Thermally Limited Force Microscopy on Optically Trapped Single Metallic Nanoparticles

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 in both the confined regime and the freely diffusing regime of the optical trap. In the latter, we measure induced displacements of 10⁻¹¹ m on the trapped nanoparticle spatially confined within less than 25 nm along the optical axis.