Screening effects of photogenerated carriers on terahertz radiation from coherent GaAs-like longitudinal optical phonons in (11 n )-oriented GaAs/In_0.1Al_0.9As strained multiple quantum wells

We investigated terahertz radiation from coherent GaAs-like longitudinal optical (LO) phonons in (11 n )-oriented GaAs/In_0.1Al_0.9As strained multiple quantum wells for clarifying the screening effects of photogenerated carriers. We observed the intense quasi-monochromatic terahertz wave from the coherent GaAs-like LO phonon, which originates from the initial polarization enhanced by the strong piezoelectric field. The intensity of the coherent GaAs-like LO-phonon band exhibited a saturation behavior as the pump power was increased. We evaluated the saturation behavior in terms of excitation efficiency of the terahertz wave from the coherent GaAs-like LO phonon using the parameter of unit-power intensity. From the pump-power dependence of the unit-power intensity, we conclude that the screening effect of high density photogenerated carriers on the piezoelectric field causes saturation of the terahertz-wave intensity from the coherent GaAs-like LO phonon. (paper)