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Materials with optical gain in the infrared are of paramount importance foroptical communications, medical diagnostics1 and silicon photonics2,3 . Thecurrent technology is based either on costly III-V semiconductors that are notmonolithic to silicon CMOS technology or Er-doped fiber technology that doesnot make use of the full fiber transparency window. Colloidal quantum dots(CQD) offer a unique opportunity as an optical gain medium4 in view of theirtunable bandgap, solution processability and CMOS compatibility. Theirpotential for narrower linewidths5 and the lower-than-bulk degeneracy6 has ledto dramatic progress towards successful demonstration of optical gain4,stimulated emission7 and lasing8,9,10 in the visible part of spectrum utilizingCdSe-based CQDs. Infrared Pb-chalcogenide colloidal quantum dots howeverexhibit higher state degeneracy and as a result the demonstration of opticalgain has imposed very high thresholds.11,12 Here we demonstrateroom-temperature, infrared stimulated emission, tunable across the opticalcommunication band, based on robust electronically doped PbS CQDs, that reachgain threshold at the single exciton regime, representing a four-fold reductionfrom the theoretical limit of an eight-fold degenerate system and two orders ofmagnitude lower than prior reports.

Single-Exciton Gain and Stimulated Emission Across the Infrared Optical Telecom Band from Robust Heavily-doped PbS Colloidal Quantum Dots