Femtosecond‐Laser Direct‐Writing Hydrogel‐Based Microlens Arrays Customized with Multi‐Dimensional Optical Configurations for Information Encryption

Abstract Femtosecond‐laser direct‐writing lithography prints tunable 3D micro/nano‐scale optical devices based on hydrogel materials have garnered significant attention owing to their use in optical applications. Although the refractive index (RI) is an important parameter of optical devices, a quantitative measurement to determine that of femtosecond‐laser‐induced microstructures is lacking. This study proposes a novel method to measure the RI change of a tunable hydrogel device in different environments using a femtosecond‐laser‐customized multidimensional grating. As hydrogels can be tuned by changing the pH values, the specific RI changes of a hydrogel are first quantitatively investigated with different pH values such that the proposed method can be employed to precisely control the dynamic and tunable optical performance of hydrogel optical devices. To this end, a 3D hydrogel microlens array (HMA) with variable focal lengths is designed and manufactured to verify its dynamic and tunable optical performances. The HMA displays reversible and tunable imaging capabilities, confirming its application in information encryption. As a proof of concept, a customized sequence of letters “CFAE” is successfully obtained as the “output” by assigning the pre‐set pH values as the “code” and the original letter sequence as the “input.”