Prof. Liang Cheng^{} University of Electronic Science and Technology of China, Chengdu, China

Due to their distinctive electronic structure and symmetry, topological materials (TMs) show remarkable electronic and optical responses, making them exceptionally well-suited for applications in spintronics and nonlinear optics. These materials typically exhibit substantial nonlinear coefficients, evident in second harmonic generation (SHG) and terahertz (THz) emission measurements. [1, 2] In centrosymmetric non-magnetic topological materials like Dirac semimetals, the second-order nonlinear coefficient is usually zero. [3] In our study, we investigated the SHG signal of 800 nm pulses from a type-II Dirac semimetal, NiTe_{2}, under intense THz irradiation (See Figure 1). Our findings revealed that the SHG response could be modulated by the external THz electric field. Notably, the SHG signal is sensitive to the polarity of 800 nm light and the sample's azimuthal angle. Symmetry analysis of the data indicated that our observations arise from the modulation of second order nonlinear response by external electric field, and the possible mechanism is the symmetry breaking by the THz-induced current. Furthermore, we also observe the THz-driven circular dichroism of the SHG signal, suggesting a potential Dirac to Weyl phase transition. This work underscores the feasibility of ultrafast manipulation of the topological properties of solids and opens avenues for developing a nonlinear switch operating at terahertz frequencies.