Seeing Ultrafast Modal Dynamics of Non‐Separable Mixed States via Nonlinear Spatiotemporal Stokes Tomography

ABSTRACT Characterization of photonic states is critical in modern optics for generating and using light fields with specific frequency, spatial, and polarization modes. Traditional state tomography cannot resolve ultrafast modal evolution due to the relatively slow response of photoelectric detection, leading to information loss and the observation of mixed states. This study presents a novel nonlinear spatiotemporal Stokes tomography based on pump‐probe upconversion imaging polarimetry, which enables capture of instantaneous pure states of time‐varying structured light on a femtosecond timescale. By temporally accumulating these instantaneous pure states, we accurately reconstruct the mixed states observed in long‐shutter tomography, thereby clarifying the physical origin of such mixed states. This ultrafast light–matter interaction provides an interface to probe and exploit classical and quantum structured light with rapid modal evolution.