Theory of Coherent Phonon Oscillations in Semiconductors

We present a microscopic explanation of the experimentally observed coherent phonon oscillations in semiconductors excited by ultrafast optical pulses. For deformational electron-phonon coupling, only the $\mathbf{q}=0\mathrm{}$ optical phonon mode is directly coupled to the total number of photoexcited pairs. Rapid changes in the carrier density due to ultrafast optical excitation act as a driving force for oscillations of the $\mathbf{q}=0\mathrm{}$ mode. As a result, this mode becomes occupied by a macroscopically large number of coherent phonons.