Biomimetic non-uniform nanostructures reduce broadband reflectivity in transparent substrates
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esearch Letter
Biomimetic non-uniform nanostructures reduce broadband reflectivity in transparent substrates Alexandra A. Sourakov
and Ahmed Al-Obeidi, Massachusetts Institute of Technology, 77 Massachusetts Ave, Cambridge, MA 02139, USA
Address all correspondence to Alexandra A. Sourakov at [email protected] (Received 4 February 2019; accepted 25 April 2019)
Abstract The remarkable broadband and omnidirectional anti-reflectivity observed in the glasswing butterfly arises from the random array of nanopillars present on their wings. In the present study, analogous structures have been replicated on transparent substrates using a scalable, low-cost method that exploits surface dewetting of silver thin films on silica substrates to form an etch mask. Directional etching was applied with high selectivity between Ag and SiO2 using CHF3, allowing large aspect ratios to be achieved with 20 min etches. Single-sided nanostructuring of glass by this method improved the transmission of light by 2–8% for viewing angles of 25°, 45°, and 65°.
Introduction Biomimetic patterns serve as an inspiration for designing materials with various optical properties, from iridescence to transparency. In an age of screens, displays, and optoelectronic devices, there is an enduring interest in eliminating reflectivity from surfaces. Traditionally, this result has been achieved by applying layered antireflective coatings (ARCs) so that the scattering of light in the direction of its source is inhibited by destructive interference at the layer interfaces.[1] It is difficult to achieve broadband and omnidirectional antireflection with this approach, because it can only reduce reflection at a few wavelengths and at specific incident angles.[2] Inspiration for improving ARCs and arrays can be drawn from examples of non-reflectivity and transparency observed in nature, which are often a result of nanoscale structures. Recent work on the glasswing butterfly has shown that broadband, omnidirectional anti-reflective properties can be achieved by randomly sized nanopillars with a characteristic length scale below the wavelength of light.[3] We demonstrate how such structures can be replicated directly on a glass substrate, leading to a significant decrease in the reflection of incident light. Conventional ARCs involve coating the reflective surface with a thin multilayer film where the refractive indices and film thicknesses are selected such that the reflections of light occurring at each of the film’s interfaces destructively interfere with each other.[4] However, there are a number of shortcomings such as poor mechanical durability, sensitivity to thickness variations, and performance limited to a narrow range of incident wavelengths and angles.[5] An alternative strategy to achieve non-reflectivity is to eliminate the sharp change in refractive index at the surface by gradually changing the refractive index from one refractive index to another.[6] From a
technologic standpoint, this phenomenon can be achieved by varying the packing density (i.e., porosity) throughout
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