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Abstract: Reducing the size of materials down to a few nanometers is a powerful approach to controlling material properties by design. A case in point is semiconductors, where size quantization leads to a size- and shape-dependent band gap once crystal dimensions become comparable with or smaller than the exciton Bohr radius, an observation first made almost 40 years ago.

This talk explores the opportunities size reduction brings for creating new optical gain materials. Using free carrier gain in bulk semiconductors as a reference, we discuss four different model systems, each exemplifying a different mechanism to attain net stimulated emission.

First, we focus on large perovskite nanocrystals. This example helps introduce the experimental methods we use to characterize gain materials and shows that weakly confined semiconductors have gain characteristics vert similar to those of the corresponding bulk material. Next, we highlight the impact of size quantization by using stimulated emission by CdSe/CdS quantum dots as a second example, which is introduced as a unique model system of band-edge gain by quantum dots. Interestingly, we show that tweaking the core and shell dimensions provides unique possibilities to tune the optical gain characteristics of these materials.

Building on the conditions that yield the lowest gain thresholds in CdSe/CdS quantum dots, we discuss two possibilities for overcoming the intrinsic limitations of band-edge gain. First, we turn to two-dimensional colloidal nanoplatelets, where we show that stimulated emission through excitonic molecules leads to a combination of low-gain thresholds and high-gain coefficients. Finally, we propose transitions involving localized band-gap states, exemplified by HgTe quantum dots, as a way to achieve nearly thresholdless gain by colloidal semiconductor nanocrystals.

We conclude this presentation with a short outlook on the prospects and challenges of using colloidal quantum dots as a gain material for microlasers.

Bio: Zeger Hens is a professor at Ghent University, where he received his Ph.D. in applied physics. The main focus of his research is the synthesis, processing, and characterization of colloidal semiconductor nanocrystals or quantum dots for applications in opto‐electronics.