  Materials with optical gain in the infrared are of paramount importance for
optical communications, medical diagnostics and silicon photonics. The current
technology is based either on costly III-V semiconductors that are not
monolithic to silicon CMOS technology or Er-doped fiber technology that does
not make use of the full fiber transparency window. Colloidal quantum dots
(CQD) offer a unique opportunity as an optical gain medium in view of their
tunable bandgap, solution processability and CMOS compatibility. The 8-fold
degeneracy of infrared CQDs based on Pb-chalcogenides has hindered the
demonstration of low-threshold optical gain and lasing, at room temperature. We
demonstrate room-temperature, infrared, size-tunable, band-edge stimulated
emission with linewidth of ~14 meV. Leveraging robust electronic doping and
charge-exciton interactions in PbS CQD thin films, we reach gain threshold at
the single exciton regime representing a four-fold reduction from the
theoretical limit of an eight-fold degenerate system, with a net modal gain in
excess of 100 cm-1.
