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

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 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 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.