Fig. 2: Ultrashort laser-driven imaging of molecular orbitals.

a,b, Illustration of the laser-induced transitions at the tunnel junction. E(t) is the electric field at the tunnel junction. a, At a bias where the HOMO of PTCDA molecules rises close to the Fermi level of the tungsten nanotip (E_F,t), photons of the laser pulse can dipole couple the HOMO of the PTCDA molecule with the surface state of Au(111). b, When the Fermi levels of the tungsten nanotip and the Au(111) surface (E_F,s) are closely spaced, impinging photons can dipole couple either E_F,t with the LUMO of the PTCDA molecule (1) or the surface state of Au(111) with the LUMO of the PTCDA molecule (2). c, Numerical simulation showing the temporal evolution of the population of the electronic states (\(\left| 1 \right\rangle\) and \(\left| 2 \right\rangle\))—which are separated by ~1.5 eV—on exposure to a resonant 6 fs laser pulse of 1 × 10¹² W cm^–2 intensity, for a transition dipole moment of 0.15 a.u. along the nanotip axis direction (Supplementary Sections 1 and 8). d–h Spatially resolved topographic scans of a single-pulse-driven tunnelling current in a monolayer of PTCDA molecules on top of Au(111) at various biases at the STM junction, −1.8 V (d), –20 mV (e), –200 mV (f), 1.4 V (g) and 2.2 V (h). Steady-state STM imaging was performed in the constant-current mode with a set-point value of I_d.c. = 200 pA. White dashed ellipses in f are to guide the eyes of the reader to the intensity of the tunnelling current present in between the molecules. Scale bars, 1 nm. The reproducibility of data in d–h was confirmed in eight individual measurements.