Abstract

Coherent plasma oscillations in polar semiconductors like GaAs can be generated by ultrafast screening of a space charge field by exciting cold electrons and holes with femtosecond laser pulses.^1–4 Because of their high mobility, the electrons can undergo coherent oscillations following the periodically reversing electric field in the space charge region. For low excitation densities, the plasma frequency is well below the optical phonon frequency, therefore, the coupling between plasmons and optical phonons is very weak. In this range of “pure” plasmons, the frequencies obey the well known relation ω_P² = n · e²/m^* · ε · ε₀. Increasing the carrier density leads to a stronger coupling of plasmons to optical phonons when the plasma frequency ω_P approaches the phonon frequencies ω_LO and ω_TO.⁵ This coupling appears in the carrier density dependence of both the frequency and the damping time of the plasmon phonon-coupled modes.

© 1998 Optical Society of America