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We present a joint experimental theoretical study of the coupling of coherent phonons in bulk GaAs with a nonequilibrium electron-hole plasma following photoexcitation at the ${E}_{1}$ gap by ultrafast laser pulses. In contrast to prior coherent phonon experiments where photoexcitation across the ${E}_{0}$ gap generated electrons in the $\ensuremath{\Gamma}$ valley, for the ${E}_{1}$ gap excitation, the majority of the electrons are generated in the satellite $L$ valleys. This leads to a drastically different situation from the previous studies because the damping of electrons is now faster due to the higher scattering rates in the $L$ valley, and, in addition, the diffusion of carriers has a significant effect on the plasma response due to the shorter optical absorption depth of the pump-probe light. Reflectivity measurements show coherent phonon-plasmon oscillations, whose frequencies lie between the transverse and longitudinal optical phonon frequencies due to the heavy damping and change with time due to the diffusion of the plasma. We analyze the experimental data with a theoretical model that describes the time and density-dependent coupling of the coherent phonon and the electron-hole plasma as the photoexcited carriers diffuse into the sample on a subpicosecond time scale. The calculated phonon-plasmon dynamics qualitatively reproduce the experimentally observed time-dependent frequency.

Ultrafast coupling of coherent phonons with a nonequilibrium electron-hole plasma in GaAs