Dephasing of coherent phonons by lattice defects in bismuth films

We have studied the effect of point defects on coherent optical phonons in ion-implanted bismuth polycrystalline films. Ultrafast dynamics of coherent phonons and photogenerated carriers in the femtosecond time domain have been investigated by means of pump-probe reflectivity measurements. The dephasing rate of the A1g phonon increases linearly with increasing ion dose, which is explained by the additional dephasing process of the coherent phonon originated from scattering of phonons by the defects. Carrier dynamics are also found to be affected by additional scattering process mediated by point defects.