Content area
Full Text
REPORTS
The development of optical gain in chemically synthesized semiconductor nanoparticles (nanocrystal quantum dots) has been intensely studied as the first step toward nanocrystal quantum dot lasers. We examined the competing dynamical processes involved in optical amplification and lasing in nanocrystal quantum dots and found that, despite a highly efficient intrinsic nonradiative Auger recombination, large optical gain can be developed at the wavelength of the emitting transition for close-packed solids of these dots. Narrowband stimulated emission with a pronounced gain threshold at wavelengths tunable with the size of the nanocrystal was observed, as expected from quantum confinement effects. These results unambiguously demonstrate the feasibility of nanocrystal quantum dot lasers.
Semiconductor lasers have become ubiquitous. Both optically and electrically pumped lasers are widely used in fields ranging from telecommunications and information storage and processing to medical diagnostics and therapeutics. The use of semiconductor quantum well (QW) structures as optical gain media has resulted in important advances in semiconductor laser technology (1, 2). Quantum confinement in one dimension restricts carrier motion in QWs to the remaining two dimensions. Consequently, QWs have a twodimensional steplike 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 states at the band edge and an associated reduction in the lasing threshold is, in principle, possible with quantum wires and quantum dots (QDs), where the quantum confinement is in two and three dimensions, respectively (3, 4). The electronic spectrum of QDs consists of well-separated atomic-like states with an energy spacing that increases as the dot size is reduced (5, 6). In very small QDs, the spacing of the electronic states is much greater than the available thermal energy (strong confinement), inhibiting thermal depopulation of the lowest electronic states, which should result in a lasing threshold that is temperature-insensitive at an excitation level of only one electron-hole (e-h) pair per dot on average (4). Additionally, QDs in the strong confinement 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...