We present a study of the interaction between high density photoexcited carriers and coherent LO phonons in Si(001). Through high-density (10¹⁹–10²⁰ carriers/cm³) photoexcitation of Si(001) near its E₁ critical point with 10 fs duration, 400 nm laser pulses, the zone-center
coherent LO (q ≈ 0) phonon is excited. The coherent LO phonon gains a complex self-energy (i.e., frequency shift and contribution to decay rate) by interacting with the photogenerated nonequilibrium electron-hole plasma through the deformation potential interaction mechanism. We
measure the time dependent renormalization of the LO phonon frequency and dephasing time by analyzing the anisotropic transient reflectivity of variously doped Si samples over a delay time of 6 ps between pump and probe pulses. We study how both these quantities depend on the initial
photoexcited carrier density, and the level and type of doping. We further study the LO phonon excitation and detection mechanism monitoring the LO phonon amplitude change with the pump and probe polarizations orientations with respect to Si sample crystalline axes. The phonon
amplitude exhibits a sin(2Ѳ)dependence on either the pump or the probe polarization angular rotation, where θ is the angles between the pulse electric field polarization and the [110] axis of the sample in both cases. This result is consistent both with the Г25’ symmetry and the deformation potential mechanism excitation of the LO phonon of Si. We also measure the dependence of the coherent LO phonon on the excitation light wavelength. We find that the phase does not fit into the theoretical picture developed to date for the coherent LO phonon generation. Moreover, the Fourier Transforms of the time-domain signals show Fano-like profiles for the LO phonon peak, with the asymmetry strongly dependent on the excitation wavelength. The wavelength dependence of both the phase and the LO phonon lineshape suggest that the mechanism for the LO coherent phonon generation should be described by a model where the generation pathways via the Raman process and the carrier excitation are coupled through the deformation potential interaction
