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

TL;DR

This study investigates how surfactants influence the balance of optical and thermophoretic forces in plasmonic nano-optical trapping, revealing that surfactants can significantly enhance trap stiffness and offering insights for optimizing nano-tweezers.

Contribution

It demonstrates the impact of different surfactants on thermophoretic forces in plasmonic trapping, providing new guidelines for trap optimization.

Findings

SDS increases trap stiffness by 20x compared to Triton X-100.

Surfactants affect the thermophilic or thermophobic response of nano-objects.

Disentangling optical and thermophoretic forces improves trap performance understanding.

Abstract

Plasmonic nano-tweezers use intense electric field gradients to generate optical forces able to trap nano-objects in liquids. However, part of the incident light is absorbed into the metal, and a supplementary thermophoretic force acting on the nano-object arises from the resulting temperature gradient. Plasmonic nano-tweezers thus face the challenge of disentangling the intricate contributions of the optical and thermophoretic forces. Here, we show that commonly added surfactants can unexpectedly impact the trap performance by acting on the thermophilic or thermophobic response of the nano-object. Using different surfactants in double nanohole plasmonic trapping experiments, we measure and compare the contributions of the thermophoretic and the optical forces, evidencing a trap stiffness 20x higher using sodium dodecyl sulfate (SDS) as compared to Triton X-100. This work uncovers an important mechanism in plasmonic nano-tweezers and provides guidelines to control and optimize the trap performance for different plasmonic designs.