DETECTION OF LOCAL CONDUCTIVITY BY LASER-FREQUENCY MIXING IN A SCANNING FORCE MICROSCOPE

Coupling of laser radiation into a conducting tip of a scanning force microscope allows one to distinguish between conducting and nonconduct ing parts of a sample. This is demonstrated for a pattern of small met al islands on a nonconducting BaF2 substrate. In the experiment two in frared laser beams are coupled into the tip. The difference frequency is generated in the tip-sample junction, emitted, and detected by mean s of an open waveguide. Images with this signal are recorded simultane ously with the topography. Difference-frequency generation is observed only on conducting parts of the surface and at islands larger than ab out 1 mum in diameter. The size of the conducting island as well as th e tunneling distance between the tip and conducting surface determine the magnitude of the difference-frequency signal. Frequency mixing of visible laser light, using the excitation of localized plasmons for fi eld enhancement, may lead to the detection of local conductivity on mu ch smaller structures.