The index of refraction for Nd³⁺ and Eu³⁺ doped Y₃Al₅O₁₂ ceramics was measured and compared with that of bulk material. The Sellmeier equation n²(λ[μm])=A+Bλ²/(λ²-C) was found to have A, B and C equal to 1, 1.44156, 0.06716 and 1, 1.36429, 0.05849 for Nd:YAG and Eu:YAG respectively, in comparison to single Nd:YAG crystal 1, 2.2779 and 0.01142. At 614nm where Eu³⁺:⁵D₀-->⁷F₂ appears the index of refraction for ceramics is n=1.617 whereas for YAG single crystal it is n=1.830. At 909nm where Nd³⁺:⁴F_3/2-->⁴I_9/2 appears the index of refraction for ceramics is n=1.604 whereas for single YAG crystal it is n=1.819.

The Einstein’s coefficient A_i for electric-dipole transitions depends on index of refraction according to χ^ABS_ED = (n²+2)2 / 9n and according to χ^EMI_ED = n(n²+2)2 / 9 for absorption and emission respectively. Thus one may expect the absorption coefficient to be reduced in nanocrystals based materials in comparison to bulk one. The lower index of refraction measured for ceramics, should reduce the absorption coefficient down to ~84% of the value for both Nd³⁺ and Eu³⁺ doped YAG single crystal. One may also expect the lifetimes to be longer in nanocrystals based materials in comparison to bulk one. The measured indexes of refraction for Eu³⁺ or Nd³⁺ doped ceramics reduces the χ_ED for luminescence down to ~66% of the value for YAG single crystal and may lead to 1.5 times increased decay constants. These effects could be one of explanations of the long lifetimes observed by us in Nd³⁺ and Eu³⁺ doped nanopowders. The measured lifetimes for 0.5 and 1%Nd:YAG nanopowders exceeded 400μs (485 and 420μs respectively), however for the ceramics the lifetimes decreased down to 230 and 210 μs respectively. We thus suppose the reabsorption of radiation could also play a role in highly scattering medium like powders.

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