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 14 – 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 BiSbTeSe 2 . 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.