Humidity‐Induced Decolorization of 1D Photonic Multilayers for Anti‐Counterfeiting Applications

Herein, a new concept is proposed for the humidity‐induced disappearance of the photonic stop band (PSB) in multilayer films of 1D photonic crystals (PCs). These films are fabricated via alternating spin‐coating of photo‐cross‐linkable high and low‐refractive‐index polymers onto a black polyethylene terephthalate (PET) substrate, followed by surface immobilization using UV light irradiation. The sensitivity to humidity is adjusted by varying the degree of quaternization (DQ) of the P(4VP‐co‐BPA) layers. The resulting films initially display vivid colors; however, as the humidity gradually increases, the PSB undergoes a red shift accompanied by a decrease in reflectivity, thus leading to the disappearance of the color. The reduction in reflectivity of the PSB can be attributed to the infiltration of water molecules (n = 1.33) into the quaternized high‐refractive‐index layers, thus resulting in a decrease in the refractive index and an increase in layer thickness. This leads to a diminished contrast between the refractive indices of the high‐ and low‐refractive‐index layers, ultimately causing the simultaneous disappearance of the PSB and the reflection color. Additionally, two patterned films are fabricated with high humidity responsiveness for application as anti‐counterfeiting labels. These findings advance photonic crystal research and offer innovative solutions for security and authentication. This study investigates the humidity‐induced disappearance of the photonic stop band (PSB) in 1D photonic crystal (PC) multilayers. The reduction in PSB reflectivity is attributed to humidity which diminishes the refractive index contrast between layers, leading to the simultaneous disappearance of the PSB and reflection color. These findings advance understanding of PCs behavior and suggest applications in anti‐counterfeiting technologies.