The transient reflectivity of GaAs Schottky diodes is measured by femtosecond time resolved pump-probe experiments. The measured reflectivity for photon energies near the band gap reveals transient quasioscillatory behavior with frequencies up to 5.5 THz. The changes of the reflectivity are due to extremely fast changes of the carrier density within the depletion layer. We interpret the observed oscillatory signal as coherent plasma oscillations. Ensemble Monte Carlo simulations for the scenario agree well with the observed plasmon frequencies.

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