2, Yonsei University, Incheon, , Korea (the Republic of)
Antireflective (AR) nanostructures observed in nature, such as the corneal surface of Moth eye, the wing scales of butterflies etc., exhibit an excellent AR performance compared to the quarter wave thin films over a wide range of incident angles and wavelengths, thus attracting great interests in the field of optical and optoelectronic devices. Especially, anti-reflectivity for wide angles of incidence is significant in display and photovoltaic applications to improve visibility and photo-conversion efficiency, respectively. Since the AR nanostructures with 3D geometry are fabricated on a substrate, it is necessary to consider the scattering and coupling of the light due to the geometry of the nanostructures on the substrate, in dealing with the anti-reflectivity by the incident angles.
In this work, we investigate effects on the AR properties of two geometries, nanocones (NCs) and inverted nanocones (INCs) which can be generated by geometric inversion in the nano-imprinting process. We fabricate the two geometries by repetitive polymer replication processes by using photo-curable polymers and nanostructured quartz molds and evaluate the specular reflectance for visible range with various incident angles from 6° to 75°. The measured spectra are analyzed in the view of Mie scattering and guided mode resonance.
We find that, unlike the INCs, the NCs enable to maintain Mie scattering efficiency against changes in the incident angles because the scattering fields are concentrated at the apex of the NCs. This phenomenon is verified by computational simulations based on finite-difference time domain methods. The concentrated scatterings on NCs allow the more propagation of incident fields and for this reason, the NCs provide better AR performance than the INCs. We observe the presence of guided mode resonance from the measured spectra and analyze it by considering the phase matching in 2D hexagonal nano-grating structures. Additionally, we find that INCs can exhibit stronger guided mode resonance and internal reflections, which can be another reason why AR performance is degraded in the INCs. By utilizing these findings on both-sided antireflective nanocones, we achieve extremely low average reflectance (5.4 %) at very high incidence angle of 75° for entire visible range.
This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (NRF-2017R1D1A1B04033182)