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.
Highlights
The refractive index of natural transparent materials is limited to 2–3 throughout the visible wavelength range
The desired metal particle array is fabricated by block copolymer (BCP) selfassembled nanopatterning on thermal shrinkage films, and subsequent pattern shrinkage allows for period and symmetry control
The physical principle underlying our approach is based on the large difference between the Thomas-Fermi screening length (TFSL)[26] and the magnetic field penetration length in metals[27]
Summary
The refractive index of natural transparent materials is limited to 2–3 throughout the visible wavelength range. 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. The fabricated sheet of the nanoparticle array can be considered a homogeneous thin film with an effective relative permeability (meff), permittivity (eeff) and index (neff), as the array period is much smaller than visible wavelengths[25]. With this approach, an neff as high as 5.10 is experimentally confirmed with a large optical anisotropy and a spatial index gradient is demonstrated
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