Abstract
We report on terahertz electromagnetic waves from coherent longitudinal optical (LO) phonons and coherent LO-phonon plasmon coupled (LOPC) modes in (001)-, (110)-, and (111)-oriented semi-insulating GaAs single crystals. The coherent LO phonons and coherent LOPC modes are observed in the three samples. In contrast, coherent transverse optical (TO) phonons are not observed in the (110)- and (111)-oriented samples in spite of that the TO phonon is the Raman active mode. We conclude that only the coherent longitudinal oscillating polarization such as coherent LO phonons and coherent LOPC modes radiate the terahertz wave and that the Raman selection rule does not contribute to the terahertz-wave-radiation process. The photogenerated electron-density dependence of the frequencies of the coherent LOPC mode is almost the same in the three samples. This finding indicates that the coupling of the LO phonon with the plasmon is not influenced by the surface orientations of the three samples.
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