Efficient Optical Gain in Spherical Quantum Wells Enabled by Engineering Biexciton Interactions

Due to the strong confinement of carriers, electronic interactions play a major role in the performance of colloidal nanocrystals (NCs) as materials for optical gain. Therefore, understanding how to manipulate this phenomenon is essential to achieve feasible devices in the future. In this context, plane colloidal quantum wells (PQWs) have emerged as one of the most promising classes of NCs to be applied as a lasing material, mainly due to the way that carriers and light interact in these structures. Here, a new NC geometry is introduced, called a spherical quantum well (SQW), composed of a CdS/CdSe/CdS (core/well/shell) heterostructure. Similar to PQWs, in this structure it is possible to combine a long biexciton Auger recombination lifetime (>1 ns) with a large absorption cross section and near-unity photoluminescence quantum yield. Also, due to the quantum well structure, this material enables enhanced biexciton interactions even for large volumes. This boosts the gain performance, decreasing the gain threshold to an average number of excitons of ⟨N⟩T = 0.61 and enhancing the intrinsic gain coefficient. Additionally, by growing thick-shell SQWs it is possible to enhance the absorption cross section, reaching an amplified spontaneous emission (ASE) threshold of 10.6 μJ/cm2. Finally, ASE tests demonstrated that this geometry enables enhanced stability, showing no signs of degradation after 287 days of exposure to regular atmospheric conditions and sustained ASE for more than 13 h under femtosecond laser irradiation. These results demonstrate that the SQW structure combines important characteristics of both PQW and core/shell geometries, representing an important step toward a solution-processed quantum dot laser in the future.