Quantifying the Role of the Surfactant and the Thermophoretic Force in Plasmonic Nano-optical Trapping

Plasmonic nano-optical trapping has revolutionized the use of optical tweezers, and a lot of attention has been devoted to the optical gradient force. However, the metal absorbs part of the incident light, leading to a temperature gradient responsible for an additional thermophoretic force which remains largely overlooked in plasmonic trapping.

In a recent Nano Letters publication, we experimentally disentangle the thermophoretic force contribution from the optical gradient force in double-nanohole plasmonic trapping. Our approach uses different surfactants which allow to tune the thermophilic or thermophobic response of the nanoparticles and set the relative strength of the thermophoretic force. This uniquely demonstrates that the choice of the surfactant can play a determining role in the outcome of the plasmonic trapping experiment.

Significance:

- Our method to measure separately the thermal and the optical forces is very general and can be easily extended to other plasmonic designs.

- The nano-optical trap performance can be significantly improved by properly choosing the surfactant to take advantage of the thermophoretic force.

Also freely available on Arxiv 2011.10263