Optical Properties ofHo3+Ions in Yttrium Gallium Garnet and Yttrium Iron Garnet

Optical absorption and emission spectra are used to determine the energy levels of the ${}_{}{}^5{}_{}{}^{}I_8^{}{}_{}{}^{}$ ground state and ${}_{}{}^5{}_{}{}^{}I_7^{}{}_{}{}^{}$ excited state of ${\mathrm{Ho}}^{3+}$ in diamagnetic yttrium gallium garnet (YGaG) and ferrimagnetic yttrium iron garnet (YIG). The results for YGaG are consistent with the $D_2$ site symmetry of the dodecahedral site in the garnet structure. Energy levels in YIG which are separated by site inequivalency are identified from spectra observed as the ${\mathrm{Fe}}^{3+}$ magnetization is rotated in the ($1\overline{1}0$) plane. Stimulated emission is observed from ${\mathrm{Ho}}^{3+}$ in YGaG at 2.086 and 2.114 μ and from ${\mathrm{Ho}}^{3+}$ in YIG at 2.086, 2.089, and 2.107 μ. Each of the lines in YIG is composed of two superimposed electronic transitions. The exchange field in YIG makes possible magnetic tuning of coherent emission over a range of 70 Å.