Terahertz radiation from the coherent longitudinal optical phonon-plasmon coupled mode in an i-GaAs/n-GaAs epitaxial structure

We have investigated the time-domain terahertz radiation from an undoped GaAs/n-type GaAs (i-GaAs/n-GaAs) epitaxial structure using an optical gating method with a femtosecond pulse laser. We have found from the Fourier power spectra of the terahertz waveforms that two terahertz bands, the frequencies of which obviously depend on the pump power, appear in a high density excitation regime in addition to a broad band due to a surge current and a sharp band due to the coherent longitudinal optical (LO) phonon. Based on a model for the LO phonon-plasmon coupled (LOPC) mode, we reasonably explain the pump-power (photogenerated-carrier-density) dependence of the frequencies of the two bands peculiar to the high density excitation. This fact demonstrates that the two terahertz bands are assigned to the lower and upper branches of the LOPC mode. The frequencies of the LOPC mode are determined only by the pump power. Thus, the LOPC mode originates from the i-GaAs layer; namely, the plasmon component of the surge current flowing through the i-GaAs layer couples with the coherent LO phonon.