Reversible Active Switching of Fano and Fabry-Perot Resonances by Electrochromic Operation

Resonances are ubiquitous in modern photonics, with the more familiar Fano and Fabry-Perot resonators as key components of sophisticated optical devices with unique properties. However, the fundamental drawback of these devices is the difficulty in altering the resonance-related features of the underlying optical structures postfabrication. This study investigates an active electrochromic tungsten oxide (WO3)-based reconfigurable photonic structure with reversible switching between the Fano and Fabry-Perot (F-P) resonances. This remarkable resonance switching occurs as a result of a change in the WO3 film optical indices (n, k) via Li+ intercalation/deintercalation, which can be inferred from the spectral response or dynamic reflected colors. When the bottom Ag layer's thickness is decreased from 130 to 10 nm, a semitransparent structure with unique optical properties emerges. The F-P resonant structure reflects and transmits different colors before Li+ intercalation, while the Fano resonant structure reflects and transmits the same color after Li+ intercalation. Along with these unique optical properties, a trans-reflective filter and beam splitter filter are also developed based on the reversible electrochromism of a fixed optical configuration.