Amphibious Artificial Compound Eye with the Self-aligned waveguide

Abstract

Compound eyes of arthropods show outstanding functionalities such as a wide field-of-view and high sensitivity to motion detection due to their unusual optical unit, ommatidium. [1,2] Figure 1a shows the biological structure of the ommatidium which consists of a corneal lens, crystalline cone, and rhabdom. In nature, biological structures of the ommatidium have diversely evolved with their environments, such as a corneal lens for optical power and crystalline cone for optical guidance. [3,4] To mimic these functionalities, artificial ommatidium has been studied using a photosensitive polymer material. [5] However, previous efforts to mimic a natural ommatidium have not focused on the external environment such as weather conditions. Generally, optical systems lose their focusing power since the refractive power of curved lenses is changed with their external refractive index (RI). Therefore, the curved lens does not gather the optical rays efficiently once they are in the changing environment. Here, we introduce an amphibious artificial ommatidium with the waveguide via a self-writing process and flat micro-lens array (MLA), which enables the same light-gathering efficiency regardless of outer RI. As shown in Fig. 1b, the optical power of the curved lens is changed with the external medium while that of the flat lens is consistent. [6] Using the ultraviolet (UV) curing property of the photosensitive polymer (SU-8, Microchem Corporation, Newton, MA) and the light focusing property of MLA, artificial ommatidium with the self-written waveguide was formed. Figure 2 shows schematic illustrations of the fabrication process of self-aligned waveguides. Prior to the fabrication of the SU-8 MLA, a polydimethylsiloxane (PDMS) mold with a sidewall was fabricated using a quartz MLA mold etched by reactive ion etching. The temperature of SU-8 was gradually increased from 65 ℃ to 120 ℃ to completely evaporate the solvent and the optical adhesive (Norland optical adhesive (NOA), Norland Products, USA) was spin-coated on a fabricated SU-8 MLA. When the UV light source is irradiated, the exposed region is photo-crosslinked through post-exposure baking, which results the RI increase, while the unexposed region is thermal-crosslinked with the RI reduction through hard baking. As a result, the exposed and unexposed regions serve as a core and cladding of the waveguide, respectively. We have implemented the universal self-aligned waveguide by using flat MLA and the difference in RI between the photo/thermal-crosslinked regions. This artificial component allows the optical system to ensure amphibious imaging without non-essential optical loss. Our optical system could be of use in a variety of imaging applications such as medical engineering and light field imaging in all-weather.