Nonintrusively Adjusting Structural Colors of Sealed Two-Dimensional Photonic Crystals: Immediate Transformation between Transparency and Intense Iridescence and Their Applications.

Responsive photonic crystals (PCs), which can adjust structural colors in response to external stimuli, show great potential applications in displays, sensors, wearable electronics, encryption, and anticounterfeiting. In contrast, conventional structure-intrusive adjustment manners that external stimuli directly interact with the ordered arrays may lead to structural damage or longer response time. Here, a noninvasive adjustment of the structural colors of two-dimensional (2D) PCs (2D-PCs) is explored based upon diffraction theory. Sealed 2D-PCs and 2D inverse opal photonic crystal (IOPC) flexible devices are prepared. They are highly transparent in air but immediately exhibit intense viewing angle-dependent structural colors after being dipped in water. The mechanism of transparent-iridescent immediate transformation is explained by Bragg's law. The design mechanism is examined by numerical simulation and spectral shifts in different external media. We demonstrate its applications in the fields of information encryption and anticounterfeiting by using the transparent-iridescent immediate transformation of sealed 2D-PC patterns and 2D IOPC free-standing films sealed on the product surface. Because of the strong contrast between transparency and intense iridescence, reversible and immediate transformation, and durability, sealed 2D-PCs and 2D IOPC flexible devices designed by the noninvasive adjustment strategy will lead to a variety of new applications in displays, sensors, wearable electronics, encryption, and anticounterfeiting.