We demonstrate Si-based single core/shell (Si/SiO₂) nanoparticles which exhibit the Fano resonance associated with ultrahigh scattering directionality. The SiO₂ shell plays a crucial role in achieving zero backscattering at the Fano resonance wavelength along with strongly-enhanced forward scattering. As a result, the front-to-back scattering-intensity ratio is five orders of magnitude greater than that of a Si nanoparticle. Furthermore, the Fano resonance wavelength is controlled over the entire visible region by changing the core diameter. The Fano spectra also show distinctive intensity modulations depending on the index of refraction of the surrounding medium. These unique features make Si/SiO₂ nanoparticles promising for the design of low-loss nano-antennas, metamaterials, and other nanophotonic devices.

We perform a comprehensive numerical analysis on the optical binding forces of a multiple-resonant silicon nanodimer induced by the normal illumination of a plane wave in the visible region. The silicon nanodimer provides either repulsive or attractive forces in water while providing only attractive forces in air. The enhancement of the magnetic dipole mode is attributed to the generation of repulsive forces. The sign (attractive/repulsive) and the amplitude of the optical forces are controlled by incident polarization and separation distance between the silicon nanoparticles. These optomechanical effects demonstrate a key step toward the optical sorting and assembly of silicon nanoparticles.

We report light-transmittable ultrasmooth gold film enabling gap-mode tip-enhanced Raman scattering (TERS) spectroscopy measurements with extremely low background signals. An ultrasmooth Au substrate was fabricated by introducing a wetting layer of germanium (Ge). We also found that high mechanical and chemical stability of the substrate along with flexibility in chemical modification provides versatile platforms for gap-mode TERS measurements in various environments, such as air, water, and organic solvents.

Surface force analysis with atomic force microscope (AFM) in which a single amino acid residue was mounted on the tip apex of AFM probe was carried out for the first time at the molecular level on titanium dioxide (TiO2) as a representative mineral surface for prebiotic chemical evolution reactions. The force analyses on surfaces with three different crystal orientations revealed that the TiO2 (110) surface has unique characteristics for adsorbing glycine molecules showing different features compared to those on TiO2 (001) and (100). To examine this difference, we investigated thermal desorption spectroscopy (TDS) and the interaction between the PEG cross-linker and the three TiO2 surfaces. Our data suggest that the different single crystal surfaces would provide different chemical evolution field for amino acid molecules.