Ultrafast Carrier Dynamics, Optical Amplification, and Lasing in Nanocrystal Quantum Dots
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Ultrafast Carrier
Dynamics, Optical Amplification, and Lasing in Nanocrystal Quantum Dots
Victor I. Klimov and Moungi G. Bawendi Introduction Semiconductor materials are widely used in both optically and electrically pumped lasers. The use of semiconductor quantum wells (QWs) as optical-gain media has resulted in important advances in laser technology. QWs have a two-dimensional, step-like density of electronic states that is nonzero at the band edge, enabling a higher concentration of carriers to contribute to the band-edge emission and leading to a reduced lasing threshold, improved temperature stability, and a narrower emission line. A further enhancement in the density of the band-edge states and an associated reduction in the lasing threshold are in principle possible using quantum wires and quantum dots (QDs), in which the confinement is in two and three dimensions, respectively. In very small dots, the spacing of the electronic states is much greater than the available thermal energy (strong confinement), inhibiting thermal depopulation of the lowest electronic states. This effect should result in a lasing threshold that is temperatureinsensitive at an excitation level of only 1 electron-hole (e-h) pair per dot on average.1 Additionally, QDs in the strongconfinement regime have an emission wavelength that is a pronounced function of size, adding the advantage of continuous spectral tunability over a wide energy range simply by changing the size of the dots.
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QD lasers have been demonstrated previously using epitaxially grown nanoislands.2,3 As predicted, these lasers show an enhanced performance, in comparison with, for example, QW lasers, and feature reduced thresholds, improved temperature stability, and high differential gain (important for achieving high modulation rates). This success has been a strong motivating force for the development of lasers based on chemically synthesized nanocrystal quantum dots (NQDs). Direct colloidal chemical synthesis provides routine preparations of freestanding semiconductor nanoparticles (i.e., NQDs) with sub-10-nm sizes that correspond to the regime of extremely strong confinement.4 In this size range, electronic interlevel spacings can exceed hundreds of millielectronvolts, and size-controlled spectral tunability over an energy range as wide as 1 eV can be achieved. Furthermore, improved schemes for surface passivation (e.g., by overcoating with a shell of a wide-gap semiconductor),5 particularly well developed for CdSe NQDs, allow significant suppression of surface trapping and produce room-temperature photoluminescence (PL) quantum efficiencies as high as 50%, with emission wavelengths tunable across the entire visible spectrum. Additionally, due to their chemical flexibility, NQDs can be easily prepared as close-packed films (NQD solids) or incor-
porated with high densities into glasses or polymers. NQDs are, therefore, compatible with existing fiber-optic technologies and are useful as building blocks for bottomup assembly of various optical devices, inclu
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