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

are two natural approaches to studying the broadening of intraband transitions.Figure 8 shows hole-burning spectra of CdSe, InP, and ZnO nanocrystalcolloids at 10 K [57]. These spectra also exhibit the LO phonon replica.Their strength is in good agreement with the bulk electron–LO phononcoupling, with values of the Huang-Rhys factor of 0.2 for the spectrum ofCdSe nanocrystals shown in Fig. 8. An interesting result is that for CdSenanocrystals, the homogeneous width remains narrower than f10 meV at 200K for a transition at f300 meV [57]. Compared to the overall inhomogeneouslinewidth of f100 meV, this value indicates that much progress is stillpossible in improving monodispersity of colloidal samples that should leadto a more precise control of intraband absorption features.V. CONCLUSIONSAs emphasized throughout the other chapters of this book, semiconductorcolloidal quantum dots hold an interesting place as chromophores. Thischapter focuses on the mid-IR properties that arise due to their intrabandtransitions. Whereas organic molecules can be made with great control andcomplexity, the strong electron–electron interactions and large electron–vibration coupling of the conjugated carbon backbone might preclude thepossibility of efficient organic chromophores in the mid-infrared. Comparedto organic materials, inorganic semiconductor quantum dots have weakerelectron–vibration coupling and weaker electron–electron interactions. Asbriefly reviewed in this chapter, the intraband transitions are spectrally welldefined in the mid-infared, strong, size-tunable, and controllable by electroninjection. The chemically synthesized quantum dots therefore have a uniqueappeal as mid-infrared ‘‘dyes’’ and have also a potential as nonlinear opticalelements [58,59]. The improvement in the quality of nanocrystal materials,however, is still required to improve the control over the intraband transitionenergies and electronic relaxation pathways. Ultimately, the intrabandresponse of the colloidal quantum dots may find widespread applications inmid-infrared technologies.REFERENCES1. West, L.C.; Eglash, S.J. Appl. Phys. Lett. 1985, 46, 1156.2. Levine, B.F.; Malik, R.J.; Walker, J.; Choi, K.K.; Bethea, C.G.; Kleinman, D.A.;Vandenberg, J.M. Appl. Phys. Lett. 1987, 50, 273.3. Rosencher, E.; Bois, P.; Nagle, J.; Costard, E.; Delaitre, S. Appl. Phys. LettCopyright 2004 by Marcel Dekker, Inc. All Rights Reserved.