Orientation of Co2+ dipoles in magnesium–aluminum spinel crystal

The nonlinear transmission curves of [100]- and [110]-cut Co2+:MgAl2O4 crystals at 1.54 μm were measured experimentally. A phenomenological model was established to analyze the experimental results, in which resonant absorption in the Co2+:MgAl2O4 crystal was characterized by four groups of linear dipoles oriented along the four body diagonals ([111], [ $${1}\stackrel{\mathrm{-}}{1}\stackrel{\mathrm{-}}{1}$$ ], [ $$\stackrel{\mathrm{-}}{1}{\text{1}}\stackrel{\mathrm{-}}{1}$$ ] and [ $$\stackrel{\mathrm{-}}{1}\stackrel{\mathrm{-}}{1}{\text{1}}$$ ] axis) of the unit cell. The theoretical calculations were in good agreement with the experimental measurement data. This work demonstrated that the transition dipoles were parallel to the four body diagonals of the Co2+:MgAl2O4 unit cell. Meanwhile, absorption cross-sections of the ground state and excited state at 1.54 μm were calculated to be $$\sigma_{{{\text{gsa}}}} = \left( {3.6 \pm 0.3} \right) \times 10^{ - 19}$$ cm2 and $$\sigma_{{{\text{esa}}}} = \left( {4.5 \pm 0.4} \right) \times 10^{ - 20}$$ cm2, respectively.