Extended Data Fig. 3: Imposing a unipolar strong-field-induced current pulse with a d.c. bias voltage offset and qualitative waveform verification using terahertz pulse autocorrelations.
a,b, Terahertz pulse autocorrelations at VSF,pk = +13 V (red line) and VSF,pk = –13 V (blue line) with Vd.c. = 0 V, z = z0 (a) and Vd.c. = ±8 V, z = z0 + 0.9 nm (b). The left inset in a shows a set of shifted and unshifted QTHz–ESF,pk curves for the voltage calibration with this tip apex, acquired at Vd.c. = +5 V (orange circles) and Vd.c. = +8 V (purple line) and a tip height of z = z0 + 1.2 nm. The right insets in a and b are a zoom into the central region of the autocorrelation (dashed black line). c, THz-CC waveform measured at Vd.c. = 0 V, VSF,pk = +13 V and VWF,pk = –0.40 V. d, THz-CC waveforms measured at VWF,pk = +0.40 V (light blue line) and VWF,pk = +0.08 V (dark blue circles) with Vd.c. = 0 V and VSF,pk = +13 V. e,f, THz-CC waveforms measured with a d.c. bias of Vd.c. = ±2 V (red and blue lines) and Vd.c. = ±8 V (red and blue circles) applied during the measurement. The d.c. bias polarity was matched to the polarity of VTHz,pk: +9 V (light red line), +13 V (dark red circles), –10 V (light blue line) and –13 V (dark blue circles). The weak-field terahertz amplitude was –0.40 V for e and +0.40 V for f. All measurements were performed at constant height with the feedback disengaged at V0 = 20 mV, I0 = 200 pA on Au(111).
