Fig. 1: Ultrafast photon-induced electron tunnelling.
From: Real-space subfemtosecond imaging of quantum electronic coherences in molecules
a, Laser pulses with a very small offset in their carrier frequencies (<1 kHz) are generated by selecting the zeroth-order (\(\textbf E _1\)) and first-order (\(\textbf E _2\)) diffracted beam of laser pulses traversing through an acousto-optic frequency shifter (AOFS). b, A schematic of the experimental set-up is shown: a pair of orthogonally polarized CEP-stable near-infrared laser pulses at slightly different carrier frequencies—f1 (for E1) and f1 + f0 (for E2)—are focused at the apex of tungsten nanotip in tunnel contact with a monolayer of PTCDA molecules on top of a Au(111) surface. The top of the inset shows the molecular structure of a PTCDA molecule and the bottom shows the polarization axes of the two ultrashort pulses with respect to the axis of the nanotip. c, Nonlinear cross-correlation of the laser-induced tunnelling current measured on a clean Au(111) surface by orthogonally polarized laser pulses (\(\textbf E _1(f_1)\) and \(\textbf E _2(f_1 + f_0)\)) at the STM junction. The variation in the position of the z-piezo in tunnel contact (Δz, orange curve, right axis) during the cross-correlation measurement is shown. The bias and set-current at the STM (constant-current mode) junction are –1 V and 200 pA, respectively. The error bar indicates the standard deviation of the mean value from five consecutive measurements. The envelope of the cross-correlation trace is shown by the dashed green curve. d, Measured variation of the photon-induced tunnelling current as a function of rotating polarization of the laser pulses with respect to the tip-axis (0°, dashed black line) for \(\textbf E _1\), \(\textbf E _2\) and a homodyne beating signal between them. e, Variation of the laser-induced tunnelling current measured on a PTCDA molecule over a Au(111) surface as a function of the bias (ILaser–V curve). f, Comparison of the variation of the laser-induced tunnelling current and the d.c. tunnelling current as a function of the (increasing) tunnel gap width (I–Δz curve). The I–Δz curve was measured on a Au(111) surface with a d.c. bias of 1 V and a set-point current of 200 pA. The dashed portion of the red curve indicates the noise floor.
