Sixfold-degenerate valleys in Si have attracted considerable attention for valleytronics application. Using optical pump-terahertz (THz) probe spectroscopy, we study the dynamics of valley polarization in bulk Si(001) at room temperature. Linearly polarized pump pulses excite electrons and holes with asymmetric distributions in momentum space, leading to in-plane anisotropic conductivity. By varying the polarization directions of the pump light relative to the in-plane crystalline axes, the valley polarization of electrons and the momentum asymmetry of holes are separately probed by THz pulses through light-induced linear birefringence and dichroism. We demonstrate that the valley relaxation time of electrons near the conduction band minimum exceeds 1.5 ps at room temperature, in good agreement with theoretically calculated intervalley phonon scattering with $f$ process. This work paves the way for Si-based room-temperature valleytronics.

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• Photoconductivity
• Valley degrees of freedom
• Valleytronics
• Terahertz spectroscopy