Coherent optical phonons in a Bi2Se3 single crystal measured via transient anisotropic reflectivity

Coherent optical Eg phonons in a Bi2Se3 single crystal have been observed by using femtosecond transient anisotropic reflection measurements in addition to A1g1 and A1g2 phonons. Frequency, damping rate, and initial phase of coherent oscillations of phonons are obtained. The phonons with the higher frequency have the faster damping rate. The A1g2 phonons show cosine-like oscillation, while the A1g1 and Eg2 phonons show sine-like oscillation. These differences in the initial phase suggest that the nonequilibrium carrier distribution may shift the potential energy surface significantly along the A1g2 eigenvector and little along the A1g1 and Eg2 eigenvectors in the excited state.