Detection of local conductivity by laser-frequency mixing in a scanning force microscope

Abstract

Coupling of laser radiation into a conducting tip of a scanning force microscope allows one to distinguish between conducting and nonconducting parts of a sample. This is demonstrated for a pattern of small metal islands on a nonconducting BaF2 substrate. In the experiment two infrared laser beams are coupled into the tip. The difference frequency is generated in the tip-sample junction, emitted, and detected by means of an open waveguide. Images with this signal are recorded simultaneously with the topography. Difference-frequency generation is observed only on conducting parts of the surface and at islands larger than about 1 μm in diameter. The size of the conducting island as well as the 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 field enhancement, may lead to the detection of local conductivity on much smaller structures.