Limitations and Opportunities for Optical Metafluids To Achieve an Unnatural Refractive Index

Abstract

Optical metafluids have held a special position among the platforms of metamaterials, because other than the lithography-based hard approaches, the soft fluidity-based solution process not only enables their immediate practical utility but also allows for reconfigurable and adaptable nanophotonic systems. However, the fundamental limits of the available effective parameters of optical metafluids are not yet clearly defined. Of particular interest is the accessible range of the refractive index under a practically available volume fraction ϕ and the structural motifs of building blocks. In addition, previously reported theoretical works are based on an effective medium theory that excludes dipolar coupling between building blocks. Using these initial approaches, the interaction between the building blocks at a relatively higher ϕ was not accurately rationalized. In this work, we advance an effective medium theory by using the 3D dressed polarizability. Then, we successfully rationalize the dipolar coupling between each of the building blocks and systematically exploit the fundamental limits of optical metafluids in terms of accessible effective parameters. Also, for the first time, we discuss both the phase transition of metafluids and uniaxial characteristics of fluidic crystals in terms of engineering effective parameters. Thereby, the practically available range of effective parameters from the concept of an optical metafluid is realistically defined. It is revealed that an unnaturally near-zero refractive index and an ultrahigh refractive index can be attainable through optical metafluids. Given the fundamental limits defined by 3D dressed polarizability, a comprehensive perspective of the limits and merits of optical metafluids is provided.

Exploiting Colloidal Metamaterials for Achieving Unnatural Optical Refractions

“…Here, we investigate the design principle of colloidal optical metamaterials exhibiting unnaturally high and negative refractive indices, and then address the materialization aspects of the colloidal optical metamaterials. 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.…”

“…These initially randomized colloids can spontaneously become ordered when their vol% is larger than approximately 50%. [37,114] This is referred to as entropic 3D crystallization of hard colloids (the enthalpic change via interparticle interaction can be negligible). [114] The modulation of interparticle interactions (the enthalpic change is not negligible) can also be intentionally implemented into colloidal suspensions (collectively denoted as soft colloids) to induce the random aggregations, even beyond 50 vol%.…”

DNA Origami‐Guided Assembly of the Roundest 60–100 nm Gold Nanospheres into Plasmonic Metamolecules

“…The ring motif of plasmonic NP clusters can exhibit a circular displacement current and thus artificial magnetism at optical frequencies (i.e., plasmonic metamolecules) . In conventional colloid assemblies, Au NSs can be forced to pack into ring clusters by capillary action; however, symmetric tetramers are difficult to achieve using this conventional method, mainly due to the packing argument .…”

“…Our plasmonic assemblies generated strong electric and unnatural magnetic resonances due to their high structural fidelity. Looking forward, we outline deterministic strategy for engineering of optical metamaterials/metafluids by using roundest and large Au NSs in 3D DNA origami‐enabled self‐assembly.…”

Light‐Directed Soft Mass Migration for Micro/Nanophotonics

“…For example, holographic photopolymerization‐guided counter‐diffusion can organize silver bromide (AgBr) NPs (the yellow spheres in Figure c,d) into a volumetric 1D grating within a polymeric matrix that is functionalized with pendant 8‐hydroxyquinoline (8HQ) moieties (see the chemical structure in Figure c,d) . The incorporation of metallic NPs within this holographic medium could lead to an enhancement in the effective refractive index according to Maxwell‐Garnett effective medium theory . Consequently, the efficiency of the Bragg diffraction and the resultant purity of the structural colorization can be further enhanced.…”