Ultrafast coupled plasmon-phonon mode dynamics in GaAs, a combined experimental and theoretical approach*

We present results from a joint experimental and theoretical study exploring the excitation of coupled plasmon-phonon modes in GaAs. In contrast to previous coherent phonon studies in GaAs where electrons were generated primarily in the $\Gamma$ valley ($E_0$ gap), we use a pump-probe technique with a 10 fs pulse width and a shorter 400 nm laser wavelength to photoexcite electrons predominately in the L valley ($E_1$ gap). As a result: i) damping of the electron-hole plasma is faster and ii) diffusion of the carriers from the surface becomes important owing to the shorter absorption length. The probe pulses measure the time-dependent changes to the reflectivity due to the coupled plasmon-phonon modes created by the ultrafast photoexcitation and the subsequent depletion field screening. To model this, we solve for the time and density dependent coupled-mode frequencies allowing for ambipolar diffusion. Simulation of the coupled plasmon-phonon dynamics allows for comparison with, and a better understanding of experiments.