Direct determination of the zero-field splitting for a single Co2+ ion embedded in a CdTe/ZnTe quantum dot

When a Co2+ impurity is embedded in a semiconductor structure, crystal strain strongly influences the zero-field splitting between Co2+ states with spin projection Sz = ±3/2 andSz = ±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 Sz = +3/2 and Sz = −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.