The scientists developed a spherical nanoantenna that forms a near-field of circularly polarized light

June 28, 2023

(Nanowerk News) A team of researchers from the Graduate School of Engineering at Kobe University, consisting of Master student Hiromasa Negoro, Associate Professor Hiroshi Sugimoto, and Professor Minoru Fujii, have succeeded in innovating a spherical nano antenna. This antenna can generate near-field circularly polarized light, a significant development in the fields of nanotechnology and molecular biology.

The team’s findings are published in Nano Letters (“Helicity Preserving Optical Metafluid”). Schematics of an electric dipole, a magnetic dipole and a “double” nanoantenna, (Image: SUGIMOTO Hiroshi)

The essence of their research revolves around overcoming current limitations in detecting and separating chiral molecules. Chiral molecules and their mirror image isomers, known as enantiomers, exhibit different properties in living organisms.

Although there are currently photochemical detection and reaction methods that take advantage of the differences in optical absorption for left and right circularly polarized light (circular dichroism) in these molecules, they are limited by requiring high sample concentrations and long measurement times. This is largely due to negligible absorption differences. Nano-antennas that amplify electromagnetic fields have been proposed to overcome this limitation, but conventional models have struggled to maintain the circular polarization of incoming light.

The Kobe University research group overcame this hurdle by proposing and testing a new nano-antenna design. This design increases the electromagnetic field while maintaining the circular polarization of the incoming light. This was achieved by exploiting optical resonance, maintaining the circular polarization of light across a broad spectrum using colloidal silicon nanoparticles independently developed by the team.

This new approach enhances the response of chiral molecules to circularly polarized light, making it a potentially transformative development for the sensitive detection of molecular chirality and asymmetric photochemical reactions. This technique has wide application in the analysis of biomolecules, chemicals, and drugs. nanoparticles and helicity density spectra (a) Photographs of colloidal Si nanoparticles in water with different diameters. (b,d) Schematic of scattering of circularly polarized light in “non-dual” (b) and “dual” (d) nanoparticles. (c,e) Calculated and measured helicity density spectra of Au(c) and Si(e) nanoparticles. (© Nano Letters)

In their study, the team focused on the Mie resonance of dielectric nanoparticles with a high index of refraction. These particles can enhance the incident electric and magnetic fields, making them “dual” nano-antennaes. The researchers demonstrated that these particles can maintain the helicity (circularly polarized quality) of incident circularly polarized light under certain conditions, creating a near-field of circularly polarized light.

The team independently developed a colloid solution from crystalline silicon nanoparticles and used it to demonstrate this property. They successfully demonstrated that “double” nanoparticles that meet specific conditions can maintain the helicity of scattered light from incident circularly polarized light. The researchers demonstrated that conservation of the helicity of incident circularly polarized light is possible in the 550 to 750 nm wavelength, indicating the creation of a circularly polarized near field on the surface of the nanoparticles.

This development paved the way for increased interactions between light and chiral molecules, thereby increasing the circular dichroism of chiral molecules. This could lead to more sensitive detection and analysis, increasing the efficiency of asymmetric photochemical reactions with potential applications in the pharmaceutical industry. In addition, the new nanoparticle solution developed by the team could serve as a groundbreaking new medium for the control of light polarization.

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