Extended Data Fig. 2: Simulation of instantaneous current and lightwave-driven spectra.

a, The near-field waveform measured with photon-assisted emission currents^34,36 acts as transient bias voltage \({\mathcal{V}}\)_LW (t) in the junction (grey curve). Time values correspond to the mutual delay between THz and optical pulse driving photon-assisted current. The blue curve shows the steady-state I(V) curve measured on the orbital lobe. b, The instantaneous current \({{\mathcal{I}}}_{{\rm{LW}}}\left(t\right)=I({{\mathcal{V}}}_{{\rm{LW}}}\left(t\right))\) determined by solving the rate equation with 5 fs lifetime in the limit of single-electron tunnelling per THz pulse is shown (green curve) for the near-field transient in a with its peak field tuned to the first defect level (280 mV, green dashed line in a). \({\mathcal{I}}\)_LW (t) has a full width at half maximum (FWHM, grey area) of 296 fs, conventionally understood as the temporal resolution of LW-STM at a given peak field^30,31. c, The temporal resolution simulated with the rate equation model exhibits tunnelling windows of ~300 fs around D₁. d, The simulated instantaneous current for \({V}_{{\rm{LW}}}^{\,{\rm{peak}}}\) = 500 mV (pink curve, dashed line in a), based on the rate equation model, rises steeply at the crest of the bias pulse owing to the additional tunnelling channel through D₂. e-g, The peak field in the junction is calibrated by comparing the appearance of D₁ in the unpumped spectrum (data, black; data and error bars: see Fig. 1d, Se vacancy) with that in the steady-state spectrum (blue) (e), in the simulated spectrum based on the instantaneous tunnelling model (f) and the simulated spectrum based on the rate equation model (g). The calibration yields a peak field of 2.7 V, 3.2 V and 3.2 V, respectively, or (3.0 ± 0.3) V on average, consistent with photon-assisted measurements of the near-field waveform. h, The instantaneous tunnelling model without (light blue) and with taking reflections of the transient (dark blue) in the setup into account, exhibits an almost unchanged position of D₂ in the spectrum.