Raman spectroscopy of optical-trapped single particle using bull's eye nanostructure.

Surface-enhanced Raman spectroscopy (SERS) has enabled single nanoparticle Raman sensing with abundant applications in analytical chemistry, biomaterials, and environmental monitoring. Genuine single particle Raman sensing requires a cumbersome technique, such as atomic force microscopy (AFM) based tip-enhanced Raman spectroscopy; SERS-based single particle Raman sensing still collects an ensemble signal that samples, in principle, a number of particles. Here, we develop in situ Raman-coupled optical tweezers, based on a hybrid nanostructure consisting of a single bowtie aperture surrounded by bull's eye rings, to trap and excite a rhodamine-6G-dye-doped polystyrene sphere. We simulated a platform to ensure sufficient enhancement capability for both optical trapping and SERS of a single nanoparticle. Experiments with well-designed controls clearly attribute the Raman signal origin to a single 15-nm particle trapped at the center of a nanohole, and they also clarified the trapping and Raman enhancement role of the bull's eye rings. We claim Raman sensing of a smallest optically trapped particle.

Optical trapping using transverse electromagnetic (TEM)-like mode in a coaxial nanowaveguide.

Optical traps have emerged as powerful tools for immobilizing and manipulating small particles in three dimensions. Fiber-based optical traps (FOTs) significantly simplify optical setup by creating trapping centers with single or multiple pieces of optical fibers. In addition, they inherit the flexibility and robustness of fiber-optic systems. However, trapping 10-nm-diameter nanoparticles (NPs) using FOTs remains challenging. In this study, we model a coaxial waveguide that works in the optical regime and supports a transverse electromagnetic (TEM)-like mode for NP trapping. Single NPs at waveguide front-end break the symmetry of TEM-like guided mode and lead to high transmission efficiency at far-field, thereby strongly altering light momentum and inducing a large-scale back-action on the particle. We demonstrate, via finite-difference time-domain (FDTD) simulations, that this FOT allows for trapping single 10-nm-diameter NPs at low power.