Paper SP-TuP8
Periodically-pulsed Laser-Assisted Tunneling May Generate Terahertz Radiation

Tuesday, November 8, 2016, 6:30 pm, Room Hall D

Background: Periodic excitation of the tunneling junction in a scanning tunneling microscope by a mode-locked ultrafast laser superimposes a frequency comb at harmonics of the pulse repetition frequency on the DC tunneling current.¹ The power measured at the first 200 harmonics (74.254 MHz to 14.85 GHz) varies inversely with the square of the frequency—decaying only due to shunting by the stray capacitance.

Hypothesis: The tunneling junction is much smaller than the laser wavelength so effectively the laser superimposes a time-dependent voltage on the DC bias. Quasi-static conditions cause the time-dependent tunneling current to be related to the time-dependent voltage by a cubic polynomial as in the DC case. Thus, the waveform of the current in the tunneling junction is similar to the envelope of the laser radiation.

Analysis: The time-dependent voltage is modeled as a random process including pulse-jitter and finite coherence length of the laser. The current in the tunneling junction is shown to be a wide-sense stationary random process. For a laser with a pulse-width of 15 fs and pulse repetition frequency of 74.254 MHz the power spectral density in the tunneling junction has an intrinsic decay of 3 dB at the 2.4 x10⁵th harmonic of 18 THz. The power measured at the first harmonic corresponds to a peak current of 5.7 nA. But the frequency of this harmonic is low enough that the decay caused by stray capacitance is negligible so this value, adjusted for the intrinsic decay, is the peak current at each harmonic in the tunneling junction.

Results and conclusions: Under the conditions for our measurements of the frequency comb we predict that in the tunneling junction the peak current for each pulse is approximately 690 mA. This value would be higher with a laser having greater coherence length or lower timing-jitter. It appears that the finest spatial resolution so far achieved in terahertz imaging is 40 nm by the near-field confinement of plane-wave illumination at a conical metal tip.² Our simulations suggest it may be possible to achieve atomic resolution by using the terahertz radiation at the tunneling junction in periodically-pulsed laser-assisted scanning tunneling microscopy. Much higher power is expected in periodically-pulsed laser-assisted field emission because of the greater current and much lower stray capacitance.

References

¹M. J. Hagmann, A. J. Taylor, and D. A. Yarotski, Appl. Phys. Lett. 101, 241102 (2012).

²A. J. Huber, F. Keilmann, J. Wittborn, J. Alzpurua, and R. Hillenbrand, Nano Lett. 8, 3766-3770 (2008).