Helicity-Preserving Optical Metafluids: A New Frontier in Nanophotonics and Metamaterials

An optical metafluid is essentially a colloidal suspension of nanostructures that exhibit optical magnetism. This phenomenon is primarily driven by the magnetic-type Mie resonances at optical frequencies in these nanostructures. A critical component of a metafluid is a dielectric nanosphere, typically made of materials with a high refractive index. When these nanospheres are subjected to the Kerker conditions, they fulfill the electromagnetic duality symmetry condition, thereby preserving the handedness of circularly polarized incident light. Understanding the electromagnetic duality symmetry is key to grasping the significance of these findings. A system with electromagnetic duality symmetry responds symmetrically to electric and magnetic fields. In a “dual” state, such a system preserves the helicity of the incident field, meaning that if right-handed circularly polarized light is scattered by a dual nanoparticle, the scattered field remains right-handed circularly polarized in all directions. Conversely, in an “anti-dual” state, the system flips the helicity of the incident field. Achieving this duality symmetry at optical frequencies is not common in natural materials, making the study of artificially engineered nanostructures like dielectric nanospheres vital.

In a new study published in the NanoLetters Journal led by Professor Minoru Fujii and conducted by Mr. Hidemasa Negoro and Dr. Hiroshi Sugimoto, from the Department of Electrical and Electronic Engineering at Kobe University, explored the unique properties of high-refractive index dielectric nanospheres and their ability to maintain the helicity of incident light, paves the way for advanced applications in fields like chiral molecular sensing and optical component design. The primary goal of the authors was to create a colloidal suspension of photonic nanostructures (optical metafluid) that can preserve the helicity of incident light. This was achieved using high-refractive index dielectric nanospheres that exhibit magnetic-type Mie resonances at optical frequencies. They chose silicon due to its high refractive index and low extinction coefficient, making it suitable for creating well-defined electric and magnetic Mie resonances. The researchers created solutions of crystalline silicon nanospheres. These solutions were used to study the electromagnetic duality symmetry of the nanospheres and their behavior under different conditions.

The authors first addressed the theoretical aspect of electromagnetic duality symmetry in single silicon nanospheres. This involved understanding how these nanospheres interact with electric and magnetic fields and their ability to preserve the helicity of incident light. They produced solutions with silicon nanospheres having narrow size distributions. The uniformity in size is crucial for maintaining consistent optical properties across the solution. The team experimentally demonstrated that these silicon nanosphere solutions could exhibit “dual” and “anti-dual” behaviors. In a “dual” state, the solution preserves the helicity of incident light, whereas in an “anti-dual” state, it reverses the helicity. The researchers discovered that a metafluid composed of such dielectric nanospheres preserves the helicity of incident light, confirming the theoretical predictions about electromagnetic duality symmetry. In the helicity-preserving metafluid, the local chiral fields around the nanospheres were found to be strongly enhanced. This has implications for improving the sensitivity in enantiomer-selective chiral molecular sensing.

The researchers also investigated into how the size of the nanospheres and the refractive index of the medium (water, in this case) affected the helicity preservation. They found that the quadrupole modes do not significantly alter the helicity density at the lowest-order Kerker conditions, but they can degrade the helicity preservation at higher-order conditions. The successful demonstration of helicity preservation in a colloidal suspension opens up possibilities for applications in fluidic lenses, liquid light guides, optical switches, interferometers, and enhanced chiral molecular sensing. The new study is notable for being one of the first to demonstrate a liquid capable of controlling the helicity of scattered light, marking a significant advancement in the field of nanophotonics and metamaterials. In summary, Professor Minoru Fujii and colleagues presented a comprehensive exploration of helicity-preserving optical metafluids, demonstrating both theoretical understanding and experimental validation of the unique properties of silicon nanosphere-based metafluids. Their findings open up new avenues in optical technology and molecular sensing, with potential for significant practical applications.