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

systems. Of potential practical interest is the fact that one single excesselectron detectably affects the optical response of a small semiconductornanocrystal, leading to nonlinear optical and large electro-optical responsesin the infrared spectral range.One motivation for studying quantum dots as building blocks foroptical devices is that one can potentially control the energy and phaserelaxation of excited electrons, issues that are essential, for example, to theoperation of quantum-dots lasers. At present, unlike for quantum wells, thereis a weak understanding of the role that phonons play in the relaxationmechanisms in quantum dots, mainly because the energy separation betweenthe electronic states can be much larger than the phonon energy. Intrabandspectroscopy is well suited for studies of relaxation processes because it allowsone to decouple electron and hole dynamics.II.BACKGROUNDAs discussed in previous chapters of this book, the 3D confinement ofelectrons and holes leads to discrete energy states. Neglecting phonons andrelaxation processes, these states are delta functionlike, and their energiesare determined by the band structure of the semiconductor, the shape ofthe boundary, and the nature of the boundary conditions. For II-VI, III–V,and IV–VI semiconductors, the effective mass approximation based on theLuttinger model of the band structure has been used to reproduce interbandspectra [5–8]. Given the uncertainties in the shape, the boundaryconditions [9], the presence of surface charges [10] or dipoles [11], and soforth, one might expect the predictions to be rather crude, particularly forholes that are typically characterized by large effective masses. Nevertheless,the effective mass approximation, with its few adjustable parameters,provides a significant simplification over atomistic tight-binding or pseudopotentialmethods in the description of the delocalized states and opticaltransitions in quantum dots (see Chapter 3). The situation for the nondegenerateconduction band is the simplest. For a spherical box, the quantumconfinedelectronic wave functions are Bessel functions that satisfy theboundary conditions for, for example, a fixed finite external potential. Theelectronic states are described by the angular momentum (L) of the envelopefunction and denoted as 1S e , 1P e , 2S e , 1D e , based on standardconvention. The optical selection rules are the same as for atomic spectrabecause the Bloch functions are derived from identical atomic wave functions,and, therefore, the allowed intraband transitions correspond to DL=F1. These selection rules are in contrast to interband transitions for whichDL=0, F2.Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.