# Thermally limited force microscopy on optically trapped single metallic nanoparticles

## TL;DR

This paper introduces a new optical force microscope using a standing wave trap to measure forces on metallic nanoparticles with high sensitivity and minimal heating, achieving femtonewton resolution in water at room temperature.

## Contribution

It presents a novel in-situ calibrated optical force microscope based on standing wave trapping capable of high-resolution force measurements on metallic nanoparticles.

## Key findings

- Achieved 3 fN force resolution in water at room temperature.
- Demonstrated stable operation with 0.1 Hz bandwidth.
- Measured nanoparticle displacements of 10^{-11} m within 25 nm confinement.

## Abstract

We propose and evaluate a new type of optical force microscope based on a standing wave optical trap. Our microscope, calibrated in-situ and operating in a dynamic mode, is able to trap, without heating, a single metallic nanoparticle of 150 nm that acts as a highly sensitive probe for external radiation pressure. An Allan deviation-based stability analysis of the setup yields an optimal 0.1 Hz measurement bandwidth over which the microscope is thermally limited. Over this bandwidth, and with a genuine sine-wave external drive, we demonstrate an optical force resolution down to 3 fN in water at room temperature with a dynamical range for force detection that covers almost 2 orders of magnitude. This resolution is reached both in the confined and freely diffusing regimes of the optical trap. In the latter, we measure $10^{-11}$ m induced displacements on the trapped nanoparticle, spatially confined within less than 25 nm along the optical axis.