Broadband high refractive index metamaterial for visible wavelengths based on tens-of-nanometer size metal particles

Abstract

Metamaterials can achieve extraordinary properties, which does not observed in nature such as nega-tive index, zero index and high index of refraction. These unnatural properties of metamaterial comes from its meta-atom. Meta-atom has much smaller size than operation wavelength, therefore incident wave ob-serves metamaterial as a homogenous medium. Especially, metamaterial with high refractive index has vari-ous applications. It can enhance optical resolution by reducing wavelength, control spontaneous emission and absorption by changing optical density of states, and maximize index contrast to low index material for resonator, reflector, grating structure. Due to wide applicable field, high refractive index metamaterial already suggested and it was demonstrated in THz. However, it is hard to directly apply in visible wavelengths, due to difficulty in nanofabrication for the complex structure. On the other hand, in order to realize visible met-amaterial, it is needed to fabricate meta-atom which size is around tens-of-nanometer scale. Through con-ventional lithography, periodically arranged pattern with nanometer size is difficult to fabricate. Using self-assembly characteristic of diblock copolymer, it can be applied to realize metamaterial in visible wavelengths. In this work, we propose new design principle of visible metamaterial with simplicity. Using close-packed nanoparticles with tens-of-nanometer size, it is possible to achieve both enhancement of permittivity and sustenance of permeability. Refractive index is square root of product of permittivity and permeability, therefore, suggested structure can show high refractive index. Permittivity can be enhanced by confined elec-tric field between metal nanoparticles and permeability can be sustained due to small size of metal nanopar-ticle which can lead penetration of magnetic field. Through finite-difference time-domain (FDTD) simulation, numerical analysis is used to verify electromagnetic response of suggested structure. By measuring both magnitude and phase of transmission and reflection, complex refractive index is retrieved and also permittiv-ity, permeability are calculated. In order to realize suggested structure, block copolymer lithography was used to fabricate metal nanoparticles with tens-of-nanometer size, and fabrication result is analyzed through FDTD simulation result. Therefore, using close-packed metal nanoparticles with tens-of-nanometer size can achieve broadband, high refractive index.