Design of Artificial Hollow Moth-Eye Structures Using Anodic Nanocones for High-Performance Optics

Abstract

We demonstrated an innovative approach for the fabrication of self-organized, hollow-to-solid nanocone arrays, using a decoupling two-step anodization of an Al/Ta interlayer on a substrate. The porosity of the hollow nanostructures was tunable by the deposited thickness of Ta film. We also employed subwavelength nanocone arrays as Moth-eye structures for high-performance antireflection coatings. These oxide nanostructures greatly suppressed reflectance over a large range of wavelengths and angles of incidence and displayed a good mechanical stability. When a corresponding porosity was designed, these nanostructures could be applied to various substrates for an effective antireflection coating. In addition, we were able to customize antireflection coatings for various types of substrates or materials of interest based on the suitable coating of initial Ta thickness.Antireflection (AR) coating is one of the most important optical designs, and it makes up more than 50% of the total optical thin-film market. 1 It is used to suppress surface reflection and to increase transmission of light for optical applications, such as flat panel displays, lasers, photovoltaic devices, and all kinds of lenses. Rayleigh first observed a thin film whose refractive index (n) is the geometric mean of the air, and then, substrate's indices reduced the reflection on the surface of the substrate optimally. In the early 19th century, interference AR coating was discovered using one-quarter of the light's wavelength in thickness. Thus, a single layer structure was applied in general AR coatings by following the aforementioned two conditions, but it only works at a specific wavelength and a normally incident light source. Over the past few decades, the AR coatings primarily consisted of multilayer coatings comprising layers of high-and low-n. Through the algorithm design, multilayer coatings were applied to achieve a wider broadband AR across a visible region for incident angles ranging from 0°to 50°. 2 However, complicated stacks and unsatisfied angle-independence ability on multilayer AR coatings forced scientists to develop next-generation AR structures for future optical devices.According to optical theory, the refractive index of the AR layer should gradually decrease from air to the substrate. 3 Apparently, the refractive index of any dense material is not small enough, but nature provides a solution for such antireflective structures (ARS). The moth eye consists of quasi-close-packed nipples having heights and spacings typically of less than 300 nm, which reduces reflection from it's compound eyes. 4 The ARS on the corneas of these insects gradually match the optical impedance of one medium with its neighbor across the interface. 5 This concept has been applied to realize a variety of photonic systems, particularly semiconductor photovoltaic devices. Many techniques based on top-down etching like lithography, 6 self-organized templates, 7-9 and mask-less etching 10,11 have been used to fabricate tiplike nanostructure for ARS surfa...

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