Soft Plasmonic Assemblies Exhibiting Unnaturally High Refractive Index

Abstract

The increases in refractive indices (n) of materials are crucial for transformative optical technologies. With the progress of monolithic lithography, large advances have been achieved with several semiconductors, including silicon, germanium, and gallium arsenide, which generally provide higher n of ∼4.0 compared to those of other elements. Nevertheless, above this upper limit of naturally available n, the range of light–matter interactions could be unprecedentedly expanded, which in turn enriches the possible applications. Here, we present a soft self-assembly of polyhedral Au colloids as a promising method to achieve unnaturally high n values. The interfacial assembly of Au nanocubes provides n of 6.4 at the resonant wavelength (near-infrared) and 4.5 in the off-resonant regimes (mid-infrared), which have not been previously reached. The soft self-assembly of polyhedral Au colloids can be a versatile and highly effective route for the fabrication of optical metamaterials with unnaturally high n values.

Exploiting Colloidal Metamaterials for Achieving Unnatural Optical Refractions

“…[51][52][53][54] The variously shaped metallic colloids acting as electric meta-atoms can be closely packed into a large-area 2D or 3D superlattice using currently available self-assembly technologies such as convective selfassembly, depletion forcing, DNA self-assembly, and interfacial assembly (e.g., Langmuir-Blodgett (L-B)). [55][56][57][58][59][60][61]72,[134][135][136][137][138][139][140] In these [51] b) In 2009, Fan and co-workers [18] theoretically suggested that the elaboration of the shape of conductive meta-atoms from a cube to a 3D "I"-beam frame can dramatically suppress the diamagnetism (M) and the modulation of permeability (μ eff ), while the facet interfaces between meta-atoms can boost the capacitive coupling between the induced EDs and resultant polarization (P)/ε eff . c) In 2011, Min and co-workers [19] found that a 3D "I"-beam frame design can be transformed into its 2D counterpart, which can facilitate the experimental realization.…”

“…In this context, the self-assembly-enabled packing of a metallic colloidal superlattice can provide an effective platform for achieving unnaturally high n eff at optical frequencies. [55][56][57][58][59][60][61] Herein, we explore the unnaturally achievable high refractive indices by packing metallic colloids into a 2D superlattice, as summarized in Figures 3 and 4. Toward this, we used the method of s-parameter retrieval, as briefly introduced in Section 2.2.…”

“…[143] The selective use of the off-resonant behavior can address this high loss problem. [18][19][20][21]54,57,60,61] Figure 4e-g summarizes the figure-of-merit (FOM, defined by n eff /k eff ) corresponding to Figure 4b-d. The optical loss at the off-resonant regime (from NIR to mid-IR) becomes negligible as evidenced by a sufficiently high FOM.…”

“…To push the working frequencies of metamaterials to optical regimes (including visible and near-infrared (NIR) frequencies), the structural scales of each individual unit, which can serve as a meta-atom or metamolecule (hereafter collectively denoted as metaunits), must be smaller than 100-200 nm, according to the an ultrasmall gap array; consequently, the ε eff and resultant n eff of this optical effective medium can be unnaturally increased at optical frequencies. [51][52][53][54][55][56][57][58][59][60][61] In addition to achieving extremely high and low refractive indices at optical frequencies, the soft fluidity uniquely makes the self-assembly of colloids advantageous over the lithographic approaches in terms of mass production and immediate practical applications.…”

“…This present progress report is a more comprehensive study that extends our previous reports on the theoretical frameworks behind colloidal optical metamaterials [37,38,54] and discusses some practical considerations for their materialization. [32,34,60,88,89,101,103] Finally, we address the remaining practical challenges in colloidal self-assembly for the materialization of optical metamaterials together with its perspective.…”