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Transmission spectra from 1.5 to 8.8 eV of undoped crystalline α-Al2O3 synthesized by five growth techniques are compared. Salient features in this below-band-gap region are attributed to extrinsic effects of absorption by impurities and color centers, and to Rayleigh scattering. Analysis suggests material with substantially less absorption is possible.
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Bruce D. Evans (1991) studied this question.
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