Theory of coherent phonon oscillations in nonpolar and polar semiconductors

Abstract

A microscopic description of coherent phonon oscillations generated by femtosecond optical excitation in both polar and nonpolar semiconductors is presented. For nonpolar semiconductors such as Ge, we show that the coherent lattice displacement is related to a quantum-mechanical average of a single phonon creation operator and we derive the equation of motion for the coherent phonon amplitude. In polar materials such as GaAs there is also another driving force which is more effective, namely, the depolarization electric field created by the separation of electrons and holes in the applied DC field and we formulate a microscopic theory of the plasmon- phonon oscillations. Results show that for an idealized situation with homogeneous plasma density that plasmon-like oscillations dominate the transient behavior. However, once the inhomogeneous density distribution is taken into account, only density-independent LO phonon oscillations are present in the transient optical response.