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.
Phonons, either incoherent or coherent, do not live long; they have typically picosecond lifetime.
In a perfect crystal at finite temperature, anharmonic coupling between normal modes provides the main path for the decay of optical phonons. The simplest anharmonic process would be the spontaneous decay of a single phonon into two phonons of lower frequency. Temperature dependence of the decay rate (or the linewidth) can reveal this anharmonic process [7].
Scattering by crystalline defects such as impurity atoms, atoms with different isotopic masses and vacancies can annihilate coherent optical phonons effectively [8].
Electron-phonon coupling can dominate the decay of coherent phonons in metals and semimetals, when the electronic temperature is extremely high under intense photo-excitation. Then the Fourier transformed spectrum of the coherent phonons shows a Fano-type lineshape [9].
In low-dimensional systems such as graphite and carbon nanotubes, high-frequency optical phonons can relax by creating electron-hole pair near the Fermi level. In such a case, moderate photo-excitation can lead to suppression of the decay of coherent phonons [10].
[7] Hase et al. Phys. Rev. B 58, 5448 (1998).
[8] Ishioka et al. Physica B, 316-317, 296 (2002); Appl. Phys. Lett. 78, 3965 (2001).
[9] Misochko et al., J. Phys.: Cond. Mater. 19, 156227 (2007).
[10] Ishioka et al., Phys. Rev. B 77, 121402R (2008).
Temperature- (top) and photoextation density- (bottom) dependences of coherent phonons of bismuth.