Subfemtonewton Force Spectroscopy at the Thermal Limit in Liquids

TL;DR

This paper presents a method for thermally limited force spectroscopy in liquids using optically trapped microspheres, achieving sub-femtonewton resolution and validating results with theoretical calculations.

Contribution

It introduces a novel approach combining evanescent wave tracking and lock-in detection for high-resolution force measurements in liquids.

Findings

Force resolution below 1 fN in 100 seconds

Agreement with Mie theory for evanescent wave forces

Achieved 2 Å rms displacement sensitivity

Abstract

We demonstrate thermally limited force spectroscopy using a probe formed by a dielectric microsphere optically trapped in water near a dielectric surface. We achieve force resolution below 1 fN in 100 s, corresponding to a 2 {\AA} rms displacement of the probe. Our measurement combines a calibrated evanescent wave particle tracking technique and a lock-in detection method. We demonstrate the accuracy of our method by measurement of the height-dependent force exerted on the probe by an evanescent wave, the results of which are in agreement with Mie theory calculations.