A Fabry‐Perot Resonant Nanocavity Film for Mechanistic Probing of Ion‐Specific Electrochromism

Electrochromism (EC) enables energy‐efficient, and non‐volatile optical modulation through reversible ion intercalation, with promising applications in smart windows, color displays, and advanced optical devices. Although numerous cations, such as H + , Li + , Na + , Mg 2+ , Zn 2+ , and Al 3+ , have been intensively examined for enhanced EC performance, a precise, quantitative understanding of how different cation species govern the evolution of optical constants ( n , k ) and the resultant coloration remains a challenge. Herein, we develop a Fabry‐Perot resonant nanocavity film (FPRNF) consisting of a WO 3 layer atop a metallic reflector. The FPRNF transduces subtle changes in optical constants ( n , k ) of WO 3 upon different cation intercalation into distinct spectral and color responses. We establish a precise quantitative relationship between these EC responses and the density of intercalated cation/charge, highlighting that the injected electrons accompanying cation intercalation are the primary determinant of coloration behavior. The intercalation of higher‐valence cations such as Al 3+ promotes substantial charge compensation, with electron injection over 400 mC/cm 2 ·µm. This significantly alters the optical constants ( n , k ), with achieved values of Δ n = 0.43 and Δ k = 0.50, and consequently drives a pronounced resonant wavelength shift of 160 nm and intensity modulation of approximately 40%, leading to greatly enhanced EC performance.