Broadband THz pulses enable ultrafast electronic transport experiments
on the nanoscale by coupling THz electric fields into the devices
with antennas, asperities, or scanning probe tips. Here, we design
a versatile THz source optimized for driving the highly resistive
tunnel junction of a scanning tunneling microscope. The source uses
optical rectification in lithium niobate to generate arbitrary THz
pulse trains with freely adjustable repetition rates between 0.5 and
41 MHz. These induce subpicosecond voltage transients in the tunnel
junction with peak amplitudes between 0.1 and 12 V, achieving a conversion
efficiency of 0.4 V/(kV/cm) from far-field THz peak electric field
strength to peak junction voltage in the STM. Tunnel currents in the
quantum limit of less than one electron per THz pulse are readily
detected at multi-MHz repetition rates. The ability to tune between
high pulse energy and high signal fidelity makes this THz source design
effective for exploration of ultrafast and atomic-scale electron dynamics.
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