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Colloidal semiconductor quantum wells have emerged as a promising materialplatform for use in solution-processable light-generation including colloidallasers. However, application relying on their optical gain suffer from afundamental complication due to multi-excitonic nature of light amplificationin common II-VI semiconductor nanocrystals. This undesirably increases theoptical gain threshold and shortens the net gain lifetime because of fastnonradiative Auger decay. Here, we demonstrate sub-single exciton level ofoptical gain threshold in specially engineered CdSe/CdS@CdZnScore/crown@gradient alloyed shell colloidal quantum wells. This sub-singleexciton ensemble-averaged gain threshold of Ng = 0.80 (per particle) resultingfrom impeded Auger recombination along with a large absorption cross-section ofquantum wells enables us to observe the amplified spontaneous emission startingat a low pump fluence of 800 nJ cm-2, at least three-folds better than thepreviously best reported values among all colloidal semiconductor nanocrystals.Moreover, long optical gain lifetimes of 800 ps accompanied with modal gaincoefficients of 2,000 cm-1 are achieved. Finally, using these gradient shelledquantum wells, we show a vertical cavity surface-emitting colloidal laseroperating at an ultralow lasing threshold of 7.5 micro-joule cm-2. Theseresults represent a significant step towards the realization ofsolution-processable electrically-driven colloidal lasers.

Sub-single exciton optical gain threshold in colloidal semiconductor quantum wells with gradient alloy shelling