Electrically Tunable Epsilon-Near-Zero Properties and Anisotropy in Conjugated Polymers

Epsilon near zero (ENZ) materials have been extensively studied in recent years due to their unique optical properties arising from extreme light matter interaction within them [1-2]. Controllable ENZ properties realized in polymer materials drawn particular interest because of their advantages such as dynamic tunability, ease of fabrication, and naturally hyperbolic dispersion [3]. The oxidation states of the conducting polymer can also be tuned between the metallic and dielectric states, depending on the applied voltage [4]. This, combined with its polymeric flexibility, makes it a promising material for metasurface applications. In this work, we demonstrate that high electrical conductivity, anisotropy, and a tunable ENZ wavelength in the visible regime can be achieved in conducting polymer PEDOT:PSS (poly(3,4-ethylenedioxythiophene) polystyrene sulfonate) by varying the sulfuric acid treatment recipe (e.g. controllable ENZ wavelength from 700 nm to 1600 nm). Furthermore, the ENZ wavelength in visible regime can be controlled in electrochemical devices made of PEDOT:PSS. This electrical tunability of ENZ wavelength and anisotropy in the visible spectrum is unique for ENZ materials and enables gate control of ENZ effects and the topological state of iso-frequency surface. We prepare our samples by spin coating of PEDOT:PSS solution. The conductivity and degree of anisotropy is controlled by adding DMSO (dimethyl sulfoxide) into the solution and using post-deposition acid treatment. We observed that the thin films exhibit uniaxially anisotropy with its in-plane direction having ENZ properties, while the out-of-plane direction remains dielectric after the acid treatment. The ellipsometry characterization of the material can be perfectly describe using an anisotropic Drude-Lorentz model. Electrochemical devices are assembled with solid polymer electrolyte and transparent conducting oxide as electrodes. The reflection spectrum of the device in the visible range exhibits a 60% change in reflectivity upon application of a voltage. This change is due to the gate control of the polymer’s redox state, making it either dielectric or metallic. The optical behavior of the device was simulated using commercial software, highlighting the potential for designing hyperbolic metasurfaces for various applications and controlling the topological state of iso-frequency surfaces using external fields.<br/><br/>1.Liberal, I.; Engheta, N. Nature Photonics, 11(3), 149 (2017).<br/>2.Y. Yang et al., High-harmonic generation from an epsilon-near-zero material. Nature Physics <b>15</b>, 1022-1026 (2019).<br/>3.Chen, S., Kang, E.S.H., Shiran Chaharsoughi, M. et al. Conductive polymer nanoantennas for dynamic organic plasmonics. Nat. Nanotechnol. 15, 35–40 (2020).<br/>4.Julian Karst et al. Electrically switchable metallic polymer nanoantennas.Science 374,612-616(2021).