Electrically Tunable Steganographic Nano-Optical Coatings

Thin film coatings with tunable colors have a broad rangeof applications,from solid-state reflective displays to steganography. Here, we proposea novel approach to chalcogenide phase change material (PCM)-incorporatedsteganographic nano-optical coatings (SNOC) as thin film color reflectorsfor optical steganography. The proposed SNOC design combines a broad-bandand a narrow-band absorber made up of PCMs to achieve tunable opticalFano resonance in the visible wavelength, which is a scalable platformfor accessing the full-color range. We demonstrate that the line widthof the Fano resonance can be dynamically tuned by switching the structuralphase of PCM from amorphous to crystalline, which is crucial for obtaininghigh-purity colors. For steganography applications, the cavity layerof SNOC is divided into an ultralow loss PCM and a high index dielectricmaterial with identical optical thickness. We show that electricallytunable color pixels can be fabricated using the SNOC on a microheaterdevice.