Focused plasmonic trapping of metallic particles

TLDR

Scattering forces in focused light beams repel metallic particles, making conventional optical tweezers ineffective for Mie‑sized metallic particles, though they can manipulate dielectric particles. The study investigates how plasmonic tweezers, using a radially polarized beam to excite surface plasmons in a high‑NA microscope, attract and trap metallic particles, attributing this contrary behavior to distinct force contributions. Plasmonic tweezers employ a radially polarized beam focused by a high‑numerical‑aperture microscope to excite surface plasmons that attract metallic particles. Unlike conventional tweezers, plasmonic tweezers attract metallic particles because gradient and scattering forces act together, and theory, simulation, and experiments confirm trapping of particles up to 2.2 µm in diameter.

Abstract

Scattering forces in focused light beams push away metallic particles. Thus, trapping metallic particles with conventional optical tweezers, especially those of Mie particle size, is difficult. Here we investigate a mechanism by which metallic particles are attracted and trapped by plasmonic tweezers when surface plasmons are excited and focused by a radially polarized beam in a high-numerical-aperture microscopic configuration. This contrasts the repulsion exerted in optical tweezers with the same configuration. We believe that different types of forces exerted on particles are responsible for this contrary trapping behaviour. Further, trapping with plasmonic tweezers is found not to be due to a gradient force balancing an opposing scattering force but results from the sum of both gradient and scattering forces acting in the same direction established by the strong coupling between the metallic particle and the highly focused plasmonic field. Theoretical analysis and simulations yield good agreement with experimental results. Focused light beams can be used as optical tweezers for manipulating small dielectric particles, but they normally repel metallic ones. By exploiting surface plasmons excited by a radially polarized beam, Min et al.show that it is possible to trap metallic particles with diameters up to 2.2 μm.