Disappearance of the Photonic Stop Band in 1D Photonic Crystal Films through Humidity-Induced Refractive Index Matching for Anti-Counterfeiting Applications
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초록
내용
In this study, we fabricate one-dimensional photonic crystal (PC) films that exhibit a humidity-induced disappearance of the photonic stop band (PSB). The films consist of photo-crosslinkable high- and low-refractive-index polymers. The sensitivity to humidity is adjusted by varying the degree of quaternization of the high-refractive-index polymer. The PC films display vivid colors at low humidity; however, as the humidity increases, the PSB undergoes a red shift accompanied by a decrease in reflectivity. This phenomenon occurs when water molecules (n=1.33) penetrate the high-refractive-index layers, resulting in a decrease in the refractive index and an increase in layer thickness. Consequently, the contrast between the refractive indices of the high- and low-refractive-index layers is diminished, causing the disappearance of the PSB and the reflection color. This phenomenon is utilized in the application of anti-counterfeiting films.