Colloidal quantum dots are nanometer sized semiconductor crystalllites obtained by solution-based synthesis. They exhibit unique optical and electronic properties that depend on their size, shape and composition. In combination with their suitability for solution processing, many applications are found for these novel materials, ranging from lasing, lighting, photovoltaic devices to bio-imaging. Due to their tunable photoluminescence, colloidal quantum dots are promising optical gain materials for lasers. Moreover, the low cost and ease of processing provide substantial benefits over classical, epitaxially grown semiconductors. However, high pumping thresholds for photo-excitation and low material gain remain limiting factors.
Here, we have investigate optical gain in CdSe nanoplatelets, also called ‘solution processable quantum wells’ using ultrafast pump-probe spectroscopy. As platelets are 2D materials, they have advantages like large absorption cross section, slow non-radiative recombination rates and narrow emission linewidth. Recent studies have shown gain in these platelets under continuous wave optical pumping, but it remains unclear under what conditions – single/bi-exciton or multi-excitons – optical gain is achieved. We address this point in a quantitative transient absorption study on CdSe platelets of different thickness. We observe optical gain in 3 monolayer (MLs) thick CdSe platelets, yet we find a gain threshold that corresponds to approximately 160 excitons per platelet, a number substantially higher than the bi-exciton regime. These results question the interpretation of the optical properties of CdSe nanoplatelets in terms of quantum-dot like models.