Surface-plasmon-based optical trapping of hard nanoparticles: two- dimensional closely packed assembly of polystyrene nanospheres on a metallic nanostructure

摘要

Localized surface plasmons (LSPs) have been investigated for applications such as highly sensitive spectroscopies and the enhancement of photochemical reactions. These applications are enabled by the enhancement effect of an incident resonant electromagnetic field (EMF) at the surfaces of noble metallic nanostructures. In particular, the application of LSP has recently attracted much attention for achieving the effective optical trapping of nanoparticles; this is called LSP-based optical trapping (LSP-OT). LSP-OT possesses several advantages; (ⅰ) the EMF enhancement effect of LSP enables the incident light intensity to be significantly reduced for stable LSP-OT, (ⅱ) a nano-sized object can be trapped in a nano-space whose volume is much smaller than that of conventional optical tweezers (diffraction limit), (ⅲ) a large and complicated optical set-up is not necessary, and (ⅳ) this technique can potentially be combined with microfluidic devices. That is, plasmonic substrates can work as "double-functional" devices where biomolecules trapped by LSP-OT can subsequently be analyzed on the basis of SERS or fluorescence enhancement. Thus, LSP-OT could enable a new technique for manipulating not only nanoparticles, but also smaller molecules such as polymer chains, proteins and DNA. Here, we will present the demonstration of LSP-OT of fluorescent-labeled polystyrene nanospheres. We discuss multiple optical trapping in which a closely packed 2D hexagonal assembly appeared on a metallic nanostructure.