Scanning force sensing at sub-μm-distances from a surface using optically trapped nanospheres

In vacuum, optically levitated dielectric nanoparticles 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 surface and use a piezo-driven mirror to scan the two dimensional space parallel to the mirror surface, achieving attonewton level force sensing at room temperature and moderate vacuum. This method enables three-dimensional 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.