Giant Helicity-Dependent Second Harmonic Generation in Type-2 Weyl Semimetal

Second harmonic generation (SHG) has emerged as a powerful probe of fundamental geometric and topological attributes of the band structure of the Weyl semimetals (WSMs) through the photocurrent generation (shift, injection, and anomalous currents). Notably, circular photocurrents, such as injection and anomalous currents, are sensitive to the tilt of the Weyl cones, making circularly polarized light-induced SHG a tilt-inversion symmetry-specific probe of the Weyl topology. Here, we investigate the helicity-dependent nonlinear optical response in the type-2 Weyl semimetal WTe_{2} using equilibrium and ultrafast time-resolved SHG (TR-SHG). A large circular dichroism (CD) of 11% is observed in equilibrium SHG, attributed to the tilt of the Weyl cone. TR-SHG dynamics reveals a positive sign with biexponential decay that corresponds to the fast (∼10 ps) and long-lived (∼ns) components, along with an oscillation corresponding to the A_{1} coherent optical phonon mode. Importantly, a giant helicity-dependent modulation (CD∼40%) is observed in the TR-SHG dynamics, corresponding to the asymmetric photocurrent generation in a tilted cone. A quantitative analysis of the helicity-dependent equilibrium and TR-SHG signals using quantum kinetic theory reveals that the injection current dominates the SHG response in an overtilted Weyl cone. Most importantly, control experiments on TaAs (type-1 WSM) show no helicity dependence, confirming that the observed effect in WTe_{2} is intrinsic to the overtilted Weyl cones. Altogether, these findings not only underscore the power of SHG to monitor photoinduced evolution of symmetry and topology in real time, but also lay the groundwork for helicity-controlled, ultrafast optoelectronic applications that harness topological band structures.