The change in position of fluorescent beads captured inside the focal volume of optical tweezers is
monitored using fluorescence emission induced by two-photon absorption of a continuous-wave
Nd:YAG laser ~l51064 nm!. The displacement of a bead due to interactions with its environment
leads to a fluorescence intensity variation that is used to design a novel spatial sensor. We determine
changes in the axial position of a CY3-labeled latex bead with a diameter of 1.03 mm to a precision
better than 10 nm. At an intensity of 600 mW/ mm2 the two-photon bleaching rate is lower than 50%
per 2000 s.