Publication date: 15th December 2025
Colloidal nanocrystals (NCs) are a Nobel-prize winning class of nanomaterials generating widespread attention due to their size-tunable properties and solution-processable nature, making them promising for opto-electronic applications such as LEDs, photo-detectors and lasers. Their potential has been well-established in the visible to near-infrared (NIR) range (400 - 1600 nm) with commercial impact realized via displays and NIR photo-detectors today1.
Until recently, applications of NCs for the mid-infrared spectral range (MIR, 2 - 10 μm) focused on their use in photo-detection based on Pb- and Hg-chalcogenides. When it comes to MIR stimulated emission of light, the premise to build printable lasers, no reports thus far indicated that this would be feasible with NCs. In part, this originates from a lack in development of techniques capable of probing NC electronic structure and ultrafast carrier dynamics in the challenging MIR range. One such technique is infrared transient absorption spectroscopy, a pump-probe method that enables us to study light-matter interactions on a femto - to nanosecond time scales. As used extensively in the visible, such a toolbox would not only allow us to understand how the excited states of narrow MIR band gap NCs fill up and deplete via potentially fast recombination pathways, but would also enable the direct and quantitative observation of stimulated emission and net optical gain.
Here, we implemented quantitative ultrafast transient absorption spectroscopy in the MIR spectrum to study the ultrafast carrier dynamics of multi-excitons in chalcogenide PbS and HgTe NCs with band gaps in the MIR range (2000 – 4000 nm). First, we demonstrated that PbS nanocrystals, when pushed into the bulk regime with sizes up to 28 nm, exhibited stimulated emission up to 2.9 μm with gain lifetimes up to 400 ps and intrinsic gain coefficients reaching up to 103 cm-1. Our findings indicate that PbS NCs are potentially suitable as gain media for MIR lasers. We also showed a first exploration of HgTe MIR NCs, demonstrating their potential as well. Our results show that chalcogenide NCs can impact the MIR spectrum through optical gain, as the latter can outcompete non-radiative energy transfer pathways limiting their spontaneous emission quantum yield.
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