Design for Approaching Cicada-Wing Reflectance in Low- and High-Index Biomimetic Nano structures

摘要

Natural nanostructures in low refractive index Cicada wings demonstrate <= 1% reflectance over the visible spectrum. We provide design parameters for Cicada-wing-inspired nanotip arrays as efficient light harvesters over a 300-1000 nm spectrum and up to 60 degrees angle of incidence in both low-index, such as silica and indium tin oxide, and high-index, such as silicon and germanium, photovoltaic materials. Biomimicry of the Cicada wing design, demonstrating gradient index, onto these material surfaces, either by real electron cyclotron resonance microwave plasma processing or by modeling, was carried out to achieve a target reflectance of similar to 1%. Design parameters of spacing/wavelength and length/spacing fitted into a finite difference time domain model could simulate the experimental reflectance values observed in real silicon and germanium or in model silica and indium tin oxide nanotip arrays. A theoretical mapping of the length/spacing and spacing/wavelength space over varied refractive index materials predicts that lengths of similar to 1.5 mu m and spacings of similar to 200 nm in high-index and lengths of similar to 200-600 nm and spacings of similar to 100-400 nm in low-index materials would exhibit =1% target reflectance and similar to 99% optical absorption over the entire UVvis region and angle of incidence up to 60 degrees.