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

high-threshold e–h plasma mechanisms. Thermal dissociation of excitons atroom temperature (the exciton binding energy in CdSe is 16 meV) results in asignificantly increased threshold for light amplification. Because of a largeinterlevel spacing, ‘‘quantum-confined’’ excitons in NQDs are more robustthan bulk excitons, allowing one to excite room-temperature ASE at pumplevels comparable to those at cryogenic temperatures. This is an illustrativeexample of the enhanced temperature stability of optical gain expected forstrongly confined NQDs.B. Modal Gain StudiesOne of the most direct ways of quantifying optical-gain properties ofmaterials is by using ‘‘variable-stripe-length’’ measurements of ASE [61]. Inthese measurements, the excitation beam is focused onto the sample with acylindrical lens into a narrow stripe of variable length l and the emissionintensity is monitored as a function of the stripe length (see inset in Fig. 25a).This experiment allows one to determine net or modal gain ( g m ), whichrepresents a difference between material gain ( g) and linear optical losses due,for example, to light scattering.Figure 25a shows a set of room-temperature emission spectra for a CdSeNQD film (mean dot radius R = 2.5 nm) recorded using a ‘‘variable-stripelength’’configuration. The development of ASE (a narrow peak at f635 nm)occurs at a threshold stripe length, l th , of f0.06 cm. The ASE threshold isclearly seen in the plot of the emission intensity (I) versus the stripe length as asharp increase in the slope at l>l th (Fig. 25b). To derive the magnitude ofmodal gain, the I-versus-l dependence is usually analyzed using the expressionI = A(e g ml 1)/g m , in which A is a constant proportional to the spontaneousemission power density [61]. This dependence should, in principle, describeboth an initial, linear intensity growth below the ASE threshold (spontaneousemission regime) and a fast, exponential increase above it (stimulatedemission regime). However, in the case of NQD samples, this simple expressiondoes not allow one to simultaneously fit both regimes (see fit shown bydashed line in Fig. 25b). This discrepancy is fundamentally linked to the factthat in NQDs, spontaneous and stimulated emission arise from different typesof excitation.As indicated by the analysis in previous sections, optical gain and hence,stimulated emission in NQDs are dominated by quantum-confined biexcitons(doubly-excited dots). However, because of their short lifetimes, which arelimited by nonradiative Auger recombination, biexcitons are not pronouncedin spontaneous emission. Simple estimations based on the comparison ofefficiencies of Auger decay and radiative recombination indicate that thecontribution of biexcitons to time-integrated spontaneous emission is lessCopyright 2004 by Marcel Dekker, Inc. All Rights Reserved.