Optical Absorption of Y3Al5O12 from 10- to 55 000-cm−1 Wave Numbers

Abstract

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 ${\mathrm{cm}}^{-1\mathrm{}}$ at 300°K. The wave-number range 10 to 900 ${\mathrm{cm}}^{-1\mathrm{}}$ has been studied at 4°K. The 17 theoretically predicted absorption peaks of the ${\mathrm{Y}}_3$ ${\mathrm{Al}}_5$ ${\mathrm{O}}_{12}$ lattice at 300°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 ${\mathrm{cm}}^{-1\mathrm{}}$. The fourteenth, fifteenth, and sixteenth peaks are believed to be associated with the ${\overline{\nu }}_3$ vibrations of the ${\mathrm{AlO}}_4$ groups in the lattice. The multiphonon lattice absorption bands extend upward to 2400 ${\mathrm{cm}}^{-1\mathrm{}}$ wave numbers. Beyond 2400 ${\mathrm{cm}}^{-1\mathrm{}}$ the crystal is transparent, with an absorption coefficient <0.1 ${\mathrm{cm}}^{-1\mathrm{}}$ up to $\overline{\nu }=30\mathrm{} 000$ ${\mathrm{cm}}^{-1\mathrm{}}$. From 30 000 to 52 000 ${\mathrm{cm}}^{-1\mathrm{}}$ the absorption appears to be dominated by trace impurities. The optical absorption increases rapidly for wave numbers above 52 000 ${\mathrm{cm}}^{-1\mathrm{}}$ and reaches an absorption coefficient of ${10}^{+3\mathrm{}}$ ${\mathrm{cm}}^{-1\mathrm{}}$ at 54 300 ${\mathrm{cm}}^{-1\mathrm{}}$ wave numbers (6.73 eV or 1840 Å). 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.