All-optical polarization control in time-varying low-index films via plasma symmetry breaking

Controlling the polarization state of light with sub-picosecond speed and sub-wavelength precision remains a key challenge for next-generation nanophotonic devices. Conventional methods, such as birefringent crystals, liquid crystals or electro-optic Pockels cells are limited in speed, compactness and energy consumption. While structured materials and two-dimensional heterostructures show some promise for on-chip ultrafast performance, all-optical control at the nanoscale remains an open issue. Here we introduce an all-optical scheme that uses femtosecond pumping of low-index, sub-wavelength isotropic films to achieve ultrafast control over birefringence, dichroism and optical activity within a single-material platform. When the material is probed at its crossover wavelength, linearly polarized pumping induces a transient phase retardation between opportune orthogonal components as large as 0.1π μm^-1, accompanied by a dichroic absorption ratio of ~1.2. When, instead, circularly polarized excitation is employed, the probe experiences non-reciprocal optical activity, leading to polarization rotation reaching 1.1° μm^-1. These transient values are orders of magnitude larger than what is recorded from alternative nanophotonic systems and can be quantitively reproduced by a specialized model, which highlights the critical role of time-varying damping in photoexcited carrier plasma. Our combined experimental and theoretical study establishes a reconfigurable, deep-sub-wavelength polarization-control mechanism operating on sub-picosecond timescales. This approach is ideally suited for compact ultrafast modulators, dynamic metasurfaces and tunable non-reciprocal photonic devices, with broad implications for quantum optics, ultrafast logic and time-resolved sensing.