Ultra-small excitonic fine structure splitting in highly symmetric quantum dots on GaAs (001) substrate

We prepare symmetry-controlled GaAs/AlGaAs quantum dots (QDs) on (001) GaAs substrates by infilling GaAs into AlGaAs nanoholes. For the most symmetric QDs, we measure an average excitonic fine structure splitting (FSS) of only (3.9 ± 1.8) μeV. The FSS and polarization direction of the two bright excitonic recombination lines directly reflect the degree of the QD symmetry. Since the FSS is comparable to typical homogeneous linewidths of excitonic recombination, these strain-free GaAs/AlGaAs QDs might offer a practical platform to generate entangled photons in future quantum devices.