Nonlinear helicity-dependent photocurrents have been reported in three-dimensional topological materials lacking inversion symmetry. Here, we theoretically study the charge and spin photocurrents generated by linear and circularly polarized radiation in ultrathin topological insulator films. Using time-dependent perturbation theory and detailed balance equations, we find that helical transverse spin currents are generated when the symmetry between the top and bottom film surfaces is disturbed. Such spin currents are invariant under in-plane mirror transformations but have $s$-wave and $d$-wave components in regard to transformations under $\pi /2$ rotations. Spin-current phototransistors and amplifiers can be based on these effects.

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• Charge dynamics
• Light-induced magnetic effects
• Photocurrent
• Spin dynamics
• Spin-orbit coupling
• Spintronics
• Surface states
• Topological insulators