OPTICAL MIXING OF CO2-LASER RADIATION IN A SCANNING TUNNELING MICROSCOPE

Two infrared laser beams coupled into the tunneling junction of a scanning tunneling microscope lead to the generation of a signal at the difference frequency. In this article it is described that two different frequency mixing mechanisms are responsible for this process. One part of the signal is generated through a mixing process owing to the nonlinearity in the static current-voltage characteristic. Another part has its origin in a nonlinear susceptibility at the surface; it therefore corresponds to frequency mixing in nonlinear optics. It will be shown that the difference-frequency signals generated by the two processes can be separated owing to their different dependence on the tip-sample distance. (C) 1995 American Institute of Physics.