Optical tweezers as sub-pico-newton force transducers

We have experimentally demonstrated the application of optical tweezers as a non-invasive force transducer. Specifically, we have measured the transverse trapping force (with sub-pico-Newton resolution) as a function of the lateral position of the trapped particle (with sub-micron resolution) relative to the optical axis. The trapping force was measured by the conventional dragging technique, whereas the position of the trapped particle was determined by monitoring with a charged coupled device camera the forward scattering pattern of a weak auxiliary probing beam. The experimental results are in excellent agreement with the theory based on the ray-optics model for optical trapping of Mie particles. (C) 2001 Elsevier Science B.V. All rights reserved.