Carrier-Induced Coherent Phonon Softening in Bismuth

With performing time-resolved pump-probe experiments with ultrashort pulses from a mode-locked Ti:sapphire laser, we report on the carrier-induced softening and dephasing of coherent phonon oscillations in the bismuth crystal. As the lattice displacement becomes severe at high pump power, a frequency shift can occur due to the lattice anharmonicity term. We clarified from the measurements done by varying the pulse duration that the phonon softening at high pump intensity in bismuth originates from the electron-interaction of photoexcited carriers not from the phonon anharmonicity. We also find that the coherent phonon dephasing is accelerated at high carrier densities.