The Ultrasmall-Quantum-Dot Paradox: Light Amplification and Lasing from Non-Emissive Species

Optical gain in ultrasmall (sub-10 nm) semiconductor nanocrystals (nanocrystal quantum dots) relies on emission from multi-exciton states. However, the decay of these states is dominated by nonradiative Auger processes rather than by radiative recombination, which makes them nominally non-emissive species. One approach to forcing these non-emissive multi-excitons to lase is through increasing the dot density in the sample until the rate of stimulated emission becomes greater than that for the Auger decay. Although this straightforward approach does work [1], the suppression of the Auger recombination remains an important current challenge in the field of nanocrystal lasing. In our work we explore the "geometrical" effects (e.g., nanocrystal shape control) for controlling the rates of the multi-particle decay. In particular, we study the effect of the zero- to one-dimensional (1D) transformation on multi-particle Auger recombination using series of elongated semiconductor nanocrystals (quantum rods). We observe an interesting new effect, namely, the transition from the three- to two-particle recombination process as the nanocrystal aspect ratio is increased. This transition implies that in the 1D confinement limit, Auger decay is dominated by Coulomb interactions between 1D excitons that recombine in a bimolecular fashion. One consequence of this effect is strongly reduced decay rates of higher multi-particle states that lead to the increased optical gain lifetime and efficient light amplification due to transitions involving excited electronic states. These unique rod properties suggest that shape control may be key to developing practical lasing applications of nanocrystals.

[1] V. I. Klimov, A. A. Mikhailovski, S. Xu, A. Malko, J. A. Hollingsworth, C. A. Leatherdale, H.-J. Eisler, and M. G. Bawendi, Science 290, 314 (2000).