Exploiting Metallic Colloidal Assemblies to Achieve Unnatural Light-Matter Interaction.

Since the concept of metamaterials (photonic metamaterials) were first suggested, the scaling down of structural units acting as a meta-atoms or metamolecules has been a long-lasting issue. According to the effective medium theory, which is foundational to the concept of metamaterials, the structural sizes of meta-atoms or metamolecules should be much smaller than that of the working wavelengths (<< 1/5 wavelength). At the relatively low frequency regimes (e.g., microwave and terahertz (THz)), conventional monolithic lithography (e.g., photolithography) can readily address the materialization of metamaterials. However, it is still challenging to fabricate optical metamaterials (metamaterials working at optical frequencies such as visible and near infrared (NIR) regimes) through such lithographic approaches. This has served as rationale for using self-assembly for the realization of optical metamaterials. The self-assembly of metallic colloids can address various critical issues associated with the materialization of optical metamaterials, such as achieving metallic nanogaps, increasing true 3D structural complexities, and processing large-scale and cost-effective materials, which is difficult to attain through monolithic lithography. Nevertheless, this self-assembly of metallic colloids has barely been used and still remains an underutilized method. In this work, we theoretically exploited the challenge of selfassembly technology for the fabrication of optical metamaterials, together with its future research opportunities. TOC: We explore the unnatural refractive indices which can be obtained by using chemically synthesizable colloidal particles. We propose two cases of structure to achieve an unnatural optical property: One is a metamolecule cluster which has unnatural magnetism and is made of silver spheres. The other is an assembled monolayer with an unnaturally high refractive index made of silver plate. Finally, we estimate the realistic limitations and opportunities of the two cases.