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

ottleneck’’ [14,28]. Further reduction in the energy loss rate is expected in theregime of strong confinement, for which the level spacing can be much greaterthan LO phonon energies and, hence, carrier–phonon scattering can onlyoccur via weak multiphonon processes.In II–VI NQDs, the energy separation between electron states is muchgreater than the separation between hole states; therefore, the ‘‘phononbottleneck’’ was expected to affect electron intraband dynamics more stronglycompared to hole intraband dynamics. Electron intraband dynamics inCdSe NQDs were extensively studied using an ultrafast transient absorption(TA), pump-probe technique [29–31]. TA is a nonlinear optical method whichallows for the monitoring (in both time and spectral domains) of absorptionchanges caused by sample photoexcitation. In the ultrafast TA experiment,nonequilibrium charge carriers are rapidly injected into a material with ashort, typically subpicosecond pump pulse. Absorption changes (Da) associatedwith photogenerated carriers are monitored with a second short probepulse which can be derived from either a tunable, monochromatic (e.g., opticalparametric amplifier), or broadband [e.g., femtosecond (fs) white-lightcontinuum] light source.One mechanism for carrier-induced absorption changes in NQDs isstate filling that leads to the bleaching of optical transitions involving electronicstates occupied by nonequilibrium carriers (a detailed analysis ofmechanisms for resonant optical nonlinearities can be found in Refs. 32and 33). Because of a high spectral density of valence-band states, TAbleaching signals are dominated by the filling of electron quantized levels.Therefore, by monitoring the dynamics of transitions associated, for example,with 1S and 1P electron states, one can directly evaluate the rate of the 1P-to-1S intraband relaxation.An example of such dynamics recorded for CdSe NQDs with R = 4.1nm at the positions of the 1S(e)–1S 3/2 (h) and 1P(e)–1P 3/2 (h) transitions (thesetransitions are referenced later as 1S and 1P, respectively) is displayed inFig. 2a. These dynamics indicate a fast population decay (f500 fs timeconstant) of the 1P state, which is complementary to the growth of the 1Sstate population. The observed fast relaxation is despite the very large 1S–1Penergy separation of about eight LO phonon energies. Interestingly, intrabandrelaxation becomes even faster with decreasing NQD size, as evidentfrom a comparison of the 1S state population dynamics for NQDs of differentradii shown in Fig. 2b. These data indicate a decrease in the 1S builduptime [s b (1S)] with decreasing NQD radius: s b (1S) is 530 fs for R = 4.1 nm andshortens down to 120 fs for R = 1.7 nm, roughly following a linear sizedependence.Extremely fast electron relaxation, as well as a confinement-inducedenhancement in the relaxation process, clearly indicate that energy relaxationCopyright 2004 by Marcel Dekker, Inc. All Rights Reserved.