Switching and measuring a force of 25 femtoNewtons with an optical trap

An experiment is described, where an ultra-weak force of 25 femtoNewtons (fN = 10-15 N) displaces a 0.53microm latex bead captured in a soft optical trap. The displacement is measured by exploiting the phase anomaly of the trapping beam, resulting in a small phase shift of the scattered light. The 25fN force stems from the radiation pressure of a nearly collimated second laser beam, which is switched by an accousto-optical modulator. To the best of my knowledge, this is the smallest switchable force which has ever been directly measured.