Confined coherent acoustic modes in a tubular nanoporous alumina film probed by picosecond acoustics methods

Coherent GHz acoustic phonon dynamics in a tubular nanoporous alumina film is studied with picosecond acoustic 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 a 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 either enhanced or reduced to zero following a selection rule. In particular, we demonstrate that detection configurations exist where the coherent acoustic phonon and probe light sensitivity functions are orthogonal, leading to cancellation of the photoelastic light scattering mechanism. This has never been reported so far in the physics of the interaction of coherent acoustic phonons with light.