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Abstract

Ultrafast optical- or magneto-optical Kerr responses exhibiting THz frequency oscillations are commonly assumed to trace coherent low-energy quasi-particles, such as phonons or magnons. Here, we decode the complex nonlinear polarization response of lead halide perovskites (LHPs) by developing two-dimensional optical Kerr spectroscopy (2D-OKE). In contrast to the quasi-particle interpretation, we unveil a unified origin for fully inorganic and hybrid LHPs, governed by anisotropic and highly-dispersive light propagation in the vicinity of the optical bandgap. Finally, to directly trace the ultrafast lattice or molecular polarization, we extend this investigation to the THz Kerr effect.