4:00–5:00 pm
Gordon Center for Integrative Science
Room W301
Stimulated Emission by Colloidal Quantum Dots
Zeger Hens, Ghent University
Reducing the size of materials down to a few nanometer is a powerful approach to control material properties by design. A case in point are semiconductors, where size quantization leads to a size- and shape-dependent band gap once crystal dimensions become comparable 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 4 different model systems, each exemplifying a different mechanism to attain net stimulated emission.
First, we focus on large perovskite nanocrystals. This example helps introducing the experimental methods we use to characterize gain materials and shows that weakly confined semiconductors have gain characteristics highly similar to the corresponding bulk material. Next, we highlight the impact of size quantization 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 to overcome intrinsic limitations of band-edge gain. First, we turn to two-dimensional colloidal nanoplatelets, were 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 by a short outlook on the prospects and challenges on using colloidal quantum dots as a gain material for microlasers. Host(s): Philippe Guyot-Sionnest, 2-7461; Email-pgs@uchicago.edu & Dmitri Talapin, 4-2607; Email - dvtalapin@uchicago.edu. Persons with a disability who may need assistance please contact Brenda Thomas at 2-7156 or via email at bthomas@uchicago.edu.