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

초록

This study demonstrates the developments of self-assembled optical metasurfaces to overcome inherent limitations in polarization density (P) and high refractive indices (n) within naturally occurring materials. The Maxwellian macroscopic description establishes a link between P and n, revealing a static limit in natural materials, restricting n to approximate to 4.0 at optical frequencies. Previously, it is accepted that self-assembly enables the creation of nanogaps between metallic nanoparticles (NPs), boosting capacitive enhancement of P and resultant exceptionally high n at optical frequencies. The work focuses on assembling 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 systematic comparisons between each differently shaped Au NP monolayers elucidate the significance of capacitive coupling in achieving an unnaturally high n. The achieved n of 10.12 at optical frequencies stands as a benchmark, highlighting the potential of polyhedral Au NPs in advancing optical metasurfaces. This work demonstrates that achieving an unnaturally high refractive index at optical frequencies is possible through colloidal self-assembly. By utilizing polymeric brush-assisted balancing between attractive and repulsive potentials, polyhedral gold nanoparticles (i.e., nanocubes) are self-assembled into a close-packed monolayer with high macroscopic integrity and crystallinity. This represents the first realization of an optical metasurface with a refractive index of 10-plus, which is out of reach thus far. image

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