The authors demonstrate a method to probe thermal diffusion at megahertz frequencies with nanometer lateral resolution in a thin opaque film on a transparent substrate. They map photothermally induced megahertz surface vibrations in an atomic force microscope using tightly focused optical illumination from the substrate side. By comparison with a theoretical model of the surface displacement field, the authors derive the thermal diffusivity of a thin chromium film on a silica substrate. ©2007 American Institute of Physics