Variable Repetition Rate THz Source for Ultrafast Scanning Tunneling Microscopy

Abstract

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.

Lightwave-driven scanning tunnelling spectroscopy of atomically precise graphene nanoribbons

“…As further corroboration of our model and the fidelity of the PES waveform, and as a test for multielectron and dynamic effects, we perform autocorrelation measurements 8 , 9 , 11 , 12 , 16 , 21 . The terahertz pulses are split in half, a delay is introduced, and Q THz is recorded as a function of delay time for three different E THz,pk values at both the anti-node and node (Supplementary Figure 6 ).…”

“…4 for further details). As terahertz field enhancement is affected by the microscopic tip profile 17 , 21 —which must be carefully prepared over the substrate for high-resolution THz-STM imaging—we use PES to measure the waveform shape but determine the terahertz voltage calibration from THz-STM and THz-STS. We select between an even or odd number of terahertz-pulse reflections before coupling the pulses to the tip, and hence invert the polarity of the terahertz voltage transient at the apex without changing its temporal shape or amplitude, as confirmed by PES in Figs.…”

“…To simulate Q THz − E THz,pk curves we map onto the conductance determined by the model d I /d V where is modeled by a normalized waveform measured with PES, multiplied by a scalar coefficient, . The peak bias applied to the junction, V pk , is a product of the field strength of the incident terahertz pulse and a voltage calibration factor that scales with field enhancement at the microscopic tip apex 17 , 21 . For height-dependent modelling we assume that the voltage calibration is constant at all tip-sample distances, unless a tip change is experimentally observed.…”

“…Scanning probe techniques that visualize orthogonal material properties on the atomic scale are also emerging, including electro- and photoluminescence STM 3 , 4 , electron spin resonance STM 5 , and tip-enhanced Raman microscopy 6 , 7 . Lightwave-driven STM 8 – 21 opens another dimension: atomically resolved microscopy on ultrafast timescales. It has enabled femtosecond pump-probe experiments 8 , 9 , 13 , 19 , field-driven electroluminescence 20 , and the application of atomic-scale ultrafast forces 15 , 17 .…”

“…It has enabled femtosecond pump-probe experiments 8 , 9 , 13 , 19 , field-driven electroluminescence 20 , and the application of atomic-scale ultrafast forces 15 , 17 . Ultrafast fields can also operate in regimes inaccessible to conventional static STM fields, for example through lightwave-control of extreme tunnel currents 10 – 12 , 16 , 21 . The natural next step is to bring these capabilities to bear on novel material systems to inform design and synthesis.…”