Characterizing dielectric surfaces by detecting electron tunneling. An apparatus includes an atomic force probe. A mechanical actuator is connected to the atomic force probe. A mechanical modulator is connected to the mechanical actuator. The mechanical modulator modulates the mechanical actuator and the atomic force probe at the resonant frequency of the atomic force probe. An electrical modulator is connected to the atomic force probe. A feedback sensing circuit is connected to the mechanical modulator to detect movement of the atomic force probe and provide information about the movement of the atomic force probe to the mechanical modulator allowing the mechanical modulator to modulate the atomic force probe at the resonant frequency of the atomic force probe as the resonant frequency of the atomic force probe changes. An FM detector is connected to the feedback circuit detects changes in the resonant frequency of the atomic force probe.