[1] [1] Schweizer T, Hewak D W, Samson B N, et al. J. Lumin., 1997, 72-74: 419.

[2] [2] Heo J, Shin Y B. J. Non-Cryst. Solids, 1996, 196: 162.

[3] [3] Tanabe S, Hanada T, Watanabe M, et al. J. Am. Ceram. Soc., 1995, 78(11): 2917.

[4] [4] Turnbull D A, Aitken B G, Bishop S G. J. Non-Cryst. Solids, 1999, 244: 260.

[5] [5] Schweizer T, Hewak D W, Samson B N, et al. Opt. Lett., 1996, 21(19): 1594.

[6] [6] Shin Y B, Heo J. J. Non-Cryst. Solids, 1999, 253(1-3): 23.

[7] [7] Wei K, Machewirth D P, Wenzel J, et al. Opt. Lett., 1994, 19: 904.

[8] [8] Carter J N, Smart R G, Tropper A C, et al. J. Non-Cryst. Solids, 1992, 140: 10.

[10] [10] McCumber D E. Phys. Rev., 1964, 134(2A): A299.

[11] [11] Song J H, Heo J. J. Am. Ceram. Soc., 2004, 87(10): 1903.

[12] [12] Choi Y G, Cho D H, Kim K H. J. Non-Cryst. Solids, 2000, 276(1-3): 1.

[13] [13] Dexter D L. J. Chem. Phys., 1953, 21(5): 836.

[14] [14] Peng B, Izumitani T. Opt. Mater., 1995, 4(6): 797.

[15] [15] Marěs J, Jacquier B, Pédrini C, et al. Rev. Phys. Appl., 1987, 22: 145.

[16] [16] Yang Z Y, Li B, He F, et al. Non-Cryst. Solids, 2008, 354(12-13): 1198.

[17] [17] Jagosich F H, Gomes L, L. Tarelho V G, et al. J. Appl. Phys., 2002, 91(2): 624.

[18] [18] Weber M J. Phys. Rev. B, 1971, 4: 2932.

[19] [19] Peng B, Izumitani T. Opt. Mater., 1995, 4(6): 701.