Coherent phonon oscillations in GaAs

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Abstract
We present a microscopic theory of plasmon-phonon oscillations created by ultrafast optical excitation in polar semiconductors. Our analysis is based on the equations for the optical lattice displacement and the electronic polarization. For an idealized situation with homogeneous plasma density we find that plasmonlike oscillations dominate the transient behavior after the optical excitation. However, once the inhomogeneous density distribution is taken into account, only density-independent LO-phonon oscillations are present in the transient optical response. By using a model of electro-optical response that includes the influence of both the electric field and the lattice displacement on the refractive index, we achieve a quantitative agreement with the experiment.