Ultrafast decay of coherent plasmon-phonon coupled modes in highly doped GaAs

We report on the ultrafast decay of coherently excited plasmon-phonon coupl ed modes in a highly doped n-GaAs. Coherent oscillations of the upper branc h (L+) as well as the lower branch (L-) of the coupled modes have been obse rved by a femtosecond pump-probe technique with 20 fs ultrashort laser puls es. The decay time of the L+ mode estimated from a time partitioning Fourie r transform spectra reveals that the decay rate increases Linearly with inc reasing photoexcited carrier density. This result leads to a conclusion tha t the ultrafast decay of the coherent L+ mode is mainly caused by loss of c oherence in electron-hole plasmas when the photoexcited carrier density is higher than doping levels. [S0163-1829(99)01048-6].