Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.

function and envelope function momenta) [19]. In the notation for the holestates, the F number is usually shown as a subscript. For example, thenotation for the lowest hole state is 1S 3/2 . The hole states are (2F+1)-folddegenerate. This degeneracy is lifted if the effects of the crystal field in ahexagonal lattice [20], NQD nonspherical shape [21], and e–h-exchangeinteractions [22,23] are taken into account, leading to a ‘‘fine-structure’’ splittingof hole levels [23].Band-edge optical properties of NQDs are dominated by transitionsinvolving the lowest electron 1S state and fine-structure states derived fromthe 1S 3/2 hole level (Fig. 1a). The split-off 1S 3/2 hole states form two groups ofclosely spaced levels separated by the gap that can be as large as tens ofmillielectron volts in small-size dots. The high-energy manifold of hole statesis coupled to the 1S electron state by a strong optical transition observed inabsorption spectra as the lowest 1S absorption maximum. The low-energyhole states are coupled to the 1S(e) state by a much weaker transition thatgives rise to a band-edge PL band. The large, NQD size-dependent gapbetween the high- and low-energy manifolds of hole states (manifolds of‘‘absorbing’’ and ‘‘emitting’’ states, respectively) is observed in opticalspectra as a large Stokes shift between the lowest 1S absorption maximumand a band-edge PL detected under nonresonant excitation (Fig. 1b). Thisshift is often referred to as the ‘‘global’’ Stokes shift, in contrast to the‘‘resonant’’ Stokes shift [24] observed under quasiresonant excitation influorescence line-narrowing experiments (see Chap. 2).B. Electron Intraband RelaxationIn the case of both the optical and electrical pumping of semiconductor gainmedia, nonequilibrium charge carriers are usually injected with energies thatare greater than the material’s energy gap. Therefore, intraband energyrelaxation, leading to the population buildup of the lowest ‘‘emitting’’ transition,is an important process in the sequence of events leading to lightemission and, ultimately, to lasing. To ensure efficient pumping of electron/hole states involved in the ‘‘emitting’’ transition, energy relaxation in bothconduction and valence bands has to be more efficient than carrier recombinationdue to both radiative and nonradiative processes.In bulk II–VI semiconductors, carrier energy relaxation is dominated bythe Fro¨hlich interactions with longitudinal optical (LO) phonons that lead tofast (typically subpicosecond) carrier cooling dynamics [25–27]. In QDs, evenin the regime of weak confinement when the level spacing is only a fewmillielectron volts, the carrier relaxation mediated by interactions withphonons is hindered dramatically because of restrictions imposed by energyand momentum conservation, leading to a phenomenon called a ‘‘phononCopyright 2004 by Marcel Dekker, Inc. All Rights Reserved.