Bio‐Inspired Giant Refractive Index Gradient Lenses Based on Ionic Polymers

Transparent polymers are low‐cost, light, and flexible, making them prospective for a plethora of applications. Still, their usage in photonics is impeded by a low refractive index, usually less than 1.7. In this work, an alternative strategy is proposed for improving optical characteristics by developing poly(ionic liquids) (PILs) with a gradient refractive index (GRIN) in thin films. The obtained PILs are transparent, environmentally friendly, and possess the GRIN effect in thin films. Inspired by the architecture of the animal's eye, PILs are employed for inkjet fabrication of microlenses with a giant GRIN value of 0.8, which is up to several times higher than that in previous studies on nanolayered polymeric and 3D printed GRIN lenses. Furthermore, in terms of focusing power, lens transparency, and depth of field, these microlenses outperform the result of high refractive index polymers. Hence, the findings open a novel platform for compact optical components based on new types of ionic polymers.