Ultrafast second order response in symmetry breaking process

Second-order nonlinear responses are highly sensitive to the symmetry of materials. In static regimes, the second-order susceptibility vanishes in the presence of spatial inversion symmetry, however, the dynamics of the second-order response is unexplored during symmetry evolution. Here, we employ ultrafast spectroscopy to investigate the dynamical second-order response by inducing a transient symmetry breaking through hot electron generation and diffusion in bilayer transition metal dichalcogenides (TMDs). The resulting coherent hyper-Raman scattering signal provides insights into electron dynamics. Both the population and coherence dynamics are characterized using Second Harmonic Generation (SHG) and hyper-Raman components.