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

Two infrared laser beams coupled into the tunneling junction of a scan ning tunneling microscope lead to the generation of a signal at the di fference frequency. In this article it is described that two different frequency mixing mechanisms are responsible for this process. One par t of the signal is generated through a mixing process owing to the non linearity in the static current-voltage characteristic. Another part h as its origin in a nonlinear susceptibility at the surface; it therefo re corresponds to frequency mixing in nonlinear optics. It will be sho wn that the difference-frequency signals generated by the two processe s can be separated owing to their different dependence on the tip-samp le distance. (C) 1995 American Institute of Physics.