Laser field enhancement at the scanning tunneling microscope junction measured by optical rectification

In this letter we report the measurement of the field enhancement at the tip of a scanning tunneling microscope, by means of the detection of the optical rectification current. A field enhancement factor between 1000 and 2000 is obtained for highly oriented pyrolytic graphite and between 300 and 600 for gold. Field enhancement factors found are strongly dependent on the particular tip used. The magnitude of the emitted light at the field enhanced region, calculated from the measured optical voltage, could be easily detected by a simple photodiode.