Variable Repetition Rate THz Source for Ultrafast Scanning Tunneling Microscopy

Area of Science:

• Physics
• Materials Science
• Nanotechnology

Background:

• Broadband terahertz (THz) pulses are crucial for ultrafast electronic transport studies on the nanoscale.
• Coupling THz electric fields into devices requires specialized antennas, asperities, or scanning probe tips.

Purpose of the Study:

• To design a versatile THz source optimized for driving the highly resistive tunnel junction of a scanning tunneling microscope (STM).
• To achieve arbitrary THz pulse trains with adjustable repetition rates and high peak amplitudes for nanoscale experiments.

Main Methods:

• Utilized optical rectification in lithium niobate to generate THz pulse trains.
• Achieved freely adjustable repetition rates from 0.5 to 41 MHz.
• Induced subpicosecond voltage transients with peak amplitudes from 0.1 to 12 V in the STM tunnel junction.

Main Results:

• Demonstrated a conversion efficiency of 0.4 V/(kV/cm) from THz electric field to STM junction voltage.
• Successfully detected tunnel currents in the quantum limit (less than one electron per THz pulse) at multi-MHz repetition rates.
• Tuned the source for a balance between high pulse energy and high signal fidelity.

Conclusions:

• The designed THz source is effective for exploring ultrafast and atomic-scale electron dynamics.
• This versatile THz source advances nanoscale electronic transport experiments.
• Enables sensitive detection of quantum-limited electron currents at high repetition rates.