Direct determination of the zero-field splitting for a singleCo2+ion embedded in a CdTe/ZnTe quantum dot

When a ${\mathrm{Co}}²⁺$ impurity is embedded in a semiconductor structure, crystal strain strongly influences the zero-field splitting between ${\mathrm{Co}}²⁺$ states with spin projection ${S}_z=\ifmmode\pm\else\textpm\fi{}3/2$ and ${S}_z=\ifmmode\pm\else\textpm\fi{}1/2$. 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 ${\mathrm{Co}}²⁺$ 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 ${\mathrm{Co}}²⁺$ ion directly in the excitonic photoluminescence spectrum. Moreover, by observation of anticrossing of ${S}_z=+3/2$ and ${S}_z=\ensuremath{-}1/2\phantom{\rule{4pt}{0ex}}{\mathrm{Co}}²⁺$ spin states in a magnetic field, we determine the axial and in-plane components of the crystal field acting on the ${\mathrm{Co}}²⁺$. The proposed technique can be applied to optical determination of the zero-field splitting of other transition-metal ions in quantum dots.