When two infrared laser beams are coupled into the tip of a scanning tunneling microscope, the difference frequency is generated in the tunneling junction and can be observed via an emitted wave. In order to obtain a detectable signal also in the case of a force microscope, a locally 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-frequency signal is found to decrease with decreasing size of conducting islands. The detection is limited at present to islands larger than about 1 mum in diameter. A simple model for the difference-frequency generation in the tip-sample junction is presented. From this model, improvements of the method can be derived which lead to the possibility that still smaller structures can be investigated.