Concealed Structural Colors Uncovered By Light Scattering

Unusual structural colors are demonstrated in thin-film coatings due to a combination of optical interference and light scattering effects. These vivid colors are concealed under ambient illumination but can be observed when light is reflected from the film surface. The origin of the effect is explored computationally and it is shown that, in thin-films of lossless dielectrics coated on near-perfect conductors, incident electromagnetic waves form standing waves. Electric field intensities at the thin film interfaces are maximized for wavelengths that fulfil destructive interference conditions, while nanoscale roughness can enhance scattering at these boundaries. The interplay of these two factors yields vivid, thickness-dependent colors. This approach increases the repertoire of optical effects and perceived colors in thin coatings. When combined with traditional thin-film interference colours, dichromatic images with distinctly changing colors can be generated, which can function as a covert, optical security feature.