Photochromic sapphires show an optically controllable orange coloration under ambient temperatures, which strongly affects the color appearance and market value of gemstone sapphires. An in situ absorption spectroscopy technique using a tunable excitation light source has been developed to investigate the wavelength- and time-dependence of sapphire's photochromism. Excitations of ≤370 nm and ≥410 nm introduce and remove orange coloration, respectively, whereas there is a stable absorption band at 470 nm. Both the color enhancement and diminishing rates are proportional to the excitation intensity; hence, strong illumination can significantly accelerate the photochromic effect. Finally, the origin of the color center can be explained by a combination of the differential absorption and the opposite trends between the orange coloration and Cr emission, indicating that the origin of this photochromic effect is related to a magnesium-induced trapped hole and chromium. The results can be used to minimize the photochromic effect and improve the reliability of color evaluation for valuable gemstones.
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