Figure 1.

(a) Experimental scheme and (b) junction diagram for sampling of the THz voltage transient applied to the STM. The THz electric field emitted from the spintronic emitter (STE) acts as an instantaneous bias that modulates the photocurrent generated by 8 fs NIR laser pulses. Depending on the barrier height and distance d, the photocurrent originates from above-barrier photoemission and photoassisted tunneling through the barrier. (c) Laser-induced photocurrent–voltage characteristics and (d) tip-enhanced THz waveforms sampled at 8 V DC bias (d = 1 μm, E_p,STE = 0.6 μJ) for positive and negative THz polarity (STE magnetization M+/M−). The THz voltage (right y-axis) is retrieved from the THz-induced photocurrent change (left y-axis) via the linear I–V slope (green dashed line in c). The power law scaling of the photocurrent (inset in (c), U_DC = 8 V) reveals the operation in the multiphoton photoemission regime.