Colloidal superlattices for unnaturally high-index metamaterials at broadband optical frequencies

TL;DR

This paper demonstrates a method to create broadband optical metamaterials with unnaturally high refractive indices using metallic nanoparticle superlattices, with electrical tunability achieved through graphene integration.

Contribution

It introduces a novel design rule for nanoparticle superlattices that significantly enhances refractive index beyond natural limits at broadband optical frequencies.

Findings

Achieved refractive index of 15.7 at electric resonance

Refractive index of 7.3 at quasi-static limit

Enabled electrical control of refractive index with graphene

Abstract

The recent advance in the assembly of metallic nanoparticles (NPs) has enabled sophisticated engineering of unprecedented light-matter interaction at the optical domain. In this work, I expand the design flexibility of NP optical metamaterial to push the upper limit of accessible refractive index to the unnaturally high regime. The precise control over the geometrical parameters of NP superlattice monolayer conferred the dramatic increase in electric resonance and related effective permittivity far beyond the naturally accessible regime. Simultaneously, effective permeability, another key factor to achieving high refractive index, was effectively suppressed by reducing the thickness of NPs. By establishing this design rule, I have achieved unnaturally high refractive index (15.7 at the electric resonance and 7.3 at the quasi-static limit) at broadband optical frequencies (100 THz ~ 300 THz). I also combined this NP metamaterial with graphene to electrically control the high refractive index over the broad optical frequencies.