Optically trapped nanospheres for scanning force sensing at sub-μm-distances from a surface*

Optically levitated nanoparticles in vacuum are extremely well-decoupled from the environment, making them a powerful tool for precision measurement experiments. We trap a ~170 nm diameter silica nanoparticle in a single-beam tweezer trap and transfer it into a standing wave potential by retro-reflecting a laser beam from a gold-coated silicon mirror surface. In the transfer process, we reliably position the nanoparticle at distances of a few hundred nanometers to tens of microns from the conductive surface and use a piezo-driven mirror to scan the two dimensional space parallel to the mirror surface, achieving attonewton level force sensing at modestly low pressures. This method enables 3-D scanning force sensing near surfaces using optically trapped nanoparticles, promising for high-sensitivity scanning force microscopy, tests of the Casimir effect, and tests of the gravitational inverse square law at micron scales.