High-purity matched reflective and transmissive structural colors enabled by weakly coupled dual cavities

초록

Most optical coatings intrinsically produce contrasting transmission and reflection colors due to conventional resonance behavior. Here, we demonstrate weakly coupled dual cavities that generate identical high-purity colors in both reflection and transmission through engineered Fano resonances. The structure consists of an upper cavity that forms a broadband continuum state via absorption in a Ge layer and a lower cavity that supports a discrete Fabry–Perot resonance. Weak coupling between these states produces asymmetric Fano line shapes, enabling both spectra to peak at nearly the same wavelength. To systematically determine optimal structural parameters, an inverse design strategy based on the L-BFGS-B optimization algorithm is employed to tune layer thicknesses for maximizing the RGB color gamut while enforcing simultaneous color matching between reflection and transmission. The optimization reveals that the Ag thickness governs the coupling strength, whereas the Ge thickness controls the continuum background. Under optimized conditions, the device exhibits nearly coincident transmission and reflection colors in the CIE 1931 chromaticity diagram, closely matching the sRGB standard. The physical origin of this behavior is confirmed through resonance phase analysis, electric field distributions, and Fano line-shape fitting of experimental spectra, verifying that the asymmetric response arises from continuum–discrete interference. This strategy provides a robust platform for achieving vivid, stable, and color-accurate thin-film filters while demonstrating precise control of Fano resonances in multilayer structures with strong potential for dual-mode displays, advanced optical coatings, and multifunctional photonic applications.

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