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.

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Identifying the three-dimensional (3D) crystal plane and strain-field distributions of nanocrystals is essential for optical, catalytic, and electronic applications. However, it remains a challenge to image concave surfaces of nanoparticles. Here, we develop a methodology for visualizing the 3D information of chiral gold nanoparticles ≈ 200 nm in size with concave gap structures by Bragg coherent X-ray diffraction imaging. The distribution of the high-Miller-index planes constituting the concave chiral gap is precisely determined. The highly strained region adjacent to the chiral gaps is resolved, which was correlated to the 432-symmetric morphology of the nanoparticles and its corresponding plasmonic properties are numerically predicted from the atomically defined structures. This approach can serve as a comprehensive characterization platform for visualizing the 3D crystallographic and strain distributions of nanoparticles with a few hundred nanometers, especially for applications where structural complexity and local heterogeneity are major determinants, as exemplified in plasmonics.

Chiral plasmonics has been actively pursued since pushing its light‐matter interaction to an unnaturally extreme regime. Recently, chirality‐encoded metal–insulator–metal (MIM) motifs have promised compelling advantages in advancing chiroptic responses, as they enable a capacitive coupling and its resultant strong electric and magnetic resonances. However, the deterministic control over chiral MIM working at the visible regime is out of reach. Here, large‐area MIM resonators are demonstrated using non‐lithographically assembled chiral gold nanoparticles (helicoids) and subsequent gold layer deposition, and their boosted chiroptical responses at the visible regime are proven. The thickness of the topcoat Au and the dielectric gap can precisely tune the strength of capacitive coupling. The numerical analyses support that the optimally tuned capacitive coupling in the chiral MIM structure not only strengthens both electric and magnetic dipolar resonances but also makes their spectral positions closer to each other.

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