Ultrafast photocurrents in the Weyl semimetal TaAs (Conference Presentation)

Weyl semimetals have been the focus of intense experimental and theoretical investigation, due to their broad appeal in fundamental science and applied technology alike. More recently, several studies have centered on the nonlinear optical properties of these materials, where it is believed that characteristic features of Weyl physics can be observed. To date, these studies have been limited to static or quasi-static measurements, but new and important insights can come about through extending these nonlinear optical spectroscopies into the time domain. To do so, we use terahertz (THz) emission spectroscopy and time-resolved second harmonic generation (TR-SHG) spectroscopy to provide a contact free measure of ultrafast photocurrent dynamics in the transition metal monopnictide family of type-I Weyl semimetals. On the basis of our data, we are able to clearly distinguish between helicity-dependent photocurrents generated within the ab-plane and polarization-independent photocurrents flowing along the non-centrosymmetric c-axis. Such findings are consistent with earlier static photocurrent experiments, and demonstrate 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. Such generality in the microscopic origin of photocurrent generation in the transition metal monopnictide family of Weyl semimetals makes these materials promising candidates as next generation sources or detectors in the mid-IR and THz frequency ranges.