Coherent phonons in condensed media

The impulsive excitation and phase-sensitive detection of coherent phonons and phonon-polaritons provide a detailed insight into the dynamical properties of matter. The experiments are based on optical pump-probe techniques with femtosecond time resolution. These techniques enable the detection of amplitude and phase of the coherent lattice motion simultaneously. Frequencies in the terahertz range and dephasing times in the picosecond range can be obtained with high accuracy. Especially in semiconductors and semiconductor heterostructures, where a coherent phonon mode and free carriers are excited simultaneously, important information about carrier-phonon interaction far away from equilibrium is obtained. This chapter presents an overview of recent achievements in this lively field of condensedmatter physics.