Enhanced near-field properties of a gap of TiO ₂ nanosphere pairs for 3D photocatalytic optical trap

Localized near field on a nanostructure has been attracting much attention for a template for size-selective optical trapping beyond the diffraction limit. The near-field optical trapping has mainly been studied by using metallic substrates such as Au nanodot pairs, periodic Al nanoslits, and nanoapertures in an Au film. In this paper, we newly design a Miescattered near-field optical trapping scheme for size-selective photocatalysis by using pairs of poly-rutile TiO ₂ nanospheres. The optical intensity distribution in a gap between the nanospheres was simulated by a FDTD (Finite- Difference Time-Domain) method. The simulation system consists of two nanospheres of 240 nm in diameter placed on a silica substrate in water. The 400 nm excitation laser is used for both the near-field generation and the photocatalyst pumping. The optical force for the trapping was calculated based on the near-field intensity distribution. The results suggest that the optical force generated by the proposed system is sufficient for near-field optical trapping which provides size-selective photocatalysis for killing virus, etc.