Universal Self-aligned Waveguide for an Amphibious Imaging System Inspired by the Compound Eye of the Fiddler Crab

Abstract

Here, inspired by the fiddler crab's eye, we introduce an amphibious artificial ommatidium with self-aligned waveguide. The artificial ommatidium comprise a self-written waveguide (i.e., crystalline cone) and a flat micro-lens array (MLA) (i.e., corneal lens), achieved through a process involving the use of ultraviolet (UV) curing properties of a photosensitive polymer material (SU-8) and the light-focusing capability of the MLA. Prior to fabricating the SU-8 MLA, a polydimethylsiloxane (PDMS) mold with a sidewall was created using a quartz MLA mold etched through reactive ion etching. The temperature of the SU-8 material was gradually increased from 65 ℃ to 120 ℃ to ensure complete solvent evaporation, followed by spin-coating of an optical adhesive (Norland optical adhesive, NOA) onto the fabricated SU-8 MLA. Upon UV light irradiation, the exposed region underwent photo-crosslinking via post-exposure baking, resulting in an increase in RI. Meanwhile, the unexposed region experienced thermal-crosslinking through hard baking, leading to a reduction in RI (~0.008). Consequently, this process turned out to be RI change between a core and cladding structure for the waveguide (ΔnSU-8 = 0.029), with an additional spin-coating step to cover the outer layer.