Abstract
Unification of the techniques of ultrafast science and scanning tunneling microscopy (STM) has the potential of tracking electronic motion in molecules simultaneously in real space and real time. Laser pulses can couple to an STM junction either in the weak-field or in the strong-field interaction regime. The strong-field regime entails significant modification (dressing) of the tunneling barrier of the STM junction, whereas the weak-field or the photon-driven regime entails perturbative interaction. Here, we describe how photons carried in an ultrashort pulse interact with an STM junction, defining the basic fundamental framework of ultrafast photon-induced tunneling microscopy. Selective dipole coupling of electronic states by photons is shown to be controllable by adjusting the DC bias at the STM junction. An ultrafast tunneling microscopy involving photons is established. Consolidation of the technique calls for innovative approaches to detect photon-induced tunneling currents at the STM junction. We introduce and characterize here three techniques involving dispersion, polarization, and frequency modulation of the laser pulses to lock-in detect the laser-induced tunneling current. We show that photon-induced tunneling currents can simultaneously achieve angstrom scale spatial resolution and sub-femtosecond temporal resolution. Ultrafast photon-induced tunneling microscopy will be able to directly probe electron dynamics in complex molecular systems, without the need of reconstruction techniques.
Highlights
Unification of the techniques of ultrafast science and scanning tunneling microscopy (STM) has the potential of tracking electronic motion in molecules simultaneously in real space and real time
The quest to achieve atomic resolution both in real space and real time simultaneously has led to numerous efforts of integration of ultrashort laser pulses with a scanning tunneling microscope (STM) over the past 30 years.[1−16] Integration of high-energy, low-repetitionrate optical pulses at the STM junction leads to thermal instabilities, i.e., periodic contraction and expansion of the nanotip.[17,18]
The photon-driven tunneling regime is the weak-field regime, implying that incident photons gently perturb the system without distorting the potential energy landscape of the STM junction, which would be the case in the strong-field regime
Summary
Unification of the techniques of ultrafast science and scanning tunneling microscopy (STM) has the potential of tracking electronic motion in molecules simultaneously in real space and real time. The quest to achieve atomic resolution both in real space and real time simultaneously has led to numerous efforts of integration of ultrashort laser pulses with a scanning tunneling microscope (STM) over the past 30 years.[1−16] Integration of high-energy, low-repetitionrate optical pulses at the STM junction leads to thermal instabilities, i.e., periodic contraction and expansion of the nanotip.[17,18] This undesirable artifact leads to a dramatic reduction of the spatial resolving capability of the STM, besides generating an artificial laser-induced tunneling current. Experiments performed using the pioneering technique of shaken-pulse-pair excitation[1,20,21] (SPPX) have shown that the time scale of thermal effects, i.e., expansion or contraction of the nanotip of the STM on integration of low-energy optical
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