Annihilation of coherent LO phonon-plasmon coupled modes by lattice defects in n-GaAs

We have studied the effect of point defects on dephasing of coherent optical phonons in ion-implanted GaAs. Ultrafast dynamics of coherent phonons and photo-generated carriers in the femtosecond time-domain have been precisely measured by means of a reflection-type pump-probe technique. The coherent LO phonon-plasmon coupled (LOPC) modes disappear with increasing ion dose, and only the coherent LO phonon. is observed at high ion doses. The relaxation time of the photo-generated carriers is decreased by ion irradiation, and this effect is due to trapping of carriers with deep defects. Our results suggest that point defects involve deep levels below the conduction band, and as a results, carrier trapping dominates annihilation of the coherent coupled modes.