Subfemtonewton Force Spectroscopy at the Thermal Limit in Liquids
  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. 
Lulu Liu, Simon Kheifets, Vincent Ginis, and Federico Capasso
physics.optics