All‐Biomass‐Based Hierarchical Photonic Crystals with Multimode Modulable Structural Colors and Morphing Properties for Optical Encryption

Abstract Materials with structural coloration capable of multimode color manipulation are gaining growing significance for advanced encryption and high‐security anti‐counterfeiting applications. Among the most promising candidates are naturally derived biomaterials, owing to their renewable, biocompatible, and biodegradable features for developing sustainable, bio‐interfaced photonic platforms. Nevertheless, structural color encryption strategies developed from biological materials usually exhibit limited optical operation modes, lowering their encryption capability and security level. Here, an all‐biomass‐based photonic crystal platform is reported that hierarchically integrates chiral nematic and inverse opal structures through a combination of colloidal assembly, silk protein self‐assembly, and chiral self‐assembly of cellulose nanocrystals, enabling multiplex structural color manipulation in 2D and 3D spaces. The platform's Janus‐style integration brings specular and diffuse reflection, direction‐dependent reflection, circular dichroism, and birefringence into a single form, thereby facilitating multimode structural color tuning in a 2D plane by altering the illumination‐viewing modes. The inherent shape plasticity of silk proteins allows the subsequent creation of 3D photonic platforms with diverse configurations, offering additional spatial flexibility for color encoding. It is demonstrated that this all‐biomass‐based photonic framework exhibits versatile, multilevel, and high‐capacity encryption capability in 2D and 3D spaces, representing an innovative solution to bolster security measures against counterfeiting for future technologies.