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

continuous spectral tunability over a wide energy range simply by changingthe size of the dots.Quantum-dot lasing was initially realized in an optically pumped deviceusing relatively large (f10 nm) CdSe nanoparticles fabricated via high-temperatureprecipitation in glass matrices [3]. Later, this effect was observed forepitaxially grown nanoislands (epitaxial or self-assembled QDs) using bothoptical and electrical (injection) pumping [4,5]. As predicted, lasers basedon epitaxial QDs show an enhanced performance in comparison with, forexample, QW lasers and feature reduced thresholds, improved temperaturestability, and high differential optical gain (important for achieving highmodulation rates).The success of the laser technology based on epitaxial QDs has been astrong motivation force for the development of laser devices based onultrasmall, sub-10-nm chemically synthesized nanoparticles. Such nanoparticles,known also as colloidal or nanocrystal QDs (NQDs) can be routinelygenerated with narrow size dispersions (f5%) using organometallic reactions[6–8]. In the sub-10-nm size range, electronic interlevel spacings can exceedhundreds of millielectron volts and size-controlled spectral tunability over anenergy range as wide as 1 eV can be achieved. Furthermore, improvedschemes for surface passivation by, for example, overcoating NQDs with ashell of a wide-gap inorganic semiconductor [9] allow significant suppressionof surface trapping and produce room-temperature photoluminescence (PL)quantum efficiencies greater than 50%. Additionally, due to their chemicalflexibility, NQDs can be easily prepared as close-packed films [10] (NQDsolids) or incorporated with high densities into polymers or sol–gel glasses.[11,12]. NQDs are, therefore, compatible with existing fiber-optic technologiesand are useful as building blocks for the bottom-up assembly of variousoptical devices, including optical amplifiers and lasers.Despite a decade of research that provided some indication of opticalgain performance [13], strongly confined NQDs had failed to yield lasing innumerous efforts. Difficulties in achieving lasing have often been attributed tohigh nonradiative carrier losses due to trapping at surface defects, a directconsequence of the large surface-to-volume ratio characteristic of sub-10-nmparticles. Another concern raised in several theoretical articles is the stronglyreduced efficiency of electron–phonon interactions in the case of discrete,atomiclike energy structures, which are characteristic of small-size dots[14,15]. For discrete spectra, the availability of pairs of electronic statessatisfying energy conservation in phonon-assisted processes is drasticallyreduced compared to the quasicontinuous spectra of bulk materials. This deficiencyhas been expected to significantly lower the efficiency of carriercooling due to phonon emission (the effect known as a ‘‘phonon bottleneck’’),leading to reduced carrier flows into the lowest ‘‘emitting’’ states and, hence,reduced PL efficiencies. However, the difficulties anticipated due to carrierCopyright 2004 by Marcel Dekker, Inc. All Rights Reserved.