Coherent Absorption Reveals Colors Hidden in a Grey Fabry–Pérot Cavity

ABSTRACT Thin dielectric films are known to show distinct colors, depending primarily on the film thickness. In the present article we show that a specialized thin film Fabry–Pérot resonator, that we name conductor–dielectric–conductor (CDC) matched cavity, appears grey when observed under ordinary conditions (i.e., by analyzing the transmitted or reflected incoherent white light). Nonetheless, the matched CDC cavity still retains spectral information, that clearly appear when the cavity is analyzed by the coherent absorption technique. The CDC system is simply a dielectric thin film sandwiched by two conducting interfaces, whose conductivity shall be appropriately matched depending upon the dielectric refractive index. In practical applications, the ideal conducting interfaces can be replaced by thin metal films in a specialized MIM approach, making the system easily applicable in particular for information obfuscation (steganography) or anticounterfeiting purposes. We indeed demonstrated experimentally that a visually recognizable thin‐film color pattern can be concealed to an ordinary observer, and finally recovered through a dedicated coherent absorption decoding apparatus.