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 sample...

中文翻译：

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在（001），（110）和（111）取向的半绝缘GaAs单晶中，从相干纵向光学（LO）声子和LO-声子等离激元耦合模式辐射出的太赫兹电磁波

我们报道了在（001）-，（110）-和（111）取向的半绝缘GaAs单晶中，来自相干纵向光学（LO）声子和相干LO-声子等离子体激元耦合（LOPC）模式的太赫兹电磁波。在三个样本中观察到相干的LO声子和相干的LOPC模式。相反，尽管TO声子是拉曼激活模式，但在（110）和（111）取向的样品中未观察到相干横向光学（TO）声子。我们得出的结论是，只有相干的纵向振荡极化（如相干的LO声子和相干的LOPC模）会辐射太赫兹波，而拉曼选择规则对太赫兹波辐射过程没有贡献。在三个样本中，相干LOPC模式频率的光生电子密度依赖性几乎相同。