LASER-FREQUENCY MIXING IN A SCANNING FORCE MICROSCOPE AND ITS APPLICATION TO DETECT LOCAL CONDUCTIVITY

When two infrared laser beams are coupled into the tip of a scanning t unneling microscope, the difference frequency is generated in the tunn eling junction and can be observed via an emitted wave. In order to ob tain a detectable signal also in the case of a force microscope, a loc ally conducting area has to be opposite to the tip. This allows one to distinguish between conducting and nonconducting regions of a sample when a force microscope is used for the experiments. The difference-fr equency signal is found to decrease with decreasing size of conducting islands. The detection is limited at present to islands larger than a bout 1 mum in diameter. A simple model for the difference-frequency ge neration in the tip-sample junction is presented. From this model, imp rovements of the method can be derived which lead to the possibility t hat still smaller structures can be investigated.