Polarization spectroscopy of Cr³⁺ ions in laser host crystals: I. R lines and sidebands

This paper reports magnetic circular polarization studies of the Cr³⁺ R lines and their sidebands in MgO, Gd₃Sc₂Ga₃O₁₂ (GSGG) and Gd₃Sc₂Al₃O₁₂ (GSAG). The R lines are shown to occur by magnetic dipole transitions in MgO and electric dipole transitions in the garnets. However, although the one-phonon sidebands are induced by odd-parity vibration of T_1u and T_2u symmetry, the effects are of comparable magnitude in inducing σ₊ and σ_- polarized transitions so that at some wavelengths in the sideband the MCP signal is very small (i.e. P ∼ 0). From the field dependence of the magnetic circular polarization the ²E g values are found to be g_{{curly logical or}} = 2.60 + -0.1 and g _⊥ < 0.1 for GSGG. The g_{{curly logical or}} shift is due to spin-orbit mixing of the higher lying ²T₁ and ²T₂ levels into the ²E states.