Confined coherent acoustic modes in a tubular nanoporous alumina film probed by picosecond acoustics methods
Résumé
Coherent GHz acoustic phonon dynamics in a tubular nanoporous alumina film is studied with picosecond acoustics methods. This study has allowed the determination of the out-of-plane sound velocity and the effective optical index through a time-resolved Brillouin analysis. Several acoustic eigenmodes of the tubular nanoporous alumina film were detected. These GHz eigenmodes exhibit unusual frequency dependent long lifetimes. Their strong confinement has been explained by the presence of the thin alumina layer at the film/substrate interface which plays the role of an acoustic mirror. Finally, we show that the optical detection process of these long-living eigenmodes can be enhanced or on the opposite can be reduced to zero following a selection rule. In particular we demonstrate that some detection configurations exist where the coherent acoustic phonon and probe light sensitivity function are orthogonal leading to cancellation of the photoelastic light scattering mechanism. This has never been reported up to now in the physics of interaction of coherent acoustic phonon with light.