| Summary: | When a Co²⁺ impurity is embedded in a semiconductor structure, crystal strain strongly influences the zero-field splitting between Co²⁺ states with spin projection S_{z}=±3/2 and S_{z}=±1/2. Experimental evidence of this effect has been given in previous studies; however, direct measurement of the strain-induced zero-field splitting has been inaccessible so far. Here this splitting is determined thanks to magneto-optical studies of an individual Co²⁺ ion in an epitaxial CdTe quantum dot in a ZnTe barrier. Using partially allowed optical transitions, we measure the strain-induced zero-field splitting of the Co²⁺ ion directly in the excitonic photoluminescence spectrum. Moreover, by observation of anticrossing of S[subscript z]= ±3/2 and S[subscript z] =−1/2 Co²⁺ spin states in a magnetic field, we determine the axial and in-plane components of the crystal field acting on the Co²⁺ The proposed technique can be applied to optical determination of the zero-field splitting of other transition-metal ions in quantum dots.
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