Generation of coherent phonons in bismuth by ultrashort laser pulses in the visible and NIR: Displacive versus impulsive excitation mechanism

We have applied femtosecond pump-probe technique with variable pump wavelength to study coherent lattice dynamics in Bi single crystal. Comparison of the coherent amplitude as a function of pump photon energy for two different in symmetry EgEg and A1gA1g phonon modes with respective spontaneous resonance Raman profiles reveals that their generation mechanisms are quite distinct. We show that displacive excitation, which is the main mechanism for the generation of coherent A1gA1g phonons, cannot be reduced to the Raman scattering responsible for the generation of lower symmetry coherent lattice modes.

► We have studied coherent phonons of A1gA1g and EgEg symmetry in bismuth.
► Amplitudes of both modes were measured in visible and near infrared ranges.
► Obtained data were compared to spontaneous Raman cross sections.
► We show that displacive excitation cannot be reduced for Bi to Raman scattering.