Monoexcitonic Lasing of Colloidal Semiconductor Nanocrystals in Polymeric Parabolic Microcavities

Colloidal semiconductor nanocrystals constitute materials of choice to realize microlasers, thanks to the fine control of their optical and electronic properties and to their solution processability. However, so far, lasing has mostly been obtained under high excitation, in the biexciton regime. Here, we demonstrate instead that, in high-quality cavities, lasing naturally occurs in the single exciton regime. We describe the facile fabrication of nanocrystal-coated polymeric microcavities and study their lasing characteristics. We show that the lasing threshold lies in the submonoexcitonic excitation regime for CdS/CdSe/CdS quantum shells in particular, thanks to the high quality factor of the cavities and high transition cross sections of the quantum shells. We explain these results using a comprehensive model of nanocrystal optical properties. The model correctly reproduces lasing characteristics near and above the threshold. In this regime, in contrast to previous studies, the performance of the microlasers relies mostly on the transition cross section and the Stokes shift and not on biexciton recombination or exciton-biexciton splitting. This improved understanding will enable rational design of colloidal nanocrystals for low-threshold lasing applications.