| Summary: | Yttrium aluminum garnet (Y<sub>3</sub>Al<sub>5</sub>O<sub>12</sub>, YAG) is one of the most important optical materials with many existing and potential future applications in laser, illumination, and scintillation. X-rays, g-rays, and UV light can induce ionization and significant changes in the valency of impurities and defects in YAG single crystals which may lead to the formation of color centers. In fact, the use of YAG crystals in laser and scintillation devices involves significant formation of color centers and requires full understanding for their characteristics and effects on the material properties. In this work, the formation and characteristics of color centers in undoped and rare-earth doped YAG single crystals was investigated mainly through optical absorption spectroscopy. An increase in the absorption over a broad range of wavelengths was observed in the as-grown sample after UV irradiation. F-centers and iron impurities in the as-grown undoped crystals were found to be responsible for the formation of color centers. However, air- or oxygen-anneal seems to be effective in suppressing most color centers in the crystals.
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