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Photocarrier-Mediated Excitation of Coherent Phonons

The DECP model successfully explained the observed initial phase of the fully symmetric phonons in a number of opaque crystals [24]. The absence of the Eg mode was attributed to an exclusive coupling between the electrons photoexcited near the r point and the fully symmetric phonons. A recent density functional theory (DFT) calculation [23] demonstrated this exclusive coupling as the potential energy surface (Fig. 2.4). The minimum of the potential surface of the excited state shifted significantly along the trigonal (z) axis, [Pg.27]

As the lattice interacts with light only through electrons, both DECP and ISRS should rely on the electron-phonon coupling in the material. Distinction between the two models lies solely in the nature of the electronic transition. In this context, Merlin and coworkers proposed DECP to be a resonant case of ISRS with the excited state having an infinitely long lifetime [26,28]. This original resonant ISRS model failed to explain different initial phases for different coherent phonon modes in the same crystal [21,25]. Recently, the model was modified to include finite electronic lifetime [29] to have more flexibility to reproduce the experimental observations. [Pg.28]

In polar semiconductors, carrier-mediated generation occurs in the form of transient depletion field screening (TDFS) depicted in Fig. 2.5 [30]. The driving force in (2.1) can be expressed by the sum of the Raman term and the nonlinear longitudinal polarization [10]  [Pg.28]

The nonlinear polarization can be divided into several different contributions  [Pg.28]

The third term describes the polarization set up by ultrafast drift-diffusion currents, which can excite coherent phonons via TDFS (or via the buildup of electric Dember fields [9,10]). The first two terms represent the second- and the third-order nonlinear susceptibilities, respectively [31]. The fourth term describes the polarization associated with coherent electronic wavefunctions, which becomes important in semiconductor heterostructures. [Pg.29]


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