Realization of Structural Colors via Capped Cu-based F-P Cavity Structure

Two-dimensional transmissive structural colors for high-security information encryption

Structural colors produced from nanostructures have attracted much attention due to their promising advantages of long-term stability and high resolution. Many nanostructures like metasurfaces have been demonstrated to generate color information in the transmission or reflection mode. Here, a strategy of combining polarization-insensitive and polarization-sensitive transmissive structural color is proposed to realize convenient and diverse encrypted pattern designs. A two-dimensional metasurface, whose polarization characteristics are determined by the size of a nanobrick unit, is embedded inside an optical cavity to produce transmissive structural color. The polarization-insensitive transmissive structural color exhibits a wide color gamut and high excitation purity in all polarization states, while the polarization-sensitive transmissive structural color maintains the similar color appearance in x-direction polarization but appears nearly black in y-direction polarization. Combining these two transmissive structural colors can achieve diverse images designed at different polarizations instead of simply hiding the image in a specific polarization state. An image of "flower and flowerpot" using the generated colors is visually illustrated, which shows that the proposed transmissive structural colors would have great potential in the areas of security information encryption.

High-purity and wide-angle reflective structural colors based on an all-dielectric Fabry-Pérot cavity structure

High-purity structural colors with low fabrication cost are in demand for their commercial applications. Here, we demonstrate an all-dielectric Fabry-Pérot cavity structure consisting of four-layer lossy and lossless dielectric films alternately stacked for producing high-purity and angle-invariant reflective colors. Multiple cavity resonances function together to significantly suppress the undesired reflection with the enhanced optical absorption, leading to a distinct and saturated color with a high efficiency of ∼70%. Besides, due to the high refractive indices of constituent materials, the color appearance of the designed structure can be maintained well at ±50° incident angle for two polarization states. The excellent color performance of the proposed device together with cost-effective manufacturing convenience opens up new avenues for their large-area applications in various areas.

Structural Colors on Al Surface via Capped Cu-Si3N4 Bilayer Structure

Tunable structural colors have a multitude of applications in the beautification of mobile devices, in the decoration of artwork, and in the creation of color filters. In this paper, we describe a Metal-Insulator-Metal (MIM) design that can be used to systematically tune structural colors by altering the thickness of the top metal and intermediate insulator. Cu and Si₃N₄ were selected as the top metal and intermediate insulator layers, respectively, and various reflection colors were printed on Al. To protect the Cu surface from scratchiness and oxidation, a number of capping layers, including SiO₂, LPSQ, PMMA, and the commercially available clear coat ProtectaClear, were applied. In addition to their ability to protect Cu from a humid environment without deteriorating color quality, ProtectaClear and LPSQ coatings have minimal angle dependency. Furthermore, a bilayer of PMMA/SiO₂ can protect the Cu surface from the effects of humidity. In addition, the PMMA/SiO₂ and ProtectaClear/SiO₂ bilayers can also protect against corrosion on the Cu surface. The colors can be tuned by controlling the thickness of either the metal layer or intermediate insulator layer, and vivid structural colors including brown, dark orange, blue, violet, magenta, cyan, green-yellow, and yellow colors can be printed. The measured dielectric functions of Cu thin films do not provide any evidence of the plasmonic effect, and therefore, it is expected that the obtained colors are attributed to thin-film interference.

All-dielectric high saturation structural colors enhanced by multipolar modulated metasurfaces

A visible light depth modulation based on a metasurface consisting of TiO₂ nanorings and SiO₂ substrate is proposed to significantly enhance the saturation and structural colors' gamut. Compared with the nanostructure of the TiO₂ nanodisks, the developed TiO₂ nanorings can enhance monochromatic excitation by inhibiting the multipole mode, particularly electric quadrupole (EQ) mode at a shorter wavelength. Furthermore, when TiO₂ nanorings are combined with a refractive index matching layer - water, reflection bandwidth, and background reflection are reduced, and the brightness and color purity are significantly enhanced. The novel and unique nanostructures developed can generate a significant gamut of 140% sRGB and 103% Adobe RGB. Additionally, the color structure based on the TiO₂ nanoring metasurface is sensitive to the surrounding medium's refractive index and can be employed in sensor display and other fields, as well as to amplify color information in high resolution display and imaging applications.