Ultrafast STM is an exciting new research direction that combines the atomic resolution of scanning tunneling microscopy (STM) with the sub-picosecond time resolution of fast optics. This is a new research area and so there is a wealth of physical systems to explore. The Crommie group is actively pursuing this research topic in collaboration with the Feng Wang group. Our emphasis is on exploring electronic, structural, and magnetic excitations in molecular systems and 2D materials (Fig. 1)
One difficulty in obtaining ultrafast information from an STM is the low bandwidth of tunnel current detection (which is typically limited to audio frequencies). One way around this limitation is to illuminate the STM tunnel junction area with fast optical pulses in a pump-probe configuration. If two closely-spaced pulses are incident on the STM then the transient STM tunnel current induced by the second pulse will depend on the delay time between the two pulses. By measuring the integrated STM tunnel current (i.e., the DC current) as a function of the pulse delay time it is possible to extract fast dynamical information at the site of the tip. The time resolution is then set by the width of the pulses and the delay between them (Fig. 2). This and related techniques have been used by STM practitioners to explore dynamical behavior in a variety of different physical systems.1-3
Our ultrafast STM system integrates pump/probe optics with a cryogenic ultrahigh vacuum STM. The laser source includes a mode-locked Ti:sapphire oscillator and a broadly tunable optical parametric oscillator capable of generating independently tunable output pulses (Fig. 3)
The Crommie and Wang groups have previously collaborated to develop a new technique for performing infrared (IR) spectroscopy on molecules using the tip of an STM (IRSTM).4,5 This technique involves illuminating an adsorbate-decorated surface with a frequency-tunable IR laser and measuring changes in tunnel current as the laser frequency is swept. When the laser is on-resonance with a molecular mode then the molecule absorbs optical energy and the molecule/surface expands. STM tunnel current is a very sensitive detector of this type of expansion and so can measure molecular IR resonances with high resolution (Figs. 4a, b). We implemented IRSTM by integrating a homemade tunable mode-hop-free IR laser with a cryogenic UHV STM (Fig. 4c). An IRSTM spectrum of tetramantane diamondoid molecules on Au(111) is shown in Fig. 4d. Six molecular vibrational modes are clearly observed (black curve). Comparison of these modes to a conventional STM inelastic tunneling spectrum (IETS) (blue curve) shows that IRSTM has an energy resolution that is better than IETS by a factor of ~30 at this temperature (T = 13 K).4
Analysis of IRSTM spectra using DFT-based first principles simulations provides insight into how molecule-molecule and molecule-substrate interactions affect molecular modes observed by IRSTM (Fig. 5). Comparison of theoretical calculations to our experimental IRSTM spectra shows that environmental interactions cause some vibrational modes of adsorbed molecules to significantly red-shift relative to isolated gas phase molecules (theory performed by the Cohen and Louie groups).5
1) Y. Terada, S. Yoshida, O. Takeuchi & H. Shigekawa,"Real-space imaging of transient carrier dynamics by nanoscale pump–probe microscopy", Nature Photonics 4, 869 (2010).
2) T. L. Cocker, D. Peller, P. Yu, J. Repp & R. Huber,"Tracking the ultrafast motion of a single molecule by femtosecond orbital imaging", Nature 539, 263 (2016).
3) Shaowei Li, Siyu Chen, Jie Li, Ruqian Wu, and W. Ho, "Joint Space-Time Coherent Vibration Driven Conformational Transitions in a Single Molecule", Phys. Rev. Lett., 119, 176002 (2017).
4) I. V. Pechenezhskiy, X. Hong, G. D. Nguyen, J. E. P. Dahl, R. M. K. Carlson, F. Wang & M. F. Crommie,"Infrared Spectroscopy of Molecular Submonolayers on Surfaces by Infrared Scanning Tunneling Microscopy: Tetramantane on Au(111)", Physical Review Letters 111, 126101 (2013).
5) Y. Sakai, G.D. Nguyen, R.B. Capaz, S. Coh, I.V. Pechenezhskiy, X. Hong, F. Wang, M.F. Crommie, S. Saito, S.G. Louie, and M.L. Cohen. Intermolecular interactions and substrate effects for an adamantane monolayer on a Au(111) surface. Phys. Rev. B 88, 235407 (2013).