Effects of photogenerated carrier scattering on the decay process of coherent longitudinal optical phonons in an undoped GaAs/n-type GaAs epitaxial structure investigated by terahertz time-domain spectroscopy

Abstract

The authors investigated the scattering effect of photogenerated carriers on the decay of the coherent longitudinal optical (LO) phonons in an undoped GaAs/n-type GaAs epitaxial structure using a terahertz time-domain spectroscopic technique. The terahertz wave from the coherent LO phonon was observed together with those emitted both by the ultrafast photocurrent and by the coherent LO-phonon-plasmon coupled (LOPC) mode. The simultaneous observation of the coherent LO phonon and coherent LOPC mode originates from the fact that the photogenerated carrier density laterally distributes around the surface of the undoped GaAs layer owing the Gaussian profile of the pump beam. The authors found that the terahertz wave from the coherent LO phonon lives up to 5.0 ps, whereas those from the coherent LOPC mode and ultrafast photocurrent disappear within 1.0 ps. The decay time of the coherent LO phonon monotonically decreases with an increase in the pump power. This finding indicates that the dephasing of the terahertz wave from the coherent LO phonon is dominated by the photogenerated carrier scattering in the time range shorter than 1.0 ps in which photogenerated carriers remain.