Ultrafast optical detection of coherent acoustic phonons emission driven by superdiffusive hot electrons

Ultrafast laser-excited hot electrons in metals can transport energy supersonically far from the region where they are initially produced. We show that this ultrafast energy transport is responsible for the emission of coherent acoustic phonons deep beneath the free surface of a weak electron-phonon coupling copper metal sample. Special attention is taken to investigate the interaction between superdiffusive hot electrons at the bi-metallic buried interface (Cu-Ti). To discuss the underlying physics and the ultrafast transient optical properties, several configurations developed in the frame of ultrafast optical pump-probe technique have been used. In particular, we have performed backward and forward detection of both coherent acoustic phonons and superdiffusive hot electrons. From an original probe wavelength dependence study of the optical detection process, we clearly establish the signature of superdiffusive hot transport within the copper film and the link with the acoustic phonon emission. A comparison with a strong electron-phonon coupling metal, like titanium, where there is no superdiffusive transport is also provided. These results and observations are important to quantify the role of superdiffusive carriers in ultrafast energy transport, which is involved in different processes in solid state physics or femtochemistry.