Design for approaching Cicada-wing reflectance in low- and high-index biomimetic nanostructures.

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° 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 ∼ 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 ∼ 1.5 μm and spacings of ∼ 200 nm in high-index and lengths of ∼ 200-600 nm and spacings of ∼ 100-400 nm in low-index materials would exhibit ≤ 1% target reflectance and ∼ 99% optical absorption over the entire UV-vis region and angle of incidence up to 60°.