Polarization dependence of coherent LO phonon excitation in Si

The coherent LO phonon mode excitation dependence on the angle $\theta$ between pump and probe polarizations is systematically investigated by transient electro-optic sampling measurement for both $\Gamma_{25}$ and $\Gamma_{12}$ symmetries\footnote{M. Hase, M. Kitajima, A. M. Constantinescu, and H. Petek, \textit{Nature} \textbf{426}, 51 (2003).} We find that for $\Gamma_{25}$ symmetry the phonon exhibits a sin(2$\theta)$ dependence in the amplitude, while for $\Gamma_{12}$ symmetry the signal is considerably weaker and has a cos(2$\theta)$ dependence. The LO phonon maximum amplitude for $\Gamma_{25}$ is larger than the maximum amplitude for $\Gamma_{12}$ by about a factor of 6. We will discuss the results in the context of coherent phonon excitation mechanisms.