Highly tunable refractive index visible-light metasurface from block copolymer self-assembly

TLDR

Natural transparent materials have refractive indices limited to 2–3 across the visible, prompting a need for broader tunability for applications such as nanoimaging and integrated photonics. The method uses nanoscale objects smaller than the skin depth to independently tune permeability and permittivity. Self‑assembled nanopatterns with tunable period and symmetry yield a visible‑range refractive index up to 5.10, staying above 3.0 over a 1,000‑nm bandwidth and enabling spatially graded and anisotropic indices through symmetry design.

Abstract

Abstract The refractive index of natural transparent materials is limited to 2–3 throughout the visible wavelength range. Wider controllability of the refractive index is desired for novel optical applications such as nanoimaging and integrated photonics. We report that metamaterials consisting of period and symmetry-tunable self-assembled nanopatterns can provide a controllable refractive index medium for a broad wavelength range, including the visible region. Our approach exploits the independent control of permeability and permittivity with nanoscale objects smaller than the skin depth. The precise manipulation of the interobject distance in block copolymer nanopatterns via pattern shrinkage increased the effective refractive index up to 5.10. The effective refractive index remains above 3.0 over more than 1,000 nm wavelength bandwidth. Spatially graded and anisotropic refractive indices are also obtained with the design of transitional and rotational symmetry modification.