Biomimetic Surfaces for High‐Performance Optics

Abstract

Periodic optical microstructures are abundant in biological systems, and have provided enormous inspiration for scientists to mimic them for optical applications. Some nocturnal insects use arrays of non-close-packed nipples of sub-300-nm size as antireflective structures (ARS) to reduce reflection from their compound eyes. The ARS on the corneas of these insects can also increase light transmission in dark conditions, which improves sensitivity of light vision. In practical applications, simultaneously suppressing surface reflection and increasing transmission of light is crucial to the performance of an optical system, especially when there are several optical components involved. For flat-panel displays, antireflective surfaces are usually employed to increase transmission and eliminate ghost image or veil glare due to reflection from the optical surfaces. Despite the technical importance, actual material solutions for improved performance are limited, especially in the ultraviolet spectral region. To date, two kinds of approaches are available for fabricating antireflective surfaces. One is coating porous or multilayered films on the surface of devices. The other is fabricating subwavelength ARS (moth’s eye structures) directly on the surface of devices. The ARS surfaces exhibit distinct advantages compared to coatings. First, the ARS surfaces exhibit higher mechanical stability and better durability than coatings because no foreign materials are involved when they are used over a broad thermal range. Second, the material of the ARS surfaces is the same as that of devices, while coating materials with appropriate refractive index are rare in nature. Last, the ARS surfaces have many tunable factors, such as the spacing, depth and cross-sectional geometry. Similar to the nipple arrays in insects’ compound eyes and wings, the ARS surfaces with nipple-like or tapered profiles exhibit a gradient in refractive index between air and substrate. Therefore, they can dramatically suppress the reflection losses at the interface over a large range of wavelengths and a large field of view. Many techniques based on top-down lithography, such as electron-beam etching, fast atom beam and interference lithography have been applied to fabricated ARS surfaces. To avoid scattering from the optical interface, the structure dimension at an optical interface has to be smaller than the wavelength of the incident light. For ultraviolet and visible light applications, feature sizes below 200 nm are always necessary. In such small size ranges, conventional top-down lithographic technologies require sophisticated equipment, are timeconsuming and expensive to fabricate over the large areas required for practical applications. Moreover, processing of non-planar lenses, especially with a small curvature radius, is almost impossible due to the difficulty of fabricating masks on non-planar surfaces. Recently, ARS surfaces on silicon prepared by nanosphere lithography have been reported, but there are few ARS surfaces on fused silica or on non-planar lenses. Spatz and co-workers have prepared fused silica hollow pillar arrays; such arrays exhibit high performance antireflective properties in the deep ultraviolet region, although the antireflective properties in the visible region is suboptimal due to the pillars having a smaller spacing than the wavelength of visible light. The nipple arrays on the compound eyes of insects exhibit both antireflective and self-cleaning properties. Recently, multifunctional antireflective surfaces have attracted a lot of interest because the additional functions can improve the performance of optical devices. For instance, we have successfully prepared high aspect ratio silicon hollow-tip arrays for antireflective and water-repellent surfaces. Such antireflective surfaces with water-repellent properties enable applications in humid environments. In practical applications, the high transmission and antifogging surfaces are highly desired, such as eyeglass, swimming goggles, and lens in laparoscopic surgery. Superhydrophilic coatings can significantly suppress fogging behavior by rapid spread and flow of water droplets on their surfaces and eliminate light scattering caused by water droplets. Nowadays, antifogging and antireflective surfaces based on porous films have been prepared by LBL technology, which is usually used as a general means to create multifunctional coatings. In this Communication, we report a simple and time-efficient method to prepare large area fused silica cone arrays on planar fused silica substrates and planconvex lenses for high performance ARS and antifogging surfaces. The ARS surfaces introduce a gradient in the refractive index between air and the silica substrate, which dramatically suppresses the surface reflective loss from the ultraviolet to near-infrared regions with an incidence angle up to 458. Besides, such surfaces exhibit high performance antifogging properties. The antifogging and ARS surfaces exhibit higher mechanical stability and better durability than coatings because no foreign materials are involved.