Bilayer Heterostructure Photonic Crystal Composed of Hollow Silica and Silica Sphere Arrays for Information Encryption.

Utilizing photonic crystals to fabricate information encryption materials has attracted widespread interest due to their tunable optical properties and responsiveness to external stimuli. In most of the previously reported systems, the information is hidden at a specific angle and the angle-dependent invisibility is a limitation. Meanwhile, poor structural stability is still a key issue that needs to be solved for potential applications. In this paper, a bilayer heterostructure photonic crystal containing ordered hollow silica inverse opal arrays, amorphous silica opal arrays, and poly(vinyl alcohol) (adhesive) is successfully constructed. It makes the information highly invisible at any angle and also achieves information encryption. With this strategy, the information can be hidden by the noniridescent structural color derived from the strong scattering effect of light from the top layer of amorphous silica sphere arrays. After wiping with ethanol or a refractive-index-matching solvent, the scattering effect vanishes and the amorphous silica sphere arrays become transparent. The reflected light of the bottom layer caused by the increasing refractive index contrast between the inside and outside of the hollow silica spheres could rapidly reveal the hidden information. The bilayer photonic crystal exhibits robust structural stability, and the hiding/revealing process is completely reversible, which shows great potential applications in steganography and information encryption.