Light amplification in the single-exciton regime using exciton-exciton repulsion in type-II nanocrystal quantum dots

Optical gain in ultrasmall semiconductor nanocrystals requires that some of the nanoparticles in the ensemble be excited with multiple electron-hole pairs (multiexcitons). A significant complication arising from this multiexciton nature of optical amplification is the ultrafast gain decay induced by nonradiative Auger recombination. Here, we develop a simple model for analyzing optical gain in the nanocrystals in the presence of exciton-exciton (X-X) interactions. This analysis indicates that if the X-X interaction is repulsive and its energy is large compared to the ensemble line width of the emitting transition, optical gain can occur in the single-exciton regime without involvement of multiexcitons. We further analyze theoretically and experimentally X-X interactions in type-II heteronanocrystals of CdS (core)/ZnSe (shell) and ZnTe (core)/CdSe (shell) and show that they can produce giant repulsion energies of more than 100 meV resulting from a significant local charge density generated as a result of spatial separation between electrons and holes. We observe that the dynamical and spectral properties of optical gain in type-II nanocrystals are distinctly different from those of multiexciton gain in traditional type-I nanocrystals and are consistent with those expected for the single-exciton regime. An important implication of these results is the possibility of a significant increase in the optical-gain lifetime, which could simplify applications of chemically synthesized nanocrystals in practical lasing technologies and perhaps allow for lasing using electrical injection.