All-optical tunable plasmonic nano-aggregations for surface-enhanced Raman scattering.

Interparticle forces play a crucial role in nanoparticle-based nanoscience and nanoengineering for synthesizing new materials, manipulating nanoscale structures, understanding biological processes and ultrasensitive sensing. Complicated by the fluid-dynamical and chemical nature of the liquid environment of nanoparticles, previous attempts are limited to electromagnetic and chemical methods. Alternatively, optically induced forces provide a convenient and fabrication-free route to manipulate nanoparticles at the nanoscale. Here we demonstrate a new double laser trapping scheme for metallic nano-aggregation by inducing strong near-field optical interparticle forces without any chemical agents or complicated fabrication processes. These induced optical forces arising from strong localized plasmon resonance strongly depend on the interparticle separation well beyond the diffraction limit and the polarization of the incident laser field. We examine such sub-resolved interparticle separation in trapped nanoaggregates by measuring surface-enhanced Raman scattering, and further demonstrate the single-molecule sensitivity by implementing such nanostructures. This new technique opens a new avenue for all-optical manipulation of nanomaterials as well as ultra-sensitive bio-chemical sensing applications.