Uncorrelated Optical Transitions
In a semiconductor, there is a wealth of possible transitions between electronic states mediated by a photon. Figure 4.1 summarizes a number of them. As typical photon wave vectors are negligible with respect to the extent of the Brillouin zone, the photon looks practically like a vertical line in the energy dispersion diagram.1 Direct interband transitions (Fig. 4.1a,b), direct inter-valence-band transitions (Fig. 4.Id) and similarly direct inter-conduction-band transitions, and photoemission (Fig. 4.If) directly connect an occupied initial state (filled circles) to an empty final state (open circles). Intra-conduction-band (Fig. 4.1c) and similarly intra-valence-band transitions, and indirect interband transitions (Fig. 4. 1e) on the other hand, require a phonon to be emitted or absorbed in addition. Clearly, at low temperatures, only emission remains (only this channel has been drawn in Fig. 4.1). Transitions requiring both a photon and a phonon are generally much weaker than the others and are usually only of relevance when other transitions are absent in a particular energy regime.
KeywordsCoherence GaAs Convolution Germanium
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