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