Abstract

The search for tunable solid state lasers has lead to a resurgence of interest in Cr³⁺ -doped materials. Crystals with an intermediate or low ligand field environment for the Cr³⁺ ion are of particular interest because of their potential as broadly tunable, 4-level, red and near-infrared lasers. These hosts are qualitatively different from high field materials such as ruby and luminesce in a broad band because the ⁴T₂ state is comparable or lower in energy than the ²E state. We present the results of optical and electron paramagnetic resonance (EPR) spectroscopic investigations of a family of Cr³⁺ -doped germanium garnets with the formula Ca₃M₂(GeO₄)₃, where M = Al, Ga, or Sc. The Cr³⁺ ion substitutes for the M³⁺ and occupies a site of distorted octahedral symmetry. We have found significant variation among these three germanates with regard to ligand field strength and concentration quenching for Cr³⁺. Moreover, the small ionic radii of Al³⁺ and Ga³⁺ enable these analogs to form high concentrations of a native defect believed to be a complex of the F⁺ center. The presence of this color center makes it difficult to control the oxidation state of the Cr dopant. This defect does not form in the Sc germanate and Cr remains in the 3⁺ oxidation state.

© 1984 Optical Society of America