Surface imaging beyond the diffraction limit with optically trapped spheres.

Optical traps play an increasing role in the bionanosciences because of their ability to apply forces flexibly on tiny structures in fluid environments. Combined with particle-tracking techniques, they allow the sensing of miniscule forces exerted on these structures. Similar to atomic force microscopy (AFM), but much more sensitive, an optically trapped probe can be scanned across a structured surface to measure the height profile from the displacements of the probe. Here we demonstrate that, by the combination of a time-shared twin-optical trap and nanometre-precise three-dimensional interferometric particle tracking, both reliable height profiling and surface imaging are possible with a spatial resolution below the diffraction limit. The technique exploits the high-energy thermal position fluctuations of the trapped probe, and leads to a sampling of the surface 5,000 times softer than in AFM. The measured height and force profiles from test structures and Helicobacter cells illustrate the potential to uncover specific properties of hard and soft surfaces.