Crystallization and nonlinear optical properties of transparent glass-ceramics with Co:Mg(Al,Ga)2O4 nanocrystals for saturable absorbers of lasers at 1.6-1.7 µm

Transparent glass-ceramics (GCs) containing nanocrystals of Co:Mg(Al,Ga)2O4 spinel and Co:magnesium gallium aluminotitanate solid solutions are synthesized by secondary heat-treatments of the magnesium aluminosilicate glass nucleated by TiO2 and doped with Ga2O3 and CoO. Optical spectroscopy confirms that Co2+ ions located in the initial glass in octahedral and tetrahedral positions, enter the Mg(Al,Ga)2O4 nanocrystals predominantly in tetrahedral sites and the fraction of tetrahedrally coordinated ions increases gradually with the heat-treatment temperature. The feature of these GCs is a long-wavelength shift of the absorption band related to the 4A2(4F)→4T1(4F) transition of Co2+ ions (up to ~1.67 µm) as compared with that for Co:MgAl2O4 single crystals. An increase of the heat-treatment temperature from 800 to 950 °C leading to an increase of the fraction and size of spinel crystals allowed increasing the peak absorption within this band while keeping the saturation intensity within the range 0.5…0.7 J/cm2 and the recovery time in the range 240…335 ns. The developed GCs are promising as saturable absorbers for erbium lasers emitting at 1.6-1.7 µm.