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.