  Coupled colloidal quantum dot molecules are an emerging class of
nanomaterials, introducing new degrees of freedom for designing quantum
dot-based technologies. The properties of multiply excited states in these
materials are crucial to their performance as quantum light emitters but cannot
be fully resolved by existing spectroscopic techniques. Here we study the
characteristics of biexcitonic species, which represent a rich landscape of
different configurations, such as segregated and localized biexciton states. To
this end, we introduce an extension of Heralded Spectroscopy to resolve
different biexciton species in the prototypical CdSe/CdS coupled quantum dot
dimer system. We uncover the coexistence and interplay of two distinct
biexciton species: A fast-decaying, strongly-interacting biexciton species,
analogous to biexcitons in single quantum dots, and a long-lived,
weakly-interacting species corresponding to two nearly-independent excitons
separated to the two sides of the coupled quantum dot pair. The two biexciton
types are consistent with numerical simulations, assigning the
strongly-interacting species to two excitons localized at one side of the
quantum dot molecule and the weakly-interacting species to excitons segregated
to the two quantum dot molecule sides. This deeper understanding of multiply
excited states in coupled quantum dot molecules can support the rational design
of tunable single- or multiple-photon quantum emitters.
