Extended Data Fig. 1: Linearly polarized THz emission from directional photocurrent.
a, Measured (solid line) and approximated (solid fill) THz time-domain waveforms for a uniformly oriented metasurface under circularly polarized femtosecond laser excitation, assuming a Gaussian photocurrent pulse (dashed grey line; 600 fs full width at half maximum) with ETHz ∝ −dj/dt. The true current and THz field is expected to be faster than shown here, owing to bandwidth limitations of the photoconductive antenna detector (see Supplementary Note 1). Inset, spatially mapped THz field showing the beam with approximately uniform linear polarization. A small y-polarized contribution and corresponding roughly 10° angular deviation from x axis may be attributed in part to residual sample tilt and imperfect alignment of the THz imaging system. b, The x (red), y (blue) and total (purple) THz field amplitudes measured (solid lines with data markers) as a function of sample orientation under circularly polarized incident optical pulses, compared with the expected cos(θ) (red fill), sin(θ) (blue fill) and constant (purple fill) behaviours, respectively. The skew in the y dataset is attributed to imperfect circular polarization and residual misalignment in the parabolic mirrors.