Abstract
Colloidal quantum dots (QDs) raise more and more interest as solution-processable and tunable optical gain materials. However, especially for infrared active QDs, optical gain remains inefficient. Since stimulated emission involves multifold degenerate band-edge states, population inversion can be attained only at high pump power and must compete with efficient multi-exciton recombination. Here, we show that mercury telluride (HgTe) QDs exhibit size-tunable stimulated emission throughout the near-infrared telecom window at thresholds unmatched by any QD studied before. We attribute this unique behaviour to surface-localized states in the bandgap that turn HgTe QDs into 4-level systems. The resulting long-lived population inversion induces amplified spontaneous emission under continuous-wave optical pumping at power levels compatible with solar irradiation and direct current electrical pumping. These results introduce an alternative approach for low-threshold QD-based gain media based on intentional trap states that paves the way for solution-processed infrared QD lasers and amplifiers.
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Acknowledgements
This research is funded by Ghent University (Special Research Fund BOF), BelSPo (IAP 7.35, photonics@be), EU-FP7 (Navolchi), NWO (Vidi grant, No. 723.013.002) Horizon 2020 ITN Phonsi and ERC-ULPICC and ERC-PoC Interdot. P.G. acknowledges the FWO Vlaanderen for a postdoctoral fellowship. S. Flamee is acknowledged for TEM imaging of the QDs and R. Van Deun and P. Smet are acknowledged for the use of the steady-state and time-resolved photoluminescence set-up and the cryogenic spectroscopy, respectively.
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P.G. carried out the steady-state and time-resolved photoluminescence, and the ultrafast experiments, analysed the data, aided in theory development and wrote the manuscript; A.J.H. supervised the experiments, aided in theory discussions and wrote the manuscript; I.I. and F.Z. performed the DFT calculations and wrote the manuscript; L.K.S. synthesized the HgTe QDs and performed structural characterization (TEM, XRD); C.D. and G.A. carried out the tight-binding simulations and aided in theory development; D.V.T. aided in theory discussions and supervised the research; Z.H. initiated and supervised the research, aided in the theory development and wrote the manuscript.
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Geiregat, P., Houtepen, A., Sagar, L. et al. Continuous-wave infrared optical gain and amplified spontaneous emission at ultralow threshold by colloidal HgTe quantum dots. Nature Mater 17, 35–42 (2018). https://doi.org/10.1038/nmat5000
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DOI: https://doi.org/10.1038/nmat5000