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

Controlling the polarisation state of light with sub-picosecond speed and subwavelength precision remains a key challenge for next-generation nano-photonic 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 2D 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, subwavelength 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 polarised pumping induces a transient phase retardation as large as 0.1pi/um, accompanied by a dichroic absorption ratio of about 1.2. When instead circularly polarised excitation is employed, the probe experiences non-reciprocal optical activity, leading to polarisation rotation reaching 1.1 deg/um. 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 photo-excited carrier plasma. Our combined experimental and theoretical study establishes a reconfigurable, deep-subwavelength polarisation-control mechanism operating on sub-picosecond timescales. This approach is ideally suited for compact ultrafast modulators, dynamic metasurfaces, and tunable nonreciprocal photonic devices, with broad implications for quantum optics, ultrafast logic, and time-resolved sensing