Optical Absorption of Y 3 Al 5 O 12 from 10- to 55 000- cm − 1 Wave Numbers

The optical absorption coefficient of synthetic single crystals of the garnet ${\mathrm{Y}}_{3}$${\mathrm{Al}}_{5}$${\mathrm{O}}_{12}$ has been measured from photon wave numbers of 10 to 55 000 ${\mathrm{cm}}^{\ensuremath{-}1}$ at 300\ifmmode^\circ\else\textdegree\fi{}K. The wave-number range 10 to 900 ${\mathrm{cm}}^{\ensuremath{-}1}$ has been studied at 4\ifmmode^\circ\else\textdegree\fi{}K. The 17 theoretically predicted absorption peaks of the ${\mathrm{Y}}_{3}$${\mathrm{Al}}_{5}$${\mathrm{O}}_{12}$ lattice at 300\ifmmode^\circ\else\textdegree\fi{}K have been found at wave numbers of 123, 163, 223, 255, 293, 332, 373, 395, 430, 457, 510, 530, 570, 690, 722, 784, and 830 ${\mathrm{cm}}^{\ensuremath{-}1}$. The fourteenth, fifteenth, and sixteenth peaks are believed to be associated with the ${\overline{\ensuremath{\nu}}}_{3}$ vibrations of the ${\mathrm{AlO}}_{4}$ groups in the lattice. The multiphonon lattice absorption bands extend upward to 2400 ${\mathrm{cm}}^{\ensuremath{-}1}$ wave numbers. Beyond 2400 ${\mathrm{cm}}^{\ensuremath{-}1}$ the crystal is transparent, with an absorption coefficient 0.1 ${\mathrm{cm}}^{\ensuremath{-}1}$ up to $\overline{\ensuremath{\nu}}=30 000$ ${\mathrm{cm}}^{\ensuremath{-}1}$. From 30 000 to 52 000 ${\mathrm{cm}}^{\ensuremath{-}1}$ the absorption appears to be dominated by trace impurities. The optical absorption increases rapidly for wave numbers above 52 000 ${\mathrm{cm}}^{\ensuremath{-}1}$ and reaches an absorption coefficient of ${10}^{+3}$ ${\mathrm{cm}}^{\ensuremath{-}1}$ at 54 300 ${\mathrm{cm}}^{\ensuremath{-}1}$ wave numbers (6.73 eV or 1840 \AA{}). Some comments are also made on the relationship of the lattice modes of yttrium aluminum garnet (YAG) to the phonon sidebands of electronic transitions of impurity ions in YAG.