Illuminating a femtosecond light pulse on solid materials can initiate collective atomic motions, which keep step (or are “in phase”) with the neighbors -- the coherent phonons. Here’s how they are created, and come eventually to the end of their lifetime.
Atoms in materials are always in motion. In periodically structured solid materials (crystals), the atomic motions must satisfy periodic boundary condition and become collective -- phonons.
Usually, the atomic motions at different locations in a crystal are at different stages of a periodic motion -- or at random phase.
Only when the atoms receive impulsive force do they initiate to keep pace with the neighbors -- or become in-phase. These atomic motions are called coherent phonons, because they can inferfer each other in a wave-mechanics sense.
Phonons in general are divided into two categories -- acoustic and optical. Coherent acoustic phonons are ballistic wavepackets of compressive/tensile stress. Coherent optical phonons are standing waves of in-phase atomic oscillations, in which the adjacent atoms swing against each other.
In our group, we study coherent optical phonons experimentally to look into the ultrafast electron-phonon coupling dynamics.
Reflectivity change of graphite in picosecond (ps) time scale is modified by periodic oscillations at two different periods. Fast one with a period of 0.021 ps is in-plane C-C stretching; slow one with a period of 0.77 ps is interlayer shear. Both are examples of coherent optical phonons created by impulsive stimulated Raman scattering.