Coherent Phonon Detection in Ge with a Femtosecond Laser

A highly sensitive femtosecond photodeflection spectroscopy technique was first developed experimentally.Using femtosecond laser pulses we have detected ultrafast coherent phonon emission in a Ge plate.It is found that the ultrafast femtosecond laser pulses generate coherent phonons in the plate via the electronic mechanism.Supersonic expansion of photoexcited electron-hole plasma is observed.