Fig. 3: Ultrafast nonlinear polarization coupling in a time-varying AZO film.

Experimentally measured and theoretically reconstructed polarization-resolved probe transmission through time-varying, 900-nm-thick aluminium zinc oxide (AZO) layer. a, Raw experimental intensity \({I}_{\exp }(\tau ,\theta )\) as a function of analyser angle θ and pump–probe delay τ, with the probe polarized at 45° and the pump at 0°. b, Simulated intensity map based on fitted complex indices, accurately reproducing the measured transmission with root mean square error below a few per cent. c, Real part of the transient refractive index retrieved from fitting the measured intensity, showing anisotropic changes along the x (red) and y (blue) directions. d, Imaginary part of the refractive index as retrieved from fitting, showing transient, polarization-dependent absorption. e, Extracted polarization angle of the transmitted probe from both experimental centroid analysis (green circles) and model (black dashed line), demonstrating excellent agreement. The theoretical evaluation is attained by using the formula reported in the inset, where the angled brackets indicate a time average integral over absolute time t. f, Retrieved ellipticity of the transmitted pulse as a function of time. The plot indicates induced nonlinear birefringence, whereas the inset shows the reconstructed polarization ellipse (at scale) for maximum birefringence, with ellipticity approaching a 1:7 ratio. The case for the unpumped polarization is also reported by the black bi-directional arrow (length is set to scale with the transmitted intensity).