We investigate the optical properties of neutral excitons in CdTe/ZnTe quantum dots containing a single ${\mathrm{Ni}}^{2+}$ ion. We show that the photoluminescence spectra provide a direct spectroscopic signature of strain-induced mixing of the ${\mathrm{Ni}}^{2+}$ spin states. A misalignment between the principal axis of the local strain tensor and the quantum dot growth direction reorients the spin quantization axis of the magnetic ion, reducing the $\mathrm{hole}\text{−}{\mathrm{Ni}}^{2+}$ exchange interaction at low magnetic field and giving rise to photoluminescence replicas around the partially linearly polarized bright-exciton transitions. A longitudinal magnetic field restores the circularly polarized optical selection rules, allowing the three spin projections $S_z=0,\pm 1$ of the ${\mathrm{Ni}}^{2+}$ ion to be spectrally resolved. Dark-exciton emission appears on the low-energy side of the spectra and is dominated at low field by transitions involving spin flips of the magnetic ion. An effective spin Hamiltonian including strain orientation and valence-band mixing reproduces the magnetic-field evolution of both bright- and dark-exciton spectra. These results highlight the key role of the local strain environment in determining the spin-exciton coupling of transition-metal dopants in semiconductor quantum dots.

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• Excitons
• Magneto-optics
• Optical & microwave phenomena
• Quantum dots