 Acoustic frequencies up to the terahertz range in solids and up to ~ 10 GHz in liquids have been reported.Apart from thermal expansion, generation through the deformation potential or through piezoelectricity is possible.Picosecond ultrasonics is currently used as a thin film metrology technique for probing films of sub-micrometer thicknesses with nanometer resolution in depth, that sees widespread use in the semiconductor processing industry.