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arXiv:2510.13637 · Advanced Optical Materials

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

Giuseppe Emanuele Lio, Giulio Carotta, Lorenzo Lavista, Andrea Camposeo, Giacomo Venturi, Agnese Guernieri, Alessandro Pitanti, S. A. R. Horsley, Giuseppe Carlo La Rocca, Alessandro Tredicucci, Simone Zanotto

Published Oct 5, 2026• 0 citations

Abstract

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.

DOI: 10.1002/adom.71876

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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.

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First reports of absorptive all-optical bistabilities in reflectance and luminescence of a Fabry-Perot cavity consisting of thin (0.70 ,cem) ZnSe film are presented. We demonstrate that the observation of bistability in the reflected beam does not require necessarily the appearance of bistability in the transmitted beam. Luminescence bistabilities, however, are observed both in the light captured Lrom the incident side 0L the film and in the light emitted from the transmission side. The bistabilities in reflection and luminescence are explained quite satisfactorily by the principle of increasing absorption due to photo-irradiation and the dependence of the luminescence intensity on a photo-thermally induced temperature change, respectively. A11 experiments were carried out at room temperature with the 488 nm line of an argon laser.

The electrodynamic and nonlinear optical properties of an interface between two media, one of which is nonlinear, have been studied in the context of optical switching and bistability. In this paper we consider the case in which a thin-film nonlinear material is sandwiched between two higher-index dielectric media; i.e., there are two interfaces. For an optical beam incident near the total internal re- flection state the large reflectivity of the two inter- faces gives rise to Fabry-Perot transmission/re- flection. This together with the nonlinear phase shift that can be induced in the thin film makes it possible for the system to perform switching and bistable operations.

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