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

Figure 5 Transient IR absorbance at 2.9 Am for f1.9-nm-radius CdSe nanocrystalswith different surface passivations (indicated in the figure); the pumpwavelength is 532 nm.crystals [13] and pyridine-capped nanocrystals [27] exhibit remarkably longlivedelectrons in the 1S e state, in excess of 1 ms, for a small fraction of thephotoexcited nanocrystals. It is certain that longer times are achievable withspecifically designed charge-separating nanocrystals, which might then findapplications, for example, in optical memory systems.An important issue in quantum-dot research is the mechanism oflinewidth broadening and energy relaxation. Indeed, electronic transitionsare not purely delta functions because there is a coupling between thequantum-confined electronic states and other modes of excitations such asacoustic or optical phonons and surfaces states. The efficiency of thesecoupling processes affects the optical properties of the quantum dots. Forband-edge laser action in quantum dots, it is beneficial to have a fastintraband relaxation down to the lasing states. For intraband lasers, we havethe opposite situation, and one would benefit from slow intraband relaxationbetween the lasing states. Thus, intraband relaxation is a subject of greatcurrent interest.Although lasers emitting at band-edge spectral energies have been madeusing quantum dots [28,29], the observed fast intraband relaxation is not wellunderstood. In particular it is four to five orders of magnitude slower than theCopyright 2004 by Marcel Dekker, Inc. All Rights Reserved.