Extended Data Fig. 3: Terahertz field calibration.

The probability of switching triggered by individual terahertz pulses is measured as a function of terahertz field strengths (blue data points). The field strength is controlled by transmitting the pulses through a pair of crossed polarizers and plotted on a linear scale with initially undetermined scaling factor. As the field increases, the peak field of the pulses reaches the LUMO resonance. Assuming a linear relationship between the current resulting from tunnelling through the LUMO and the switching probability, the observed onset in switching probability with increasing field should roughly resemble the onset of the tunnelling current with increasing bias voltage in time-integrated scanning tunnelling spectroscopy. The Gaussian-shaped LUMO-related peak in time-integrated differential conductance (dI/dV) spectra translates into an error-function-shaped onset of current with increasing voltage. Indeed, an error function (erf) fits the experimental data very well (blue fit curve), the derivative of which (red curve, normalized to the peak) reproduces the equivalent of a LUMO-related peak in dI/dV spectra. Time-integrated scanning tunnelling spectroscopy shows that the LUMO resonance is centred at a voltage of 1.2 V, allowing for a calibration of the applied terahertz peak fields in terms of the transient’s peak voltage.