Ultrafast cascade charge transfer in multi bandgap colloidal quantum dot solids enables threshold reduction for optical gain and stimulated emission

Achieving low-threshold infrared stimulated emission in solution-processed quantum dots is critical to enable real-life application including photonic integrated circuits (PICs), LIDAR application and optical telecommunication. However, realization of low threshold infrared gain is fundamentally challenging due to high degeneracy of the first emissive state (e.g., 8-fold) and fast Auger recombination. In this letter, we demonstrate ultralow-threshold infrared stimulated emission with an onset of 110 uJ.cm-2 employing cascade charge transfer (CT) in Pb-chalcogenide colloidal quantum dot (CQD) solids. In doing so, we investigate this idea in two different architectures including a mixture of multiband gap CQDs and layer-by-layer (LBL) configuration. Using transient absorption spectroscopy, we show ultrafast cascade CT from large band-gap PbS CQD to small band-gap PbS/PbSSe core/shell CQDs in LBL (~ 2 ps) and mixture (~ 9 ps) configuration. These results indicate the feasibility of using cascade CT as an efficient method to reduce optical gain threshold in CQD solid films.