Laser-frequency mixing in a scanning tunneling microscope at 1.3 mu m

The radiation of two single-mode diode lasers at 1.3 mu m is focused into t he tunneling junction of a scanning tunneling microscope, and gigahertz dif ference-frequency signals radiated from the tip are detected. Simultaneous measurements of the bias-voltage dependence of the mixing signal and the tu nneling current for different surface samples show that the mixing process is due to the nonlinearity of the static current-voltage characteristic of the tunneling junction. The coupling of the laser radiation into the juncti on conforms to antenna theory. The experimental results are compared with p revious measurements at a laser wavelength of 9.3 mu m. Surface images prod uced by means of the difference-frequency signal show the chemical contrast between micron-sized Au islands and a graphite substrate. (C) 1999 America n Institute of Physics. [S0021-8979(99)08109-8].