Materials with optical gain in the infrared are of paramount importance for
optical communications, medical diagnostics1 and silicon photonics2,3 . 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 medium4 in view of their
tunable bandgap, solution processability and CMOS compatibility. Their
potential for narrower linewidths5 and the lower-than-bulk degeneracy6 has led
to dramatic progress towards successful demonstration of optical gain4,
stimulated emission7 and lasing8,9,10 in the visible part of spectrum utilizing
CdSe-based CQDs. Infrared Pb-chalcogenide colloidal quantum dots however
exhibit higher state degeneracy and as a result the demonstration of optical
gain has imposed very high thresholds.11,12 Here we demonstrate
room-temperature, infrared stimulated emission, tunable across the optical
communication band, based on robust electronically doped PbS CQDs, that reach
gain threshold at the single exciton regime, representing a four-fold reduction
from the theoretical limit of an eight-fold degenerate system and two orders of
magnitude lower than prior reports