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

from a long lifetime due to the phonon bottleneck). Further studies alongthese lines will require a better understanding of the electronic coupling ofmolecular surface ligands to the quantum states.In another experiment by Klimov et al. [39], the strategy was similarexcept that the last pulse was an intraband probe of the 1S 3/2 –1S e and 1P 3/2 –1P e excitons, and the temporal resolution was f0.3 ps. The samples comparedwere (CdSe)ZnS nanocrystals, which were expected to have no holetraps, and CdSe-pyridine, which were expected to have strong hole traps. Thebleach time constants were 0.3 ps and 3 ps, respectively, which seems toconfirm the role that the hole plays in the intraband relaxation. However,unlike the previous experiment, no slow component was observed. Onepossible explanation for the discrepancy is the smaller size of the nanocrystalsin the second study, enhancing the role of the surface in the relaxation process.There have been many more studies investigating the phonon bottleneckin epitaxial quantum dots. Although most reported a fast relaxation(100 ps) intraband relaxation for photoexcitedInGaAs/GaAs quantum dots prepared with a single electron and no hole [42].Furthermore, stimulated intraband emission has also been recently achieved[43,44]. Semiconductor colloidal quantum dots may also become efficientmid-IR ‘‘laser dyes’’ after one learns how to slow down the intraband relaxation,which will probably require better control of the surfaces states [33].An attractive characteristic of quantum dots is their narrow spectralfeatures. Narrow linewidths and long coherence times would be appealing inquantum logic operations using quantum dots [45]. On the other hand, at leastfor maximizing gain in laser applications, it is best if the overall linewidth isdominated by homogenous broadening [46,47]. Given that the methods tomake quantum dots all lead to finite size dispersion, efforts to distinguishhomogenous and inhomogeneous linewidths in samples have been pursuedfor more than a decade.The first interband spectral hole burning [48–50] or photon-echo [51]measurements of colloidal quantum dots, performed at rather high powersand low repetition rates, yielded broad linewidths, typically >10 meV at lowtemperature. These results are now superseded by more recent low-intensitycontinuous-wave (CW) hole-burning [52] and accumulated photon-echoexperiments, which have uncovered sub-meV homogeneous linewidths forthe lowest interband absorption in various quantum-dot materials [53–55].In parallel, single CdSe nanocrystal photoluminescence has yielded a linewidthof f100 AeV for the emitting state [56]. For intraband optical applications,the linewidth of the 1S e –1P e state is of interest, but the broadeningof the 1P e state in the conduction band cannot be determined by interbandspectroscopy because of the congestion of the hole states and hole dynamicsthat affect the linewidth broadening. Intraband hole burning or photon echoCopyright 2004 by Marcel Dekker, Inc. All Rights Reserved.