Two Biexciton Types Coexisting in Coupled Quantum Dot Molecules

Coupledcolloidal quantum dot molecules (CQDMs) are anemergingclass of nanomaterials, manifesting two coupled emission centers andthus introducing additional degrees of freedom for designing quantum-dot-basedtechnologies. The properties of multiply excited states in these CQDMsare crucial to their performance as quantum light emitters, but theycannot be fully resolved by existing spectroscopic techniques. Herewe study the characteristics of biexcitonic species, which representa rich landscape of different configurations essentially categorizedas either segregated or localized biexciton states. To this end, weintroduce an extension of Heralded Spectroscopy toresolve the different biexciton species in the prototypical CdSe/CdSCQDM system. By comparing CQDMs with single quantum dots and withnonfused quantum dot pairs, we uncover the coexistence and interplayof two distinct biexciton species: A fast-decaying, strongly interactingbiexciton species, analogous to biexcitons in single quantum dots,and a long-lived, weakly interacting species corresponding to twonearly independent excitons. The two biexciton types are consistentwith numerical simulations, assigning the strongly interacting speciesto two excitons localized at one side of the quantum dot moleculeand the weakly interacting species to excitons segregated to the twoquantum dot molecule sides. This deeper understanding of multiplyexcited states in coupled quantum dot molecules can support the rationaldesign of tunable single- or multiple-photon quantum emitters.