Dynamic Switching of Optical Contrast in Structural Colors Based on Rayleigh Anomalies by Li-Ion Insertion

To demonstrate dynamic on/off switching of structural colors, Li ion intercalation of WO₃ in Fabry-Perot cavities and gap plasmon resonators has been explored by many groups. Various structural colors can be prepared by controlling the arrangement or dimension of such resonators while the input and output of Li ions into the resonators can change the colors from dark to bright. One of the challenges has been achieving near 100% optical switching contrast, which can be achieved with a completely black off state. In this study, we prepare vibrant structural color pixels composed of WO₃ nano gratings and demonstrate various colors based on the Rayleigh anomaly (RA) under a cross-polarized excitation and detection configuration. From RCWA calculations, high switching contrast (>80%) between the intercalated and deintercalated states was confirmed at specific grating heights. To experimentally demonstrate these findings, we fabricated an electrochemical cell with a WO₃ grating on a metal working electrode and ITO counter electrode sandwiching an electrolyte including Li ions dissolved in propylene carbonate. The grating was oriented 45 degrees with respect to the input polarized and the refection was viewed with a separate orthogonal polarizer. With the appropriate grating parameters, this configuration enables one to view colors from the RA. With an applied cathodic bias, the Li_xWO₃ gratings appear black. The original colors from different gratings returned with an applied anodic bias. Based on these calculated and measured results, we suggest the use of this structural color pixel in high contrast dynamic reflective displays.