All-optical polarization control in time-varying low-index films via plasma symmetry breaking
Authors
Wallace Jaffray
Sven Stengel
Alexandra Boltasseva
Vladimir M. Shalaev
Maria Antonietta Vincenti
Domenico De Ceglia
Michael Scalora
Carlo Rizza
Marcello Ferrera
Abstract
Controlling the polarization state of light with sub-picosecond speed and subwavelength 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 efficiency. While structured materials and two-dimensional heterostructures offer potential for on-chip ultrafast performance, achieving all-optical control remains an open problem. Here we introduce an all-optical scheme that employs 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 near its crossover wavelength, linearly polarized pumping induces a transient phase retardation up to 0.1π per micrometer, accompanied by a dichroic absorption ratio of approximately twenty. Under circularly polarized excitation, the probe experiences nonreciprocal optical activity, leading to polarization rotation of about 1.1 degrees per micrometer. A universal hydrodynamic model quantitatively reproduces these effects and attributes them to pump-induced symmetry breaking in the photoexcited carrier plasma. This symmetry breaking enables coupling between orthogonal probe polarization components, mediated by a modified time-dependent damping term, which connects to the inverse Faraday effect induced by a circularly polarized pump. Our combined experimental and theoretical study establishes a reconfigurable, deep-subwavelength polarization-control mechanism operating on sub-picosecond timescales, suitable for compact ultrafast modulators, dynamic metasurfaces, and tunable nonreciprocal photonic devices, with implications for quantum optics, ultrafast logic, and time-resolved sensing.
