Broadband and wide-angle antireflective metasurfaces with complementary patterns.

Dielectric metasurfaces with complementary shapes for the nanopatterns are fabricated using a polymer-based low-cost soft lithography approach, using a single master mould. The master mould is also fabricated in-house using a highly reliable inward-growing self-assembly process using colloids, thus making the over-all fabrication process to be simple and easy. Antireflective and anti-wetting properties of these metasurfaces, which possess graded index profiles, are studied. Antireflection characteristics of the metasurfaces are studied over a wide wavelength range of 400-2000 nm using experimental and numerical techniques. Nanodimple patterns present in the metasurfaces lower the specular reflection of glass by 67.5% in the visible range, whereas the nanobump patterns reduce the reflection by 80% throughout the wavelength range of 400 to 2000 nm. The graded index profile is calculated for both the metasurfaces from the fill fractions of the solid medium, by assuming the ideal shape of the pattern as well as from the real shape of the pattern obtained from the microscopic images of the surface. These index profiles provide the insight for the difference between nanobump and nanodimple patterns in realizing effective antireflection. The reflection spectra numerically calculated from the graded index profiles of the surfaces provide an excellent match with the measured spectra, and confirm the relative advantage of nanobump patterns over nanodimple patterns for superior antireflection even with shallow patterns. The anti-wetting and self-cleaning features of these metasurfaces are obtained from contact angle measurements. The potential application areas of these metasurfaces are in reducing the reflection losses and thus in improving the efficiency of optoelectronic devices such as the solar cells, light emitting diodes and low-light imaging sensors.