Polarized Fine Structure in the Photoluminescence Excitation Spectrum of a Negatively Charged Quantum Dot

We report polarized photoluminescence excitation spectroscopy of the negative trion in single charge-tunable $\mathrm{InAs}/\mathrm{GaAs}$ quantum dots. The spectrum exhibits a $p$-shell resonance with polarized fine structure arising from the direct excitation of the electron spin triplet states. The energy splitting arises from the axially symmetric electron-hole exchange interaction. The magnitude and sign of the polarization are understood from the spin character of the triplet states and a small amount of quantum dot asymmetry, which mixes the wave functions through asymmetric $e\mathrm{\text{−}}e$ and $e\mathrm{\text{−}}h$ exchange interactions.