Temperature dependence of coherent A1g and Eg phonons of bismuth

Bismuth has been a model material in the study of femtosecond dynamics of coherent lattice oscillations. The generation mechanism was first proposed to be displacive for the symmetric A1g mode, which was the only mode observed as a coherent phonon. The absence of the other Raman active mode Eg has not been fully explained, but was phenomenologically attributed to the exclusive coupling of the hot electrons at k∼0 and high symmetry phonons. In the present study, we demonstrate that both A1g and Eg modes are excited as coherent phonons at low temperature and confirm that the coherent phonons are generated via a Raman process in bismuth. We found a puzzling π∕2 difference in the initial phases of the two coherent phonons, which suggests that the initial phase cannot be a clear-cut index for the generation mechanism in absorbing media.