Fig. 5: Pump-induced transient optical activity and overall polarization transformations on Poincaré sphere.

a, Polarization rotation angle for an LCP pump, which is extracted from the experimental data (green circles) and from the theoretical fit (black dashed line). The theoretical evaluation is attained by using the formula reported in the legend, where the angled brackets indicate a time average integral over absolute time t. b, Polarization rotation angle for an RCP pump, which is extracted from the experimental data (green circles) and from the theoretical fit (black dashed line). As expected, the RCP-pumped nonlinearity has the opposite effect to the LCP-pumped case. The absolute rotation reaches a maximum value of approximately 1.1° μm⁻¹ in both cases. c, Diagram of our nonlinear ultrafast polarization transformations on the Poincaré sphere. In the top left corner the inset shows the full Poincaré sphere to provide context and eliminate ambiguity on orientation of the sphere itself. The main image provides a zoom of our time-varying polarization transformations. Starting from a linear polarization (black bi-directional arrow), we show that a linearly polarized pump imparts both ellipticity and dichroism (see red ellipse icon). Meanwhile the left- and right-handed circular pumps rotate the linear probe polarization along the equator in opposing directions. Polarization states are reported up to scale, with major diagonal length indicating associated transmission. The dotted black line pertains the temporal dynamics of our polarization transformation under linear pumping.