Seeking for the new opportunities to efficiently excite GHz-THz coherent acoustic phonons by femtosecond
lasers is an active field of research. Several fundamental objectives have to be addressed in order to achieve this
acoustic phonons manipulation by femtosecond laser. Among them, the understanding of femtosecond generation
of coherent acoustic phonons remains a key route. Several electron-phonon, photon-phonon and phonon-phonon
interaction mechanisms are involved in the processes of generation and remain only partially understood up
to now. In this paper, we will present a survey of ultrafast photo-generation of coherent acoustic phonon in
semiconductors. We will focus first on the generation of the phonons by fs-laser excitation through the photoinduced
modifications of nanoscopic internal electric fields (deformation potential) in non-piezo-active  GaAs
semiconductor. We will show secondly how it is possible to develop more efficient sources by using piezo-active
, [-1-1-1] and  GaAs semiconductors. In that case, generation of GHz acoustic phonon due to inverse
piezoelectrical effect is based on ultrafast light-induced screening of the near surface built-in electric field.
We will review recent progress on the generation and detection of picosecond shear acoustic waves. Examples will be
shown in which the transverse isotropic symmetry of the sample structure is broken in order to permit shear wave
generation through sudden laser heating. As an illustration of the technique, picosecond longitudinal and shear acoustic
waves have been successfully employed to probe structural dynamics in nano-sized solids (gold
nano-crystals assemblies) and nano-sized liquids (glycerol and water).