Collective Interactions\nof Quantum-Confined Excitons\nin Halide Perovskite Nanocrystal Superlattices

Collective optical properties can\nemerge from an ordered\nensemble\nof emitters due to interactions between the individual units. Superlattices\nof halide perovskite nanocrystals exhibit collective light emission,\ninfluenced by dipole–dipole interactions between simultaneously\nexcited nanocrystals. This coupling changes both the emission energy\nand rate compared to the emission of uncoupled nanocrystals. We demonstrate\nhow quantum confinement governs the nature of the coupling between\nthe nanocrystals in the ensemble. The extent of confinement is modified\nby controlling the nanocrystal size or by compositional control over\nthe Bohr radius. In superlattices made of weakly confined nanocrystals,\nthe collective emission is red-shifted with a faster emission rate,\nshowing the key characteristics of superfluorescence. In contrast,\nthe collective emission of stronger quantum-confined nanocrystals\nis blue-shifted with a slower emission rate. Both types of collective\nemission exhibit correlative multiphoton emission bursts, showing\ndistinct photon bunching emission statistics. The quantum confinement\nchanges the preferred alignment of transition dipoles within the nanocrystal\nand switches the relative dipole orientation between neighbors, resulting\nin opposite collective optical behaviors. Our results extend these\ncollective effects to relatively high temperatures and provide a better\nunderstanding of exciton interactions and collective emission phenomena\nat the solid state.