Physical coloration without chemicals offers a pathway to develop pollution-free coloration technology, and can be applied to colorimetric sensing of gases, toxic and chemical agents. In this paper, we report on realization of a high-purity red color using mechanism of mode-selective absorption in a thin-film optical cavity. By placing an ultra-thin absorber layer at the antinode of a targeted spectral band in a Fabry-Perot cavity, its otherwise conventional dichroic reflection spectrum is shaped into a broad rectangular flat-bottom one that gives the desired vivid red. The purity of our demonstrated red color reaches 76%, which is increased by 16% compared with those reported in prior thin-film structures. Our method of mode-selective absorption is adaptable to more general-purpose spectral shaping, and could be applied in producing other target colors as well as broadband light absorption for energy harvesting and infrared detection.
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