  Semiconductor nanocrystals can be synthesized using inexpensive, scalable,
solution-based techniques, and their utility as tunable light emitters has been
demonstrated in various applications, including biolabeling and light-emitting
devices. By contrast, the use of colloidal nanocrystals for optical
amplification and lasing has been limited by the high input power densities
that have been required. In this work, we show that colloidal nanoplatelets
(NPLs) produce amplified spontaneous emission (ASE) with pump-fluence
thresholds as low 6 uJ/cm2 and gain as high as 600 cm-1, both a 4-fold
improvement over the best reported values for colloidal nanocrystals; in
addition, gain saturation occurs at pump fluences two orders of magnitude
higher than the ASE threshold. We attribute this exceptional performance to
large optical cross-sections, slow Auger recombination rates, and the narrow
emission linewidth of the NPL ensemble. The NPLs bring the advantages of
quantum wells as an optical gain medium to a colloidal system, opening up the
possibility of producing high-efficiency, solution-processed lasers.
