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

Compound eyes of arthropods (e.g., insects) have remarkable features such as a wide field-of-view and high sensitivity to motion detection. [1] These characteristics are due to their unique optical unit, ommatidium, which consists of a corneal lens, crystalline cone, and rhabdom. In nature, the ommatidia have diversely evolved with their environments, such as a corneal lens for optical power and crystalline cone for optical guidance. Researchers have attempted to mimic the functionality of natural ommatidia using photosensitive polymer materials to create artificial ommatidia. [2] However, previous studies have not considered the effects of different environments, such as air and underwater, on the imaging performance of these artificial ommatidia.<br/>In typical optical systems, the refractive power of curved lenses is affected by the external refractive index (RI), leading to a loss of focusing power. In other words, curved lenses are not efficient in gathering optical rays when exposed to changing environments. Some imaging systems have addressed this limitation by modifying the lens shape or displacement (e.g., liquid lens). However, these optical accommodation systems tend to be bulky and complex, which is not suitable for compact configurations. To achieve more efficient vision in both air and water, an imaging system requires accommodation-free optical component. The fiddler crab, a semi-terrestrial crab, has compound eyes with flat corneal micro-lenses that possess a graded RI. Unlike traditional curved lenses, which rely on changes in shape or displacement for optical accommodation, the fiddler crab's flat micro-lenses maintain consistent optical power across various environments without the need for accommodation.<br/>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 °C to 120 °C 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 (Δn = 0.029), with an additional spin-coating step to cover the outer layer.<br/>We successfully implemented the universal self-aligned waveguide by using flat MLA and the RI difference between the photo/thermal-crosslinked regions. The optical performance of the artificial ommatidium was evaluated, demonstrating its amphibious imaging capability. This amphibious artificial vision system represents a significant advancement in imaging applications, particularly in the field of amphibious motion detection.<br/><br/><b>References</b><br/>[1] Song, Young Min, et al. "Digital cameras with designs inspired by the arthropod eye." Nature 497.7447 (2013): 95-99.<br/>[2] Jeong, Ki-Hun, Jaeyoun Kim, and Luke P. Lee. "Biologically inspired artificial compound eyes." science 312.5773 (2006): 557-561.