Towards zero-threshold optical gain using charged semiconductor quantum dots


Colloidal semiconductor quantum dots are attractive materials for the realization of solution-processable lasers. However, their applications as optical-gain media are complicated by a non-unity degeneracy of band-edge states, because of which multiexcitons are required to achieve the lasing regime. This increases the lasing thresholds and leads to very short optical gain lifetimes limited by nonradiative Auger recombination. Here, we show that these problems can be at least partially resolved by employing not neutral but negatively charged quantum dots. By applying photodoping to specially engineered quantum dots with impeded Auger decay, we demonstrate a considerable reduction of the optical gain threshold due to suppression of ground-state absorption by pre-existing carriers. Moreover, by injecting approximately one electron per dot on average, we achieve a more than twofold reduction in the amplified spontaneous emission threshold, bringing it to the sub-single-exciton level. These measurements indicate the feasibility of ‘zero-threshold’ gain achievable by completely blocking the band-edge state with two electrons.

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Figure 1: Concept of zero-threshold optical gain.
Figure 2: ‘Interface-engineered’ core/alloy/shell (C/A/S) CdSe/CdSexS1−x/CdS QDs.
Figure 3: Absorption spectra and PL dynamics of charged CdSe/CdSexS1−x/CdS C/A/S QDs.
Figure 4: Optical gain in charged CdSe/CdSexS1−x/CdS C/A/S QDs.
Figure 5: ASE and lasing thresholds in charged QDs.


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Work on the synthesis of core/alloy/shell quantum dots and studies of Auger recombination in synthesized materials were supported by the Chemical Sciences, Biosciences and Geosciences Division, Office of Basic Energy Sciences, Office of Science, US Department of Energy. The studies of the effect of charging on quantum dot optical gain properties were supported by the Laboratory Directed Research and Development (LDRD) programme at Los Alamos National Laboratory (LANL). K.W. acknowledges support by a LANL Director's Postdoctoral Fellowship.

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J.L. synthesized the quantum dots and conducted their microstructural characterization. K.W. conducted spectroscopic studies of the effect of charging on quantum dot optical properties. K.W. and Y.-S.P. conducted measurements of amplified spontaneous emission. K.W. and V.I.K. analysed the data and performed modelling of optical-gain performance of charged quantum dots. K.W., Y.-S.P. and V.I.K. performed theoretical modelling of lasing using charged quantum dots. K.W. and V.I.K. wrote the manuscript, with input from the other authors.

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Correspondence to Victor I. Klimov.

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The authors declare no competing financial interests.

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Wu, K., Park, Y., Lim, J. et al. Towards zero-threshold optical gain using charged semiconductor quantum dots. Nature Nanotech 12, 1140–1147 (2017).

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