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. The features of both phenomena are found to be consistent with the characteristics of and attributable to the surface of Bi2Se3, which breaks both inversion symmetry and two-fold rotation symmetry and therefore permits second-order processes. Since both phenomena result from processes that reverse sign in oppositely-oriented domains of Bi2Se3, the observation of both phenomena is attributable to the presence of unequally populated twinned domains in the sample over millimeter length scales, confirmed by atomic force microscopy measurements. These results represent the first observation of intrinsic terahertz second harmonic generation in an equilibrium system, unlocking the full suite of both even and odd harmonic orders in the terahertz regime.

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