Coupling of terahertz radiations to tunneling junction: Quantitative determination of amplitude for weak fields

Lightwave-driven terahertz scanning tunneling microscopy has attracted considerable interest due to its ability to combine sub-ångström spatial resolution with sub-picosecond temporal resolution. Such measurements require accurate knowledge of the THz near-field transient generated upon coupling the far-field pulse into the junction. Conventional approaches rely on the onset of strong nonlinearity in current–voltage characteristics measured at constant tip–sample distance. Here, combining simulations with experiments exploiting the conduction-band onset of NaCl bilayers on Ag(111), we analyze the determination of the transient amplitude, with particular emphasis on the small-amplitude regime (≲0.5 V). We show that the intrinsic broadening of electronic nonlinearities significantly complicates amplitude extraction. We, therefore, introduce an alternative approach based on constant-current spectroscopy, in which the THz-induced current is measured as a function of sample voltage while the feedback loop maintains a fixed DC current. This method yields a pronounced spectral peak whose amplitude, width, and onset are all sensitive to the transient voltage amplitude, providing multiple independent observables for a more robust determination.