Laser-frequency mixing in a scanning tunneling microscope at 1.3 μm

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Abstract
The radiation of two single-mode diode lasers at 1.3 μm is focused into the tunneling junction of a scanning tunneling microscope, and gigahertz difference-frequency signals radiated from the tip are detected. Simultaneous measurements of the bias-voltage dependence of the mixing signal and the tunneling 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 junction conforms to antenna theory. The experimental results are compared with previous measurements at a laser wavelength of 9.3 μm. Surface images produced by means of the difference-frequency signal show the chemical contrast between micron-sized Au islands and a graphite substrate.