Identifying multi‐excitons in quantum dots: the subtle connection between electric dipole moments and emission linewidths

The emission linewidths of excitonic complexes confined in quantum dots (QDs) mirror their interaction with a defect‐induced, fluctuating charge environment, a phenomenon known as spectral diffusion. Interestingly, extended excitonic complexes that comprise several interacting excitons exhibit significantly smaller emission linewidths if compared to the optical fingerprint of their building block, a sole exciton. Hence, it is not the absolute, but the relative electric dipole moment that governs the directly accessible emission linewidths. Exemplarily we investigate this matter based on differing exciton and biexciton emission linewidths of single GaN QDs with varying emission energies, i.e. QD dimensions. Our results establish the full width at half maximum (FWHM) or any other linewidths criterion for the identification of excitonic complexes, a technique that can directly be applied to polar but even non‐polar QD materials. Additionally, we find an emission energy dependent trend for the FWHM ratios of the biexciton and the exciton (XX_FWHM/X_FWHM) in perfect agreement with their relative dipole moment ratios as derived from our 8‐band‐ k · p based treatment of the Coulomb and exchange interaction within these multi‐particle complexes. (© 2015 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)