Stimulus‐Responsive Photonic Crystals for Advanced Security

Abstract Counterfeiting is a growing global issue causing severe negative impacts on the social economy and posing serious security threats to individuals and communities. However, the current anticounterfeiting technologies are relatively well known to fakers and lack sufficient security owing to the development and commercialization of high‐resolution equipment. Thus, designing new anticounterfeiting materials and systems is urgently needed. Photonic crystals (PCs) with vivid structural colors are promising materials for advanced security applications because of their unique optical characteristics. In particular, responsive PCs (RPCs), whose optical signals can be readily adjusted by external stimuli, are expected to provide improved security and may be used to determine the authenticity of banknotes, documents, and drugs. In this review, the strategies are summarized for designing RPCs, including filling responsive materials into the interstitial space of periodic photonic structures and using responsive materials as construction blocks of PCs. Subsequently, the latest progress in developing RPCs constructed for optical encryption and information security is discussed in detail. Finally, the challenges and outlook of using RPCs for optical encryption technologies are presented.