Non-fluorescent nanoscopic monitoring of a single nanoparticle trapped between two nonlinear point sources

Abstract

We propose and demonstrate a novel optical nanoscopy platform that can effectively trap the single nano-particle and transform any trapped linear non-fluorescent nano-particle into a self-illuminating nonlinear light source with minimal pump power. By using highly focused photon energy in a volume of $5\times5×\times7 nm^3$ through the three-dimensionally tapered plasmonic nanoantenna, a single 4.4-nm CdSe/ZnS quantum dot (QD) is trapped. Simultaneously, the strongly focused light into a metallic nanogap generates a strong nonlinear signal and the trapped QD is monitored by self-illuminating background-free 780-nm second-harmonic signals under a fs-laser pulse pumping. The optical trapping characteristics of a trapped nanoparticle is experimentally demonstrated by controlling the landscape of optical potential wells. In particular, the Kramers transition of QD is observed which is the motion in the double-well potential formed on the two metal surfaces when the nanoparticle is trapped at the nanoantenna. By controlling the shape of the double-well potential formed in the antenna, the Kramers transition rate is modulated. This material-free, label-free, background-free detection capability and smallness of the trapping potential of our proposed scheme could open a new platform for novel bio-optical nanoscopy.