Probing the Ultrafast Nonlinear Response in the Transition Metal Monopnictide Family of Weyl Semimetals

Weyl semimetals have been the focus of intense experimental and theoretical investigation, due to their broad appeal in fundamental science and applied technology alike. 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, many of these studies have been limited to static or quasi-static measurements, but new and important insight can come about through extending nonlinear optical probes 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 photocurrents 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, transverse photocurrents generated within the ab-plane from polarization-independent photocurrents flowing along the noncentrosymmetric c-axis. By using the photocurrent response as a probe of underlying crystal symmetry, we explore the role that polarization-dependent photoexcitation has on the assignment of point group symmetry, including the possibility of transient symmetry breaking. These findings highlight the robust nonlinear response exhibited by this class of materials, making them promising candidates for next generation sources and detectors in the mid-IR and THz frequency ranges.