Tracking Ultrafast Photocurrents in the Weyl Semimetal TaAs Using THz Emission Spectroscopy

We investigate polarization-dependent ultrafast photocurrents in the Weyl semimetal TaAs using terahertz (THz) emission spectroscopy. Our results reveal that highly directional, transient photocurrents are generated along the noncentrosymmetric $c$ axis regardless of incident light polarization, while helicity-dependent photocurrents are excited within the $ab$ plane. This is consistent with earlier static photocurrent experiments, and demonstrates on the basis of both the physical constraints imposed by symmetry and the temporal dynamics intrinsic to current generation and decay that optically induced photocurrents in TaAs are inherent to the underlying crystal symmetry of the transition metal monopnictide family of Weyl semimetals.

Figure 1

(a) False color plot and (b) select time-dependent THz traces, illustrating the polarity reversal of the emitted THz waveform upon changing the helicity of the optical generation pulse. Traces shown in (b) are obtained using quarter wave plate (QWP) angles of $\pm 45°$, $\pm 22.5°$, and 0°, which correspond to right or left circular, elliptical, or linear polarizations, respectively. (c) Peak-to-peak $E$-field amplitude plotted as a function of $\alpha$ and fit with Eq. (1). The inset illustrates the relative weight of each fitting component.

Figure 2

THz emission spectra measured along the $[11\overline{1}]$ axis generated by right circular, linear, and left circularly polarized optical pulses (traces are offset for clarity). The inset shows the peak-to-peak $E$-field amplitude plotted as a function of $\alpha$.

Figure 3

THz emission spectra measured along (a) $[11\overline{1}]$ and (b) $[1\overline{1}0]$, generated from linearly or right circularly polarized light, respectively. Spectra denoted by dashed lines were obtained following a 180° azimuthal rotation of the TaAs crystal about the (112) normal. The inset in (b) illustrates the helicity-dependent THz waveform emitted along the $[1\overline{1}0]$ direction from the (001) surface at normal incidence.

Figure 4

Intensity power spectra obtained by Fourier transforming (a) time-domain THz waveforms measured along $[11\overline{1}]$ using free space EOS (inset) and (b) helicity-and-time-dependent THz pulses measured with TFISH (inset) following appropriate subtraction of the time-resolved SHG dynamics along the $[1\overline{1}0]$ axis.