Observation of terahertz second harmonic generation from Dirac surface states in the topological insulator Bi2Se3

We report the observation of second harmonic generation with high conversion efficiency $\ensuremath{\sim}0.005%$ in the terahertz regime from thin films of the topological insulator ${\mathrm{Bi}}_{2}{\mathrm{Se}}_{3}$ that exhibits the linear photogalvanic effect, measured via time-domain terahertz nonlinear spectroscopy and terahertz emission, respectively. As neither phenomena is observable from topologically trivial In-doped ${\mathrm{Bi}}_{2}{\mathrm{Se}}_{3}$, 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 ${\mathrm{Bi}}_{2}{\mathrm{Se}}_{3}$. 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 second-order 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.