Large-Area, Active Thin-Film Resonator for Dynamic Coloration Based on Redox Reaction of Polyaniline

Abstract

Optically active materials, which reversibly change their optical properties (i.e., refractive index and extinction coefficient), have attracted attention as an active medium for dynamic responses in photonic structures due to their fine tunability and reversibility. Representatively, optically active materials include inorganic materials (e.g., phase change materials (PCM) and transition metal oxide) and organic materials (e.g., polyaniline (PANI) and poly-(3,4-ethylenedioxythiophene) (PEDOT), and polypyrrole (PPy)) [1]. Since the long-term durability and thermal/chemical stability of PCMs, they have been widely used for reconfigurable photonics. However, PCMs suffer from slow transition time and energy limitations. On the other hand, conducting polymers (CPs) provide advantages such as cost-effectiveness, a simple manufacturing process, and low operating power [2]. In addition, the optical properties of CPs can be electrically controlled, encouraging to be utilized in tunable color display, metasurface, or wavefront modulator. PANI, one of CPs, is interestingly explored in several research fields. PANI has a high current density, low power consumption, and it also realizes reversible optical properties change with a low voltage range (-0.2 V ~ 0.8 V) in the visible region. However, there is a limit to the expression of full color only by a single materials substrate. To overcome the obstacle, optical resonators (e.g., plasmonics, photonic crystal, and metasurface) combined with PANI have been extensively studied [3]. In this study, an ultra-thin film resonator (i.e., Pr-Ge on Au) was adopted to realize full-color tuning through potential and Pr-Ge modulation.As depicted in Figure 1a, the proposed active thin film resonator was fabricated by synthesizing PANI on ITO and then depositing Pr-Ge and Au. The basic mechanism of an active thin film resonator is that the oxidation by potential causes proton and electron loss, and then the molecular structure is converted, resulting in changing refractive index. When the voltage varies from -0.2 V to 0.8 V, oxidation occurs twice in the entire cycle, the PANI sate is replaced by leucoemeraaldine to emeraldine (Figure 1c). Figure 1e is an experiment result, showing the rich color tuning of the active thin film resonator according to the voltage variation. Therefore, our structure allows rich color expression subject to voltage control, and so it can be utilized as a display.