Ultrafast Structural Dynamics and Optical Nonlinearity in a Topological Semimetal

Using first-principles simulations of photoexcited Td-WTe₂, we investigate how femtosecond laser excitation drives transient layer shear and stacking reconfiguration between the non-centrosymmetric Td and centrosymmetric 1T* phases. The calculations capture the anharmonic coupling between shear phonons and electronic excitation, revealing strain-dependent pathways consistent with time-resolved diffraction experiments. By combining real-time density-functional dynamics with perturbative nonlinear optics, we evaluate the transient second-harmonic susceptibility during structural evolution. The computed suppression and recovery of the SHG signal track the loss and re-emergence of inversion symmetry, offering quantitative insight into ultrafast optical control of symmetry and topology in 2D semimetals. The results highlight how coupling of structural and optical degrees of freedom enables dynamic tuning of nonlinear response in layered quantum materials.