Spectral characteristics of neodymium-activated yttrium aluminum garnet in the ultraviolet and visible ranges

The absorption, photoluminescence, and x-ray luminescence spectra of pure and neodymium-activated YAG crystals grown by the horizontal directional crystallization method were investigated at 295$^\circ$ K in the wavelength range 190–900 nm. A particular study was made of wide strong absorption bands with maxima at 224 and 205 nm due to introduction of neodymium into YAG crystals. These bands were attributed to the $4f\to5d$ transitions in Nd$^{3+}$ ions localized at normal and anomalous structural positions. A system of luminescence bands observed in the 260–620 nm range excited by x-ray and hard ($\epsilon>4.79$ eV) photoexcitation of YAG:Nd was attributed to transitions from the $^2$F$2_{5/2}$ ($\epsilon=37850$ cm$^{-1}$) level to levels of all the lower neodymium multiplets. In the case of excitation by softer photons ($3.22\lesssim\epsilon\lesssim 4.68$ eV), the luminescence spectrum was due to transitions from another metastable level $^2P_{3/2}$ ($\epsilon=25990$ cm$^{-1}$) to levels of lower terms. A new structural scheme was proposed for the formation of anomalous rare-earth ion centers in laser garnets.