Refractive‐Index‐Matching‐Based Encryption of Photonic Crystal Prints with Multistage and Reconfigurable Information

Abstract

make PCs applicable in the fields of displays, [6][7][8][9][10][11][12] printings, [13][14][15][16][17][18][19] pigments, [20][21][22][23] biology, [24,25] sensing, [26][27][28][29][30][31][32][33][34][35][36][37] photocatalysis, [38] and anticounterfeiting. [39][40][41][42][43][44][45][46][47][48][49][50][51][52][53][54][55][56] As a member of PCs, the encrypted photonic crystals (EPCs) that can hide and decrypt the information under normal circumstances and specific stimuli respectively are important and highly desired due to their applications in anticounterfeiting and information protection.Up to now, several strategies and methods have been developed to fabricate EPCs based on the modification of the lattice constant, wettability, or refractive index of premade photonic crystals by selective crosslinking, [57][58][59] chemical modification, [60][61][62] bi/multilayer films, [43,63] fixing of ordered structures, [42,64,65] assembly of functional particles, [31,46,66] and reconfigurable absorption process. [47] The information of the EPCs fabricated by the above approaches can be hidden at normal conditions but revealed when specific stimulus, such as solvents, [42,43,47,57,61,67] humidity/ gas, [31,60,62,68] mechanical force, [58,59,65] magnetic field, [64] UV light, [46] or variation of view angles, [63,66,69] are applied. However, only limited information can be encrypted into these EPCs show due to the lack of fine control of the variation of the structural colors of PCs with simple decryption methods. The simple design and fabrication of EPCs with multilevel information are still challenging. Besides, for practical usage, the reuse of the EPCs is always necessary, which fulfills the low-resource consuming and environmentally friendly characteristics.Here, EPCs with multistage and reconfigurable information were fabricated by introducing the solvents with a desired refractive index into the PCs self-assembled of silica particles. The refractive index between the opal structures and solvents is crucial to the encryption process. The information of the EPCs can be hidden at normal conditions since the solvents have a similar refractive index comparable to that of silica particles. In contrast, multiple information can be decrypted by simply heating the EPCs for different times. The switch of the encryption-decryption states is fully reversible. By taking these characteristics, the information can be reconfigured repeatedly through removing the encrypted information by heating and then re-encrypting the new information with other solvents. The refractive-index-match strategy provides new insight for the fabrication of EPCs with complex and erasable information and will promote their applications especially in the field of anticounterfeiting, displays, and information protection. Encryption of photonic prints with multiple information is highly desired due to their potential applications in high security of anticounterfeiting and information protection. Here, a new refractive-index-matching-based technology is...

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