Generation of a frequency comb and applications thereof

An apparatus for characterizing a semiconductor sample comprising:

• a scanning tunneling microscope comprising:
• control electronics;
• a source for generating a DC bias voltage;
• a tunneling junction having a chosen diameter, a DC current generated from said DC bias voltage, and a shunting capacitance, said tunneling junction being effective for producing optical rectification, said tunneling junction comprising a tip electrode and a semiconductor sample electrode having a surface disposed a chosen distance therefrom and a band gap energy;
• and
• a scanner for adjusting the chosen distance and for rastering said tunneling junction over the surface of said semiconductor electrode;
• and
• a mode-locked laser having a pulsed output having a chosen mean photon energy less than the band gap energy focused onto said tunneling junction with a chosen repetition rate;
• whereby pulses having a chosen spacing are superimposed on the DC current of said tunneling junction such that a microwave frequency comb is produced having a fundamental frequency and harmonic frequencies at integer multiples of the pulse repetition rate of said laser;
• and whereby the bias voltage creates a depletion region in said semiconductor sample causing thereby a frequency-dependent attenuation of said microwave frequency comb at each tunneling junction location, from which dopant concentration of the depletion region is determined.