Speaker
Description
Coherent phonon spectroscopy is a powerful tool to monitor ultrafast lattice dynamics in solids under nonequilibrium conditions and provide insight into microscopic inter¬actions that govern macroscopic material properties. In spatially inhomogeneous materials, the excitation and relaxation of coherent phonons will be susceptible to the local nanoscale environment, calling for real-space observation of ultrafast lattice dy¬namics. Here we demonstrate nanoscale coherent phonon spectroscopy by optical two-pulse excitation of a plasmonic junction inside a scanning tunneling microscope (STM). In contrast to ‘lightwave’-STM, we operate the ultrafast laser-excited STM in the weak excitation regime, in which the tunnelling of photoexcited hot carriers through the static tunneling barrier can give rise to an ultrafast photoinduced current that can be used for imaging with ‘photon’-STM.
We locally excite coherent phonons in ultrathin zinc oxide films by the tightly-confined gap plasmon, and probe them via the photoinduced tunneling current through an electronic resonance of the ZnO film. Comparison of the coherent phonon spectra with tip-enhanced Raman spectroscopy allows identification of the involved phonon modes. In contrast to the Raman spectra, the relative phonon intensities obtained from time-domain spectroscopy exhibit strong nanoscale spatial variations, which correlate with changes in the local density of states recorded by scanning tunneling spectroscopy [1]. We attribute this to changes in a displacive component in the driving force due to the resonantly excited charges carriers. Our work introduces a new approach to study the ultrafast structural response at solid surfaces using optical scanning tunneling microscopy.
References
[1] S. Liu, A. Hammud, I. Hamada, M. Wolf, M. Müller, and T. Kumagai,
Science Advances (accepted).
| Abstract Number (department-wise) | PC 13 |
|---|---|
| Department | PC (Wolf) |
