Phonon dephasing

Every effort is made before each experimental run to maximize the temporal overlap (at At=0) between the Z and s laser. All the same, a temporal mismatch. Ax, of the order of < 0.5 ps is often found between the two lasers. (The mismatch can be extracted from the slight asymmetry >rhich it causes in the CARS signal in the vicinity of the peak at At=0). Finally, the relative contributions of the two components of I(At) can be experimentally specified by a quantity, Y, defined as the difference between the peak of the normalized CARS Intensity at zero time delay, I(At=0) and the intensity of the phonon dephasing component extrapolated back to At=0. This procedure is illustrated in figure 6 for a subset of the laser field polarizations indicated in Table I. (In Table I the polarization direction is given in terms of an angle measured relative to a direction in the plane of the optical table and which is coplanar with the (010) and (100) crystal directions which lie in a plane perpendicular to the optical table.)... 

This chapter presents an ab initio description of the nature and dynamics of photoexcited states in semiconductor QDs, in the energy and time domains. By combining the bulk and molecular viewpoints, the analysis elucidates the controversies and provides a unified atomistic picture of the excited state processes. These ab initio methods are used to study excited state composition, evolution and relaxation, as well as electron phonon dephasing, all with an eye towards the incorporation of QDs in solar cells. For further reading on the work featured in this chapter see publications by the Prezhdo group. ... 

Wlrile tire Bms fonnula can be used to locate tire spectral position of tire excitonic state, tliere is no equivalent a priori description of the spectral widtli of tliis state. These bandwidtlis have been attributed to a combination of effects, including inlromogeneous broadening arising from size dispersion, optical dephasing from exciton-surface and exciton-phonon scattering, and fast lifetimes resulting from surface localization 1167, 168, 170, 1711. Due to tire complex nature of tliese line shapes, tliere have been few quantitative calculations of absorjDtion spectra. This situation is in contrast witli tliat of metal nanoparticles, where a more quantitative level of prediction is possible. 

When, however, phonons of appropriate energy are available, transitions between the various electronic states are induced (spin-lattice relaxation). If the relaxation rate is of the same order of magnitude as the magnetic hyperfine frequency, dephasing of the original coherently forward-scattered waves occurs and a breakdown of the quantum-beat pattern is observed in the NFS spectrum. 

Tunnel relaxation of orientational states in the phonon field of a substrate is considered in Appendix 2). When a molecule has a single equilibrium orientation (p = 1) the deformation potential is also characterized by a well-defined barrier AU which separates the equivalent minima. That is why, the subsystem Hamiltonian (4.2.12) used in the exchange dephasing model147,148 with... 

Table 4.1. Various processes contributing to the spectral line broadening for local vibrations. Frequencies of collectivized local vibrations QK (solid arrows) are supposed to exceed phonon frequencies oiRq (dashed arrows) Ok > max oncq. For an extremely narrow band of local vibrations, diagrams A and B respectively refer to relaxation and dephasing processes, whereas diagrams C account for the case realizable only at the nonzero band width for local vibrations.

In a heavy fermion compound Yb MnSbn, the dephasing rate of the coherent optical phonons decreased with lowering temperature above Curie temperature Tc, but increased below Tc- The results were attributed to the coupling between an optical phonon mode and the Kondo effect [100]. 

If the dephasing time of the coherent phonons depend critically on the carrier density, photo-injection of carriers with the second pump pulse can annihilate them partially or completely, depending on its fluence but not on its relative timing. Such incoherent control was demonstrated for the LO phonons of GaAs [37],... 

In general, coherence destruction is due to elastic collisions, depopulation, or scattering by phonons. However, the dephasing in the context of wavepackets is not due to such collision, scattering, or depopulation processes but can be explained as follows. Transfer of a part of the wavepacket takes place at different positions near the crossing point of two potential curves 1 and 2 (Fig. 3). Since the speed of the... 

The decay of the nanoparticle plasmons can be either radiative, ie by emission of a photon, or non-radiative (Figure 7.5). Within the Drude-Sommerfeld model the plasmon is a superposition of many independent electron oscillations. The non-radiative decay is thus due to a dephasing of the oscillation of individual electrons. In terms of the Drude-Sommerfeld model this is described by scattering events with phonons, lattice ions, other conduction or core electrons, the metal surface, impurities, etc. As a result of the Pauli exclusion principle, the electrons can be excited into empty states only in the CB, which in turn results in electron-hole pair generation. These excitations can be divided into inter- and intraband excitations by the origin of the electron either in the d-band or the CB (Figure 7.5) [15]. 

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