High-harmonic generation from topological surface states

Three-dimensional topological insulators are a phase of matter that hosts unique spin-polarized gapless surface states that are protected by time-reversal symmetry. They exhibit unconventional charge and spin transport properties(1,2). Intense laser fields can drive ballistic charge dynamics in Dirac bands(3,4) or they can coherently steer spin(5) and valley pseudospin(6). Similarly, high-harmonic generation (HHG) in solids provides insights into the dynamics of the electrons in topological insulators(7-13). Despite several theoretical attempts to identify a topological signature in the high-harmonic spectrum(74-16), a unique fingerprint has yet to be found experimentally. Here, we observe HHG that arises from topological surface states in the intrinsic topological insulator BiSbTeSe2. The components of the even-order harmonics that are polarized along the pump polarization stem from the spin current in helical surface states, whereas the perpendicular components originate from the out-of-plane spin polarization related to the hexagonal wrapping effect(17). The dependence of HHG on surface doping in ambient air also suggests the presence of a Rashba-split two-dimensional electron gas, whose strength can be enhanced by an increase in the intensity of the mid-infrared pump.

Automated summary

Three-dimensional topological insulators host unique spin-polarized surface states with no energy gap, exhibiting unconventional charge and spin transport properties. High harmonic generation from the intrinsic topological insulator BiSbTeSe2 reveals that even-order harmonics polarized along the pump polarization stem from the spin current in helical surface states, while perpendicular components originate from out-of-plane spin polarization related to hexagonal wrapping effect.