A schematic of the experimental apparatus is presented in fig. S1. Laser excitation away from normal incidence was needed to allow a finite projection of the out-of-plane current in the direction perpendicular to the propagation of the THz wave (Fig. 1B) (14). The 3.1-eV ultrafast laser excitation was produced by second-harmonic generation of a mode-locked Ti:sapphire laser with pulses of 40-fs (FWHM) duration and 5.12-MHz repetition rate in a β-barium borate crystal (1 mm or 200 μm thick for data in Figs. 1 and 2 and Figs. 3 to 5, respectively). The refocused THz radiation from the heterostructure was detected using the EO effect in a noncentrosymmetric crystal (1-mm-thick ZnTe or 258-μm-thick GaP for data in Figs. 1, 2, 4, and 5 and Fig. 3, respectively). The induced birefringence in the EO crystal was recorded at different delay times by a laser probe pulse passing through a polarizing beamsplitter (Wollaston prism) and impinging on a balanced photodetector. The power imbalance (ΔI), which is proportional to the THz electric field, was fed into a lock-in amplifier synchronized with modulation of the excitation beam at 320 kHz by an acousto-optic modulator. By scanning the time delay between the excitation and probe pulses, the temporal profile of the transient THz electric field could be mapped with a time resolution down to ~40 fs.

To determine the direction of the underlying current in the heterostructure, we calibrated the polarity of the measured EO signal using the photo-Dember effect in a bulk (InSb) semiconductor crystal: The photogenerated electrons diffused into the bulk faster than the holes due to their higher mobility, generating a net transient current flowing toward the surface (+z in Fig. 1B) (28). We tuned the phase of the lock-in amplifier so that this +z current corresponded to a near–half-cycle profile with a positive maximum (Fig. 1C, inset).

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