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Optical absorption curves for nominally pure and Ce 3+ doped single crystals of Y 3 Al 5 O 12 (YAG) have been measured in the region of 5 eV–7 eV at 292 K, 78 K and 10 K. The parameters of the Urbach rule have been determined referring to those optical absorption curves which are believed to be the fundamental absorption tail of YAG single crystals at these temperatures, respectively. Optical absorption measurements on single crystals of YAG:Nd 3+ (the concentration [Nd 3+ ] L being 0.60 atm.%) have disclosed strong absorption spectra consisting of at least five Gaussian bands in the region of 5.0 eV–7.0 eV at 292 K and 78 K. These spectra have been interpreted to originate from the interconfigurational transitions 4 f 3 →4 f 2 5 d of Nd 3+ in YAG.

Anisotropic reflectivity spectra of YAlO 3 single crystals at 297 K measured with the light polarized parallel to each of the a , b and c axes are presented in the range from ∼6.5 eV to ∼120 eV. A comparison of the reflectivity spectra of YAlO 3 with those of Y 2 O 3 , Y 3 Al 5 O 12 and α-Al 2 O 3 single crystals reaffirms that the peaks at ∼35 eV are mainly associated with the Y 3+ 4 p 6 and the features beyond ∼76 eV with the Al 3+ L 2,3 . Anisotropic reflectivity spectra at 10 K are shown in the region from ∼7 eV to 15 eV. Effective optical dielectric constants calculated by the sum rule including the imaginary part of the dielectric constant up to ∼120 eV are seen to saturate at the values corresponding to 98.0% ∼ 99.7% of the literature values of the optical dielectric constant.

The spectra of optical constants of α-Al 2 O 3 single crystals at 297 K and 10 K are presented over the range of photon energies from 6 eV to 120 eV for the electric fields of incident light parallel to optic axis and normal to it. The optical constants include the complex index of refraction, conductivity, complex dielectric constant, loss functions and effective number of electrons per unit cell. The spectra in E ≥ 78 eV are interpreted in terms of the interband transitions from the O 2p and 2s bands to the Al conduction bands and the free electron-like absorption of photons by the O 2p band electrons. The volume energy loss function has a prominent peak at E ∼26.4 eV for the plasma oscillation dominantly of the 2p electrons. The spectral features in E ≤78 eV resulting from electronic transitions from the Al L 2,3 levels are presented with the L 2,3 exciton doublet, e.g. , at 78.72 eV and 79.15 eV for e 2// .

The spectra of absorption constant in the intrinsic absorption tail region of α-Al 2 O 3 single crystals at 297 K, 78 K and 10 K are presented for the electric fields of incident light parallel to the optic axis of α-Al 2 O 3 and normal to it. The absorption tail spectra obey the Urbach rule for each polarization, indicating that absorptions in the tail region result from the direct interband transitions. The optical anisotropy characterized by the Urbach rule is interpreted in terms of the bond length O 2- –Al 3+ participating in the absorption processes. The spectra of reflectivity are presented in the range of photon energies from 4.6 eV to 120 eV for electric fields parallel to and normal to it. Spectral positions of main maxima in the reflectivity spectra for each polarization are determined at 297 K and 10 K; the spin-orbit doublet of the Al 3+ L 2,3 is revealed at 78.67 eV and 79.12 eV for the first time in the case of α-Al 2 O 3 .

The reflectivity spectrum of Y 2 O 3 single crystals at 297 K has been measured from 6 eV to 110 eV, and optical functions are derived by the Kramers-Kronig transformation. The plasma loss peaks are found at 14.2 3 eV and 38.40 eV. The spectrum of e 2 ( E ) is tentatively interpreted in terms of the energy levels of Y 2+ , Y 3+ and O 2- free ions. The spectral dependence of e 2 suggests that interband transitions will be exhausted in the neighbourhood of 50 eV. In the energy region of \(E\gtrsim 50\) eV occurs the spectrum due to the free electron-like absorption alone. The accuracy of the data is discussed by the effective optical dielectric constant. The spectral dependence of the free electron-like absorption is compared with that predicted by the Drude formula. The origin of the loss peak at 38.40 eV is discussed.

The absorption spectra due to the 4 f →5 d transitions in Ce 3+ centres have been measured with the use of YAG:Ce 3+ single crystals with a lattice concentration [Ce 3+ ] L =(1.28±0.05)%. The data are almost independent of the spectral behaviour of a reference spectrum whose absorption constants are ∼2 cm -1 at the highest, since the absorption constants of 129 cm -1 and 50 cm -1 have been derived, relative to the reference spectrum, at the strongest peaks in the dγ and d e regions, respectively. The absorption area 32.3 cm -1 eV measured on the strongest dγ-band suggests that about a quarter of the Ce-concentration, [Ce 3+ ] M , added in the mixture of the starting powders is incorporated into [Ce 3+ ] L as YAG:Ce 3+ . It is found that the highest-energy lying d e band is located at 6.06 eV, in agreement with 6.07 0 eV for the corresponding band peak in the excitation spectrum and that the ratio of the absorption area of dγ to that of d e is nearly 2:3.

The reflectivity spectrum of Y 3 Al 5 O 12 (YAG) single crystals at 297 K is presented in the region of 6.734 eV ≤ E ≤116 eV. The spectra of optical constants derived from the reflectivity by the Kramers-Kronig analysis are tentatively interpreted in terms of the energy levels of the constituent atoms of YAG. Plasma resonance peaks are found at 23.3 8 eV and 36.2 0 eV in which the valence-band electrons and valence- as well as full-band electrons participate, respectively. The structure of absorption due to excitations of the electrons in the Al 3+ L 2,3 levels is found in the region of E ≥76 eV. The free electron-like absorption calculated by the Drude formula by use of the line shape parameters of the plasma resonance line is compared with the dielectric constant in the region of E ≥24 eV. The effective optical dielectric constant calculated from the optical constant e 2 is found to be in good agreement at 116 eV with the optical dielectric constant derived from the dispersion formula.