Omnidirectional / Unidirectional Antireflection-Switchable Structures Inspired by Dragonfly Wings.

Randomly arranged irregular inclined conical structure-covered dragonfly wings, distinguished from periodic conical structure-covered cicada wings, are with high optical transparency for wide viewing angles. Bioinspired by the antireflective structures, we develop a colloidal lithography approach for engineering randomly arranged irregular conical structures with shape memory polymer-based tips. The structures establish a gradual refractive index transition to suppresses optical reflection in the visible spectrum. By manipulating the configuration of structure tips through applying common solvent stimulations or contact pressures under ambient conditions, the resulting unidirectional antireflection and omnidirectional antireflection performances are able to be instantaneously and reversibly switched. The dependences of structure shape, structure inclination, structure arrangement, and structure composition on the switchable antireflection capability are also systematically investigated in this study.

Leafhopper Wing-Inspired Broadband Omnidirectional Antireflective Embroidered Ball-Like Structure Arrays Using a Nonlithography-Based Methodology.

Leafhoppers (Thaia rubiginosa) actively coat their wings with embroidered ball-like secretory brochosomes, which act as antireflective structures to enhance camouflage against predators. Inspired by the leafhoppers, we report a scalable nonlithographic approach for self-assembling nonclose-packed embroidered ball-like hierarchical structure arrays. The resulting structures create a gradual refractive index transition at the air/substrate interface, thereby suppressing the optical reflection for wide viewing angles. Compared with a bare substrate, the average reflectance of the structured substrate in the whole visible spectral region is reduced from 9 to 3% at normal incidence, and the average reflectance of that is even reduced by ca. 22% as the incident angle reaches 75°. Moreover, the dependence of the height and the shape of the hierarchical structure on the omnidirectional antireflection performance is systemically evaluated in this research.

Reversibly Erasable Broadband Omnidirectional Antireflection Coatings Inspired by Inclined Conical Structures on Blue-Tailed Forest Hawk Dragonfly Wings.

Blue-tailed forest hawk dragonfly (Orthetrum triangulare) wings, covered with inclined conical structures, are studied for their high transparency and low reflectance for large viewing angles. However, limited by existing technologies, the exquisite inclined structures are not replicated easily or applied adequately. Here, we combine a shear-induced self-assembly approach and a colloidal lithography technology to create omnidirectional antireflection structures that are inspired by dragonfly wings. Nonclose-packed colloid crystals are spin-coated and serve as structural templates in a plasma etching procedure to pattern subwavelength inclined conical structures directly on shape memory polymer-coated substrates. The dependence of the antireflection functionality on the shape and inclination of conical structures is systematically investigated in this research. Compared with a featureless substrate, the structure-covered substrate can display an approximately 8% higher average transmittance in the visible wavelength range at normal incidence and even approximately 23% higher average transmittance as the incident angle increases to 75°. Moreover, the reconfigurable structures composed of shape memory polymers can be repeatedly deformed and recovered as a result of external stimuli at ambient conditions, and the corresponding broadband omnidirectional antireflection functionality is therefore reversibly erased and restored.