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

C. Hole Intraband RelaxationAs analyzed in the previous subsection, energy transfer from electrons to holesdominates over other (e.g., phonon-related) mechanisms of intraband energyloses in the conduction band. However, because of a small interlevel spacing inthe valence band (particularly, well above the energy gap), phonon emissionby holes can provide an efficient mechanism for energy dissipation [39].Experimental studies of hole intraband dynamics are complicated bydifficulties in detecting valence-band populations. Most carrier intrabandrelaxation studies in NQDs have been performed using femtosecond TAexperiments [29,30,40]. However, holes are only weakly pronounced in TAspectra because of a high spectral density of valence-band states resulting in asignificant spread of hole populations over multiple levels. One of theapproaches to detecting hole intraband relaxation is by monitoring theultrafast dynamics of ‘‘hot’’ PL [39]. In contrast to state-filling-induced TAsignals that are proportional to the sum of the electron and hole occupationnumbers, PL is proportional to the product of these numbers. Therefore,electron and hole dynamics can, in principle, be decoupled by performingboth PL and TA studies.Experimental studies of initial PL relaxation require methods that canprovide subpicosecond time resolution. Such resolution is possible using afemtosecond up-conversion experiment [41]. In this experiment, a sampleemission is frequency mixed with an ultrashort (typically, femtosecond)gating pulse in a nonlinear optical crystal. The sum-frequency signal,spectrally selected with a monochromator, is proportional to an instant PLintensity at the moment defined by an arrival time of the gating pulse. Therefore,by scanning the gating pulse with respect to the PL pulse, one can obtaininformation about a PL temporal profile. Alternatively, by fixing the arrivaltime of the gating pulse and scanning a monochromator (and simulteneouslyadjusting a phase-matching angle in the nonlinear crystal), one can obtaintime-resolved, ‘‘instant’’ emission spectra.Figure 5(a) shows an example of ‘‘hot’’ PL spectra recorded for CdSedots with a 1.8-nm mean radius. The pump photon energy used in thesemeasurements (3 eV) is close to the energy of the 1S(e)–3S 3/2 (h) transition(f3.2 eV). Therefore, for a significant number of dots in the sample, electronsare generated directly in the lowest 1S state, whereas photogenerated holes arevery ‘‘hot’’ and carry a large excess energy of f0.9 eV. ‘‘Hot’’ hole dynamicsare well pronounced in time-resolved PL spectra (Fig. 5a) and indicate agradual relaxation of holes from high-energy states (manifested, e.g., as a‘‘hot’’ PL peak at f2.8 eV) to the lowest ‘‘emitting’’ state giving rise to theband-edge PL band.A descent of holes through the ladder of the valence-band states is easilyseen in PL time transient recorded at different spectral energies (Fig. 5b).Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.