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

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