There has long been an interest in using colloidal semiconductor nanocrystals as laser gain media, due to their tunable emission wavelengths, low cost of synthesis, and solution processability. However, early demonstrations of optical gain and lasing involved impractically high threshold excitation densities. These high thresholds were attributed to Auger processes: optical gain requires excitation of more than one exciton in each nanocrystal, but multiple excitons undergo rapid, nonradiative Auger recombination. Quantum dots that have been designed to suppress Auger recombination have resulted in reduced thresholds; however, this has come at the expense of the maximum obtainable gain, because the quantum dots have thick shells that limit their packing density. Recent innovations in colloidal synthesis have led to thin, flat CdSe nanoplatelets and CdSe/CdSe core/shell nanoplatelets, where carriers are confined quantum-mechanically in only one dimension. We have found that Auger recombination still dominates carrier dynamics over a large range of exciton densities, but the Auger rate at excitons that allow for optical gain is lower than in quantum dots emitting at the same wavelengths. Even lower Auger rates were observed when a CdS shell was grown around the CdSe core, with a minimum rate for shell thicknesses of 3 – 4 monolayers. In addition, the nanoplatelets are atomically monodisperse in thickness, virtually eliminating inhomogeneous broadening, and have large optical-absorption cross-sections. Together, these factors are expected to reduce thresholds for optical gain and lasing. We verified the superior properties of nanoplatelets by measuring thresholds for amplified spontaneous emission in films of these nanoplatelets that are lower than any previous demonstration for colloidal nanocrystals, and that are comparable to the best reported values for any solution-processed materials. Integration of nanoplatelets with a photonic-crystal cavity leads to continuous-wave lasing with the lowest reported threshold for any laser operating at room temperature.

Low-threshold optical gain and lasing with colloidal semiconductor nanoplatelets

254th National Meeting (2017)

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