Unique Color Display via High‐Order Reflectance in 1D Photonic Crystals for Advanced Security Levels in Anti‐Counterfeiting Applications

Abstract Herein, humidity‐responsive 1D photonic crystal (PC) films are developed that are capable of generating higher‐order photonic stop bands (PSBs). As a critical factor for achieving 2 ^nd ‐ and 3 ^rd ‐order PSBs, the layer thickness is modulated to achieve multi‐wavelength reflectivity within a single film. The films are fabricated using alternating layer deposition of photo‐crosslinkable poly(2‐vinylnaphthalene‐ co ‐benzophenone acrylate) (p(2VN‐ co ‐BPA)) and 52%‐quaternized poly(4‐vinylpyridine‐ co ‐benzophenone acrylate) (P4QP‐52%). Upon increasing the relative humidity (RH), the films exhibit 1 ^st ‐, 2 ^nd ‐, and 3 ^rd ‐order PSBs with visible color transitions across the spectrum. This behavior becomes evident as the thickness of the P4QP‐52% layer increases. Notably, for those films with relatively thick P4QP‐52% layers, the shift in the 2 ^nd ‐ and 3 ^rd ‐order PSBs is substantial enough to create composite colors, such as magenta, through the superposition of red from the 2 ^nd ‐order PSB and blue from the 3 ^rd ‐order PSB. This mechanism represents a significant advancement in PC technology and opens up new possibilities for colorimetric sensing and tunable optical materials. For practical applications, these films are further explored in anti‐counterfeiting applications, where their color transitions and hidden patterning are demonstrated. The films showed vivid color changes and distinct patterning in response to high humidity, offering potential for use in security and authentication applications.