Scalable, Multi-Functional Covert Polarization Display with Colorimetric Detection

Abstract

The history of coloration has been proceeded by trials and development of many technologies for selectively splitting the light. Structural coloration, among them, has grown up as a potential candidate alternating traditional pigments/dyes coloration since their great durability, sustainable production, and fine color tuning by controlling materials and dimension. As this property, structural color facilitates the creation of security features for anti-counterfeiting. However, conventionally, most of them have used metal/dielectric materials to make a resonating system at a specific wavelength and polarization condition which demands complex fabrication processes such as e-beam lithography or ion beam lithography technology. Even though this system can be suitable to an ultra-high definition structural color display, it is limited to only a few millimeter scales or less. To apply into a realistic application such as banknote, luxury products, or customer goods, it is necessary to make it over centimeter scale with a flexible medium. In this work, to break through these significant flexibility/scalability limitations, we developed polarization distinguishable covert display with ultrathin porous nanocolumns (PNCs) on a metal film by glancing angle deposition (GLAD) method. Particularly, the unique property of this covert display showed powerful functions which provide a hurdle to the inadvertent viewing of stored optical information without compromising their aesthetic. This structure with lossy PNCs on a metal film makes a non-trivial phase shift at the interfaces causing short reflection path and finally shows strong resonance [1]. Furthermore, the PNCs fabricated with GLAD produce anisotropic media and has different effective complex refractive index causing different resonating condition depending on polarization direction [2]. Using this concept, we designed polarization distinguishable color filters with a variety of materials and dimensions with rigorous coupling wave analysis (RCWA) method. In this process, we obtained an enlarged color range occupying over 80% of standard RGB area. In this color response, some showed a small color difference (?E), the other showed a large color difference following polarization angle. Additionally, we confirmed this result is originated from human eyeís spectral responses according to between tristimulus value and reflectance spectra having a single minimum. Using these results, we designed optical data label (i.e., QR code) which shows inadvertent viewing in the naked eye (under unpolarized light), on the other hand, this label can be selectively detected by polarized light. For practical application, we designed this label into daily consumer goods with the color matching between labels and products to avoid compromising aesthetics of products and successfully demonstrated onto wine bottleneck, food packaging with confirming the authentication process. As a multi-functional color display, we demonstrated a colorimetric detection. Due to the low complex refractive index of porous medium, PNCs are sensitive to subtle external environment change. Consequently, PNCs reveal a sensitive color change according to background refractive index or additional layer thickness change. Using this unique property, we presented colorimetric detection with external change for volatile organic compounds (VOCs) and humidity.