  Ultrafast laser excited hot electrons can transport energy supersonically far
from the region where they are initially produced. We show that this ultrafast
energy transport is responsible of the emission of coherent acoustic phonons
deeply beneath the free surface of a copper metal sample. In particular we
demonstrate that enough energy carried by these hot electrons over a distance
as large as 220nm at room temperature in copper can be converted into coherent
acoustic phonons. In order to demonstrate this effect, several configurations
of time-resolved optical experiments of time of flight of coherent acoustic
phonons and of hot electrons have been performed.
