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

4Intraband Spectroscopyand Dynamics of ColloidalSemiconductor Quantum DotsPhilippe Guyot-Sionnest, Moonsub Shim, and Congjun WangJames Franck Institute, Chicago, Illinois, U.S.A.I. INTRODUCTIONSemiconductor nanocrystal colloids are most striking for the ease with whichtheir color, determined by electronic absorption frequencies, can be controlledby size. Although most of the applications currently envisioned arebased on the interband transitions, one should not overlook the intrabandtransitions. In the case of conduction-band states, these transitions are easilysize-tunable through spectral regions of atmospheric transparencies (e.g., 3–5Ams and 8–10 Ams). This tunability makes semiconductor nanocrystalcolloids attractive subjects of study with potential applications in filters, detectors,lasers, and nonlinear optical elements.The investigations of intraband (also called intersubband) transitionsstarted in 1984 with studies of semiconductor quantum wells [1]. Progressin this field was rapid and led to the demonstration of photodetectors [2],nonlinear optical elements [3], and mid-infrared ‘‘quantum cascade lasers’’[4].Interest in the intraband transitions in quantum dots is more recent andis tied to the development of appropriate materials, but the essential motivationis to take advantage of the discrete transitions arising from threedimensional(3D) confinement. As reviewed in Section II, because of strongintraband transitions, semiconductor nanocrystal colloids exhibit ‘‘infrared’’(IR) optical properties that are unlikely to be achieved by organic molecularCopyright 2004 by Marcel Dekker, Inc. All Rights Reserved.