Resolving the emission transition dipole moments of single doubly-excited seeded nanorods via heralded defocused imaging

TL;DR

This study introduces a heralded defocused imaging technique to directly measure the transition dipole moments of doubly-excited states in nanorods, revealing polarization anisotropy differences linked to seed type.

Contribution

It provides the first direct characterization of multiexcitonic transition dipole moments in nanorods using a novel imaging method.

Findings

Type-I1/2 seeded nanorods show higher biexciton-to-exciton anisotropy.

Type-II seeded nanorods exhibit reduced biexciton emission anisotropy.

The anisotropy differences are explained by excitonic dynamics and refractive index effects.

Abstract

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.