[1] [1] Jin P, Lu H, Wei Y, et al. Optics Letters, 2017, 42(1): 143.

[2] [2] Sung J H, Lee H W, Yoo J Y, et al. Optics Letters, 2017, 42(11): 2058.

[3] [3] Kopylov D A, Esaulkov M N, Kuritsyn I I, et al. Laser Physics Letters, 2018, 15(4): 045001.

[4] [4] Emmett J L, Scarratt K, McClure S F, et al. Gems & Gemology, 2003, 39(2): 84.

[5] [5] Bristow J K, Tiana D, Parker S C, et al. Journal of Materials Chemistry A, 2014, 2(17): 6198.

[6] [6] Emmett J L, Stone-Sundberg J, Guan Y, et al. Gems & Gemology, 2017, 53(1): 42.

[7] [7] Moon A R, Phillips M R. Journal of the American Ceramic Society, 1994, 77(2): 356.

[8] [8] Gubelin E, Schmetzer K. Gems & Gemology, 1982, 18(4): 197.

[9] [9] Jacobs P W M, Kotomin E A. Philosophical Magazine A, 1993, 68(4): 695.

[10] [10] Kitaoka Y, Nakamura K, Akiyama T, et al. Physical Review B, 2013, 87(20): 205113.

[11] [11] Choi M, Janotti A, Van de Walle C G. Journal of Applied Physics, 2013, 113(4): 044501.

[12] [12] Hine N D M, Haynes P D, Mostofi A A, et al. The Journal of Chemical Physics, 2010, 133(11): 114111.

[13] [13] Lehmann G, Harder H. American Mineralogist: Journal of Earth and Planetary Materials, 1970, 55(1-2): 98.

[14] [14] Nikolskaya L V, Terekhova V M, Samoilovich M I. Physics & Chemistry of Minerals, 1978, 3(3): 213.

[15] [15] Ferguson J, Fielding P E. Chemical Physics Letters, 1971, 10(3): 262.