Multidimensional Emission Control of CsPbI₃ Quantum Dots Using Plasmonic Quasi-Bound States in the Continuum

Precise control over spontaneous emission is central to next-generation quantum-dot light sources, yet colloidal emitters generally radiate into weakly structured spectral, angular, and polarization channels. Quasi-bound states in the continuum (Quasi-BIC) overcome this limitation by providing high Q-factors, symmetry-defined near-fields, and controlled radiative leakage. Here we realize all three advantages in a single platform by coupling red-emitting CsPbI₃ quantum dots (QDs) to a symmetry-protected plasmonic quasi-BIC hosted by a one-dimensional gold grating. By jointly engineering the grating parameters and an atomic-layer-deposited Al₂O₃ spacer, we match the quasi-BIC (finite radiative leakage) resonance to the ∼690 nm emission of the QDs, enabling efficient near-field coupling while suppressing direct metal-induced quenching. Compared with a planar QD film, the grating-coupled structure achieves a peak photoluminescence (PL) enhancement of more than 4-fold. The emission response is strongly governed by the symmetry of the plasmonic mode: the PL intensity exhibits a pronounced dependence on the incident pump polarization, with a polarization anisotropy (PA) as high as 85.4%, while the emitted light itself reaches a degree of polarization (DOP) of 16.5%. In addition, the hybrid structure produces directional and collimated red emission with a divergence as low as 3°, accompanied by a clear wavelength-angle correlation. These results show that plasmonic quasi-BIC can serve not only as field-enhancing resonances but also as symmetry-defined optical channels for engineering QD emission. This work presents a compact strategy for integrating QD with metallic metasurfaces toward QD light sources featuring tunable emission direction and divergence, polarization sensitivity, and color selectivity.