Stimulated Emission from Below the Bandgap in Giant Quantum Shells

Colloidal semiconductor nanocrystals (NCs) have emerged as promising candidates for developing solution-processable optical gain media with potential applications in integrated photonic circuits and lasers. However, the deployment of NCs in these technologies has been hindered by the nonradiative Auger recombination of multiexciton states, which shortens the optical gain lifetime and reduces its spectral range. Here, we demonstrate that these limitations can be overcome by using giant colloidal quantum shells (g-QSs), comprising a quantum-confined CdSe shell grown over a large (∼14 nm) CdS bulk core. Such bulk-nanoscale architecture minimizes exciton-exciton interactions, leading to suppressed Auger recombination and one of the broadest gain bandwidths reported for colloidal nanomaterials, spanning energies both above and, remarkably, below the bandgap. Ultrafast transient absorption and photoluminescence measurements demonstrate that the high-energy portion of optical gain arises from states containing more than 15 excitons per particle, while the unusual sub-bandgap gain behavior results from an Auger-assisted radiative recombination, a mechanism that has traditionally been viewed as a loss pathway. Collectively, these results reveal a unique gain regime associated with bulk-nanocrystal hybrid systems, which offers a promising path toward solution-processable light sources.