  Semiconductor nanocrystal emission polarization is a crucial probe of
nanocrystal physics and an essential factor for nanocrystal-based technologies.
While the transition dipole moment of 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 post-selection of photon pairs emitted from the biexciton-exciton
emission cascade and resolving the differences in transition dipole moments.
Type-I1/2 seeded 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 transient
dynamics of the refractive index and the excitonic fine structure.
