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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., Comment: arXiv admin note: substantial text overlap with arXiv:1908.03796

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.

Defects play an important role in tailoring the optoelectronic properties of materials. Here we demonstrate that sulphur vacancies are able to engineer sub-band photoresponse into the short-wave infrared range due to formation of in-gap states in Bi2S3 single crystals supported by density functional (DF) calculations. Sulfurization and subsequent refill of the vacancies results in faster response but limits the spectral range to the near infrared as determined by the bandgap of Bi2S3. A facile chemical treatment is then explored to accelerate the speed of sulphur deficient (SD)-based detectors on the order of 10 ms without sacrificing its spectral coverage into the infrared, while holding a high D* close to 10^15 Jones in the visible-near infrared range and 10^12 Jones at 1.6 um. This work also provides new insights into the role sulphur vacancies play on the electronic structure and, as a result, into sub-bandgap photoresponse enabling ultrasensitive, fast and broadband photodetectors.