Optical Properties of<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msup><mml:mrow><mml:mi mathvariant="normal">Ho</mml:mi></mml:mrow><mml:mrow><mml:mn>3</mml:mn><mml:mo>+</mml:mo></mml:mrow></mml:msup></mml:mrow></mml:math>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 \ensuremath{\mu} and from ${\mathrm{Ho}}^{3+}$ in YIG at 2.086, 2.089, and 2.107 \ensuremath{\mu}. 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 \AA{}.

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