Achieving Optical Refractive Index of 10-Plus by Colloidal Self-Assembly

TL;DR

This paper demonstrates that self-assembled colloidal gold nanoparticle monolayers can achieve an optical refractive index exceeding 10, surpassing natural material limits, by leveraging electric dipolar resonances and capacitive coupling.

Contribution

It introduces a novel self-assembly method for polyhedral gold nanoparticles to create high-index optical metasurfaces with record-high refractive indices.

Findings

Achieved a refractive index of 10.12 at optical frequencies.

Demonstrated robust large-area self-assembly of polyhedral gold nanoparticles.

Highlighted the role of capacitive coupling in reaching high refractive indices.

Abstract

This study demonstrates the developments of self-assembled optical metasurfaces to overcome inherent limitations in polarization density (P) within natural materials, which hinder achieving high refractive indices (n) at optical frequencies. The Maxwellian macroscopic description establishes a link between P and n, revealing a static limit in natural materials, restricting n to approximately 4.0 at optical frequencies. Optical metasurfaces, utilizing metallic colloids on a deep-subwavelength scale, offer a solution by unnaturally enhancing n through electric dipolar (ED) resonances. Self-assembly enables the creation of nanometer-scale metallic gaps between metallic nanoparticles (NPs), paving the way for achieving exceptionally high n at optical frequencies. This study focuses on assembling polyhedral gold (Au) NPs into a closely packed monolayer by rationally designing the polymeric ligand to balance attractive and repulsive forces, in that polymeric brush-mediated self-assembly of the close-packed Au NP monolayer is robustly achieved over a large-area. The resulting monolayer of Au nanospheres (NSs), nanooctahedras (NOs), and nanocubes (NCs) exhibits high macroscopic integrity and crystallinity, sufficiently enough for pushing n to record-high regimes. The study underlies the significance of capacitive coupling in achieving an unnaturally high n and explores fine-tuning Au NC size to optimize this coupling. The achieved n of 10.12 at optical frequencies stands as a benchmark, highlighting the potential of polyhedral Au NPs in advancing optical metasurfaces.