True-Green Lasing in Bulk Nanocrystals through Alloying

Bulk nanocrystals (BNCs) exhibit exceptional optical gain because of strong Auger suppression and band gap renormalization effects, but this advantage comes at the cost of losing the size-dependent spectral tunability that underpins conventional colloidal nanocrystals. As a result, CdS BNCs remain largely limited to the cyan-green spectral region (480-520 nm), restricting access to technologically important wavelengths such as true green (520-550 nm). Here, we show that compositional alloying restores spectral tunability to the BNC regime while preserving its favorable gain characteristics. By incorporating selenium into CdS, we obtain CdS0.87Se0.13 alloyed BNCs that emit in the true-green region while retaining bulk-like suppression of Auger recombination. Transient-absorption spectroscopy reveals strong optical gain with multinanosecond lifetimes, including a red-shifted alloy band-edge gain feature together with a higher-energy CdS-derived contribution due to a CdS shell. Thin films of the alloyed BNCs exhibit amplified spontaneous emission with a threshold of 215 μJ/cm2. When integrated into photonic crystal surface-emitting lasers, the same material supports single-mode, highly collimated lasing across 520-540 nm under both femtosecond and quasi-continuous-wave excitation, with line widths of ∼ 0.7 nm and thresholds as low as 10 kW/cm2. These results establish alloyed BNCs as solution-processable gain media that combine bulk-like gain physics with compositional wavelength tunability, supporting the development of compact visible lasers in spectral regions inaccessible to single-composition BNC materials.