Dual Type-II Colloidal Quantum Wells for Efficient Nonlinear Optical Limiting

Colloidal II-VI nanocrystals have garnered significant research attention in nonlinear optical applications due to their low-cost synthesis, photophysical tunability, and ease of device integration. Herein, we report that dual-type II CdSe/CdTe/CdSe colloidal quantum wells (CQWs) with core/crown/crown structures achieve remarkable nonlinear optical limiting capabilities driven by an exceptionally large nonlinear absorption coefficient. Open aperture Z-scan reveals that these dual-type II CQWs exhibit a third-order nonlinear absorption coefficient of 33.1 cm/GW and an ultralow optical limiting threshold (0.71 GW/cm²), which is superior to that of any other reported colloidal semiconductor nanocrystals while also being comparable to existing two-dimensional (2D) dichalcogenide sheets. Photophysical analysis indicates that such a remarkable nonlinear optical performance in dual-type II CQWs can be primarily ascribed to the efficient excited state absorption (i.e., the sequential two-photon absorption), which benefits from the ultrafast and uniform formation of charge separation states in the dual type-II heterostructures.