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Tracking Motion of Atoms

Even though integration of ultrashort laser pulses with an STM by our group has shown its capability to capture electronic motion in a single molecule over very short time intervals, tracking and controlling the motion of atoms in a single molecule remains to be done, as one has to access much longer intervals of time while preserving high temporal resolution all over the way. This is, however, a crucial issue, since any chemical change in molecules or phase transformation in quantum materials is governed by the motion of their atomic constituents, which can be externally stimulated. This motion of atoms occurs on ultrafast time scales (tens to hundreds of femtoseconds) as revealed by nonlocal ultrafast spectroscopic measurements in bulk molecular ensembles and solids. Capturing directly the snapshots of atomic motion in a single molecule requires the unification of yet another technique, vibrational spectroscopy, in conjunction with the two techniques mentioned previously.


We have applied this technique to investigate the dynamics of vibrational wave packets in single graphene nanoribbons (GNRs), due to their potential interest in future quantum electronic devices.  We have shown that the motion of atoms (phonons) induced by broadband laser pulses in single GNRs can be probed by femtosecond coherent anti-Stokes Raman spectroscopy (CARS) performed in a scanning tunnelling microscope (STM). This allows us to determine not only the relaxation times of the coherently generated phonons, but more importantly to track and control the motion of atoms in single GNRs, which is shown to evolve on time scales as short as ~ 70 fs. We have demonstrate a new approach of launching and coherently controlling atomic motion in a determined single GNR with two delay-controlled broadband laser pulses.

Schematic of the time resolved broadband fs coherent anti-Stokes Raman spectroscopy (CARS) of a single graphene nanoribbon. Vibrational coherence generated by broadband pump (ωpu) and Stokes pulses (ωS), both ~ 50 fs long, is tracked by the probe pulses (ωpr, ~ 500 fs long), leading to anti-Stokes scattering (ωaS).

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