Reversible Active Switching of Fano and Fabry–Pérot Resonances by Electrochromic Operation

Abstract Resonances are ubiquitous in modern photonics, with the more familiar Fano and Fabry–Pérot 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 (WO 3 )‐based reconfigurable photonic structure with reversible switching between the Fano and Fabry–Pérot (F–P) resonances. This remarkable resonance switching occurs as a result of a change in the WO 3 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.