Progress in femtosecond laser fabrication of artificial compound eye

Compound eyes of arthropods are widespread and fascinating visual systems created by natural evolution. Compound eyes consist of many identical small eye units called "ommatidia", each of which is an individual imaging system composed of cornea lens, crystalline cone and rhabdome. Compared with single eyes, compound eyes have distinct advantages in small volumes, large field of view, large depth of field, and high temporal resolution. Therefore, it has sparked considerable research interests in developing various manufacturing technologies towards the fabrication of artificial compound eyes that can be applied in robot vision, three-dimension imaging, motion detection and medical examination. To date, various methods have been proposed for fabricating artificial compound eyes, such as hot embossing method, thermal reflow method, microdroplet jetting method, and ultraprecision machining method. However, these methods inherently suffer from low accuracy and multi-step processing. In particular, it is extremely difficult for these methods to acquire curved artificial compound eye lens, which is quite important for promoting the optical performance and practical applications of artificial compound eyes. In recent years, femtosecond laser direct-writing (FsLDW) has emerged as a practical and promising technique for fabricating artificial compound eyes owing to its capability of three-dimensional (3D) processing and high fabrication precision. First of all, FsLDW is performed by point by point scanning process layer by layer, making it capable of realizing various 3D structures as designed. Second, the extremely high transient intensity of femtosecond laser pulses leads to a non-linear absorption effect (two-photon absorption or multi-photon absorption) with materials in a much smaller volume than the cubic size of wavelength, which endows it with a high resolution (tens of nanometers) beyond the optical diffraction limit. Moreover, the high power of femtosecond laser pulses also provides the ability to process various materials, from soft photopolymer materials to rigid substrates as hard as diamonds. For soft materials, the fabrication mechanism is two- or multi-photon polymerization of photosensitive polymers triggered by the high energy at the localized laser focus region. Through point by point process, two-photon polymerization technology by FsLDW becomes a powerful tool for fabricating complex 3D structures. In spite of this, its shortcoming is obvious as well. The single point scanning procedure causes a relatively low efficiency, which needs further improvement. For hard materials, femtosecond laser serves as an ablation source to form micro-hole, where the local material is removed or chemically modified. Femtosecond laser ablation can be combined with a subsequent etching treatment for improving efficiency, including wet etching and dry etching. The drawback of etching assisted femtosecond laser ablation technology lies in that usually only concave structures can be obtained, further soft embossing treatment is needed in order to acquire convex compound eye lens. In this review, the latest progress in femtosecond laser fabrication of artificial compound eyes is summarized. Manufacturing of artificial compound eyes by FsLDW is introduced according to the two methods mentioned above: femtosecond laser two-photon polymerization that belongs to additive manufacturing and femtosecond laser ablation that is regarded as a subtractive manufacturing method. Then, the diverse applications of artificial compound eyes produced by FsLDW are described. At last, current challenges and future perspectives in this field are discussed.