Extending and Understanding the Optical Gain of Bulk Nanocrystals from Green to Red

Colloidal nanocrystals have been investigated for their applications in light-matter interactions as cost-effective alternatives to epitaxially-grown semiconductors. Achieving net stimulated emission , a premise to build lasers, hasbeen investigated for over 20 years.¹ In 2023, Tanghe et al.², reported exceptional optical gain coefficients and long optical gain lifetimes, ranging between 460 nm to 560 nm, by using colloidally synthesized “bulk nanocrystals” (BNC) of Cadmium Sulfide (CdS), i.e. nanocrystals with a size exceeding the Bohr diameter for strong confinement. Using the ‘bulk’ concept, it was shown that the best of both bulk photo-physics (high density of states, slower Auger recombination, ..) can be combined with nanoscale solution processing. Despite their exceptional performance, these CdS-based BNCs materials cannot be size-tuned to other visible and near-IR wavelengths. Therefore, for devices that can perform in the red to near-IR range an inevitable search for other semiconductors emerges. Recently, we investigated and reported³ the optical gain characteristics (640 -750 nm) of Cadmium Selenide (CdSe) BNCs. Similar to CdS, CdSe BNCs (diameter > 10 nm) also show remarkable optical gain coefficients and gain lifetimes. Using a bulk photo-physical model, we can explain that the specific band structure of CdSe imparts however different metrics then for CdS. The gain spectrum is for example smeared out due to high carrier temperatures over a broader energy range, diluting its absolute value to only ca. 2000 cm^-1. Using the CdSe BNCs, we also demonstrate opticaly pumped lasing under nanosecond excitation over a broad wavelength span (640 to 730 nm). Our studies show that BNCs based on CdSe can become interesting materials for optical devices in the red to near-infrared spectrum, and the BNC concept can also be extended to other solution-processable bulk nanocrystals.

In addition to these material and device characterizations at room temperature, we also investigated further the nature of the gain mechanisms in these materials at different temperatures. Combining these insights with low temperature lasing characterization enables us to develop a more complete understanding of lasing action using BNC materials.

References:

[1]: Klimov, V. I., Mikhailovsky, A. A., Xu, S., Malko, A., Hollingsworth, J. A., Leatherdale, C. A., Eisler, H., & Bawendi, M. G. (2000). Optical gain and stimulated emission in nanocrystal quantum dots. Science (New York, N.Y.), 290(5490), 314–317. https://doi.org/10.1126/science.290.5490.314

[2]: Tanghe, I., Samoli, M., Cayan, S.A. et al. Optical gain and lasing from bulk cadmium sulfide nanocrystals through bandgap renormalization. Nat. Nanotechnol. 18, 1423–1429 (2023). https://doi.org/10.1038/s41565-023-01521-0

[3]: S. A. Cayan, M. Samoli, I. Tanghe, C.-Y. Lin, D. Respekta, J. M. Hodgkiss, K. Chen, Z. Hens, and P. Geiregat, “Stimulated emission and lasing from bulk CdSe nanocrystals,” vol. 15, no. 38, pp. 9836–9843. [Online]. Available: https://pubs.acs.org/doi/10.1021/acs.jpclett.4c02167

[4]: P. Geiregat, Z. Hens, D. V. Thourhout, and I. Tanghe, “Colloidal bulk nanocrystal laser and related method,” Jun. 27, 2024 Accessed: Oct. 14, 2024. [Online]. Available: https://patents.google.com/patent/WO2024132974A1/en?q=(geiregat+bulk)&oq=geiregat+bulk