(a) Experimental setup. OPM, off-axis parabolic mirror; WP∕Babinet, quarter-wave plate or Babinet compensator; EO, electro-optic. The pump pulses are initially polarized, while the terahertz polarizer transmits -polarized terahertz radiation. (b) Coordinate system of the sample, which can be rotated around the axis. The pump pulses are normally incident, i.e., propagate in the direction.
(a) Terahertz signal from currents generated along the and axes if the QW sample is excited with light being linearly polarized along the and axes, respectively. (b) Terahertz signals of shift currents generated in the bulk GaAs sample (solid) and in the GaAs QW sample (dashed). The terahertz amplitudes differ by less than 5%, being identical if one considers the experimental uncertainty.
(a) Terahertz traces arising from currents injected along the direction in the QW sample if the sample is excited with light (solid), light (dashed), and light being linearly polarized along the  direction (dotted). (b) Terahertz peak-to-peak signal from injection currents directed along the direction vs phase difference between the two orthogonal polarization components defined by the Babinet compensator. (c) Terahertz peak-to-peak signal from currents generated along the direction in the QW sample vs rotation of the quarter-wave plate. At 0° and 180° the axis of the wave plate is parallel to the linear input polarization of the pump beam.
Normalized terahertz traces from shift (solid) and injection (dashed) currents. The shapes of both traces are identical, if one considers the experimental uncertainty.
Terahertz peak-to-peak signal vs excitation intensity for injection (filled triangles) and shift (open squares) currents in the QW sample. The solid lines represent the intensity dependence of shift currents in bulk GaAs.
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