Evanescent Optothermoelectric Trapping: Deeper Potentials at a Largescale

Abstract

Surface plasmons (SP) and their mediated effects have been widely used to manipulate micro- and nanoscale objects of dielectric and metallic nature. In this work, we show how SP excitation can be used to induce thermofluidic and thermoelectric effects to manipulate colloidal dynamics on a large scale. In an evanescent plasmonic trap, temperature gradients induce a fluid flow that can facilitate particle accumulation. However, large out-of-plane flows expel particles from the trap, resulting in a shallow trap potential. Here, we numerically demonstrate how adding thermoelectric fields can overpower the optical and hydrodynamic forces to achieve a stable nanoparticle assembly at low excitation powers. We calculate the corresponding optical, fluidic, and thermoelectric trapping forces and potentials. These potentials can be enabled with nonresonant SP excitation and do not require careful optical alignment. An experimental validation of the evanescent OTE trap demonstrates a compact assembly of colloids, implying deeper potentials. Thus, we explain the mechanism of how, despite weak optical intensities and forces, a sufficient trapping force can be supplied via the evanescent optothermoelectric trap to obtain large-scale reversible nanoparticle assemblies, irrespective of their shape, size, or material.