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 transve rse trapping force (with sub-pico-Newton resolution) as a function of the l ateral position of the trapped particle (with sub-micron resolution) relati ve to the optical axis. The trapping force was measured by the conventional dragging technique, whereas the position of the trapped particle was deter mined by monitoring with a charged coupled device camera the forward scatte ring pattern of a weak auxiliary probing beam. The experimental results are in excellent agreement with the theory based on the ray-optics model for o ptical trapping of Mie particles. (C) 2001 Elsevier Science B.V. All rights reserved.