Optically-trapped nanoparticles as scanning force sensors for conductive surfaces at sub-micron distances*

We describe a method for  3-D scanning force sensing near a conductive surface with a levitated silica nanoparticle.  Levitated nanoparticles are well decoupled from the environment, making them ideal precision sensors. 3-D positioning of a levitated nanosphere near a conductive surface could enable attonewton-level scanning force microscopy, precision tests of Yukawa-type corrections to gravity at micron distances, measurement of Casimir forces and characterization of patch potentials. We trap a ~170 nm  diameter silica nanosphere in an optical tweezer trap, and transfer it into an optical lattice by introducing a gold-coated silicon surface to retroreflect the laser beam. The nanosphere can be trapped from a quarter of the laser's wavelength to tens of microns away from the conducting surface, and a piezo-driven mirror allows us to scan in 2-D parallel to the surface while maintaining attonewton-level force sensing.