Semiconductor nanocrystal
emission polarization is a crucial probe
of nanocrystal physics and an essential factor for nanocrystal-based
technologies. While the transition dipole moment for the lowest excited
state to ground state transition is well characterized, the dipole
moment of higher multiexcitonic transitions is inaccessible via most
spectroscopy techniques. Here, we realize direct characterization
of the doubly excited-state relaxation transition dipole by heralded
defocused imaging. Defocused imaging maps the dipole emission pattern
onto a fast single-photon avalanche diode detector array, allowing
the postselection of photon pairs emitted from the biexciton–exciton
emission cascade and resolving the differences in transition dipole
moments. Type-I1/2seeded nanorods exhibit higher
anisotropy of the biexciton-to-exciton transition compared to the
exciton-to-ground state transition. In contrast, type-II seeded nanorods display a reduction of biexciton
emission anisotropy. These findings are rationalized in terms of an
interplay between the transient dynamics of the refractive index and
the excitonic fine structure.