Selective Excitation of Higher Harmonic Coherent Acoustic Phonons in a Graphite Nanofilm

Abstract

We investigate the excitation of higher harmonic coherent acoustic phonon (CAP) modes in a ∼42 nm thin graphite film. Results obtained from ultrafast electron diffraction (UED) measurements, employing single-pulse femtosecond-laser excitation as well as tailored double-pulse sequences to selectively drive the two first higher-order coherent acoustic phonon modes, are presented. Extracted harmonic contributions are compared and analyzed with the help of a one-dimensional kinematic diffraction model. Phase analysis is employed to discriminate higher-order frequencies found in the measured transient, which arise due to the presence of excited coherent acoustic phonon modes, from frequency contributions that are inherent to the ultrafast electron diffraction measurement principle.

Nanoscale Energy Transport Dynamics across Nonbonded Solid–Molecule Interfaces and in Molecular Thin Films

“…For low thicknesses, we need to consider the effects of the coherent part of the motions of the substrate atoms and the near-commensurate lattice match at the interface (Figure ). It is known that coherent acoustic phonons (CAPs) across the stacked layers of graphite are also dynamically induced following laser excitation, , which cause additional amplitudes in the vertical motions at the solid–molecule interface that can push overlaying molecules further. Such CAP motions are wavelike and may therefore incite a much faster response in adjacent molecular layers compared to the diffusion-based prediction (early time dynamics in Figure b and schematic in the lower part of Figure ).…”