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.
How can a femtosecond light pulse induce coherent (optical) phonons? The answer is not always simple.
In non-resonant excitation, in which the energy of the light does not match that of the electronic transition, coherent phonons are generated exclusively via impulsive stimulated Raman scattering (ISRS) [1].
The stimulated process utilizes a pair of light, whose energy difference matches the phonon energy. Since a femtosecond pulse has a broadband spectrum due to the uncertainty relation, the crystal can “find” multiple of such pairs. The coherent atomic motion is then induced on the ground electronic state.
Because the atoms start to move from their equilibrium position, the resulting oscillation should be a sine function of time -- zero amplitude at photo-excitation -- corresponding to the delta-function force applied by the light pulse.
Experimentally. the ISRS generation mechanism can be confirmed with the dependence of the phonon amplitude on the polarization of excitation light, if the phonon in question is an asymmetric mode.
Under resonant excitation, in which electron-hole pairs are created, at least three generation mechanisms have been proposed. In all cases, the force at work is a step-function of time rather than a delta-function.
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1)(Resonant) ISRS [2,3]: in analogy to resonant (spontaneous) Raman scattering, the efficiency of the coherent phonon generation is enhanced when the light energy matches the electronic transition -- especially for the phonon modes whose electronic coupling is strong.
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2)Displacive excitation of coherent phonons (DECP) : for phonon modes whose electronic coupling is strong, the minimum of the potential energy surface (PES) of the excited electronic state may be shifted from that of the ground state. The coherent atomic motion is then induced on the excited state, with the phonon amplitude maximum at photo-excitation (a cosine function of time).
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This happens for the A1g mode of bismuth, whose motion is associated with the Peierls distortion [4]. The Eg mode of the same crystal, on the other hand, is generated via ISRS [5].
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3)Transient depletion field screening (TDFS) : in doped polar semiconductors such as GaAs, photo-excited carriers screen the band bending in the depletion layer. This induces a sudden change of electric field, and the positive and negative ions start to move against each other [6].
[1] Dhar et al. Chem. Rev. 94, 157 (1994).
[2] Stevens et al, Phys. Rev. B 65, 144304 (2002).
[3] Riffe and Sabbah, Phys. Rev. B 76, 085207 (2007).
[4] Zijlstra et al. Phys. Rev. B 74,0220301 (2006).
[5] Ishioka et al. J. Appl. Phys. 100, 093501 (2006).
[6] Dekorsy et al. in “Light Scattering in Solids VIII”, p. 169, Springer, Berlin (2000).
