Dynamic Tunability and Anisotropy in PEDOT Films with Epsilon-near-Zero Response in the Visible Spectrum

Epsilon-near-zero (ENZ) materials have gained significant attention due to their unique optical properties arising from extreme light-matter interactions. Among these, polymer-based ENZ materials stand out for their dynamic tunability, easy fabrication, and natural hyperbolic dispersion. The conducting polymer's oxidation state can be switched between metallic and dielectric by applying voltage, making it ideal for tunable optical metasurfaces. Here, we show that PEDOT:PSS (poly(3,4-ethylenedioxythiophene) polystyrene sulfonate) achieves high conductivity, anisotropy, and a tunable ENZ wavelength (700–1600 nm). Moreover, in electrochemical devices, the ENZ wavelength in the visible range can be voltage-controlled, offering unique electrical tunability. Our PEDOT:PSS thin films, spin-coated and treated with DMSO and sulfuric acid, display uniaxial anisotropy: in-plane ENZ properties and out-of-plane dielectric behavior. Ellipsometry characterization fits well with an anisotropic Drude-Lorentz model. Devices with solid polymer electrolytes and transparent conducting electrodes show a 60% reflectivity change under voltage, driven by the polymer's redox state. Simulations demonstrate the potential for hyperbolic metasurfaces and tunable topological iso-frequency surfaces.