No abstract

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 twodimensional (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.

Experimental demonstration of light propagation with ultralow group velocity, i.e., slow light, allows for revolutionary solutions for time-domain processing and buffering of optical signals. It can spatially compress optical energy, which lessens the device footprint and enhances linear and nonlinear optical effects. Photonic crystal waveguides (PCWs) are appealing for producing slow light since they can be on-chip integrated and operated under room temperature. However, most PCW slow-light devices are restricted to the narrow spectral range of material resonance, leading to a small delay-bandwidth product, which restricts the maximum data rate, operation frequency, and storage capacity. Furthermore, the lack of broadly tunable slow light hinders practical applications in tunable photonic devices. We propose a reconfigurable slow-light device using a PCW based on a prototypical chalcogenide glass, Ge2Sb2Te5 (GST225) to solve the problems. We find that the operating wavelength of the slow light within the structure can be reversibly switched between 3575 and 4905 nm by changing the structural state of GST225 between amorphous and crystalline ones. The corresponding average group indices are 40.8 and 54.4, respectively. We experimentally illustrate that the reversible phase transition of GST225 between amorphous and crystalline ones can be realized in nanoseconds. Our proof of concept may provide a platform for actively engineering slow light that might otherwise be difficult to obtain in photonic systems. We expect it to improve the device performance in the fields of nonlinearity and sensing.