Significance Compared with single eyes, the compound eyes of natural insects are characterized by their microimaging, self-cleaning, wide-field-of-view, and high motion detection sensitivity properties due to their micro-nano multiscale structures and curved distribution of small optical units called "ommatidia". The artificial implementation of such natural imaging systems has important application prospects in cutting-edge fields of robot visual navigation, unmanned driving, and microaircraft systems; therefore, it has recently become a research hotspot. Although several methods have been proposed for fabricating the artificial compound eye, they face challenges due to some limitations. To date, as an advanced manufacturing technology, ultrafast laser has become an ideal tool for fabricating artificial compound eyes with multiscale structures owing to their good flexibility, high fabrication accuracy, and true three-dimensional processing. In addition, the high transient intensity and high ultrafast laser power give the technology a high resolution beyond the optical diffraction limit, which is enough to fabricate various materials, both hard and soft materials. This article reviews the research progress of ultrafast laser processing of various types of artificial compound eyes, including the planar microlens arrays, superhydrophobic compound eye, and wide-field-of-view compound eye. The problems and development trends of the technology in the fabrication of artificial compound eyes are analyzed, thereby providing an effective reference for further research and development of the artificial-compound-eye-based systems. Progress This study analyzes the structural characteristics of insect compound eyes and presents the advantages of using insect compound eyes in optical imaging (Fig. 1). Based on these characteristics, various artificial compound eyes were designed and fabricated. Then, the research progress for fabricating three types of artificial compound eyes using an ultrafast laser was reported and the advantages and disadvantages of different methods were analyzed. (1) Planar microlens array: currently, the use of ultrafast laser for fabricating planar distributed microlens array through laser ablation (Figs. 2 and 3) and swelling (Figs. 5 and 6) has gradually increased. For laser ablation, the technology has the advantages of high processing efficiency and filling factor, whereas for swelling, the surface quality of microlenses is relatively high and its size has high controllability. In addition, by controlling the process, various forms of microlenses such as cylindrical, dual-focus or other microlens with irregular surfaces can be fabricated (Fig. 4). (2) Self-cleaning artificial compound eye (Figs. 7-10): in terms of the self-cleaning artificial compound eye, the nonwetting nanostructures could be fabricated at the interval or on the top of microlens arrays. Both the fabricated surfaces endow good self-cleaning property for carefully controlled structures. For the former, the outside droplets are directly in contact with the optical unit and cause pollution problems. For the latter, the fully covered nanostructures easily deteriorate the transmittance of the microlens array if it is over a certain size or has a narrow distribution. In addition, to obtain neatly arranged nanostructures, combining techniques such as imprinting is necessary because the nanostructures induced by the laser are not uniform. (3) Wide-field-of-view artificial compound eye (Figs. 11-14): for programmable direct laser writing, the structures have high fidelity and the artificial compound eye can be fabricated as designed. However, the single-point scanning procedure suffers from low efficiency and the prepared eyes are generally on the micron scale. To improve the fabrication efficiency, a planar microlens array was first fabricated using the laser-based method. Subsequently, using the air- or hydraulic-assisted deformation, the planar distributed microlens array was transformed into a curved architecture. However, during the deformation of a flexible film, the height of the microlenses decreased while the spacing increased, thereby affecting the functional consistency of the imaging unit. Finally, we analyze problems and development trends of the ultrafast laser processing technology in preparing artificial compound eyes to provide necessary references for developing this field. Conclusion and prospects Artificial compound eye has several intriguing features and has been widely used in various fields. As the fabrication technology of artificial compound eyes continues to develop, ultrafast laser processing stands out because of its high processing resolution and programmable design. Thus, it allows the fabrication of artificial compound eyes with multiscale structures. With the deepening of the research, various artificial compound eyes with different shapes and arrangements have been proposed to achieve different functions and important progress has been made in laser processing technology. This article reviews the research process of three types of artificial compound eyes and comprehensively analyzes the advantages and disadvantages of the laser-based methods for preparing different compound eyes. Although several challenges hinder the fabrication of artificial compound eyes using ultrafast laser and the existing artificial compound eye vision system still has a big gap in terms of optical performance and self-cleaning ability compared with the natural one, we believe these problems will be resolved and with the continuous development of laser technology and ultrafast laser will become a powerful tool for preparing artificial-compound-eye-based vision system.
