Characterization of precise force sensing by optically-levitated microspheres

Optically levitated micro- and nano-spheres have been used for various purposes from a precise force sensor to a quantum mechanical superposition in mesoscopic systems. We have constructed a novel system to trap a 2.4 $\mu$m radius microsphere by a single upward-propagating laser beam. Position sensing based on interferometry enables us to apply feedback cooling and detect the microsphere position with a single trapping beam. We present the characterization of a precision force sensor with force sensitivity of $\sim10^{-17}$ N/$\sqrt{\rm Hz}$ for all of the three translational degrees of freedom. This force sensor is a promising system for the search for non-Newtonian gravity at the distance scale of 1-100 $\mu$m range.