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

TL;DR

This study investigates how ultrashort laser pulses generate coherent phonons in bismuth, revealing distinct mechanisms for different phonon modes through pump-probe spectroscopy.

Contribution

It demonstrates that displacive excitation primarily drives A1g phonons, unlike Raman scattering which influences lower symmetry modes, highlighting different generation mechanisms.

Findings

Displacive excitation dominates A1g phonon generation.

Raman scattering is responsible for lower symmetry modes.

Different mechanisms operate for various phonon symmetries.

Abstract

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 Eg and A1g 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 A1g phonons, cannot be reduced to the Raman scattering responsible for the generation of lower symmetry coherent lattice modes