Nonlinear absorption properties of Co2+:MgAl2O4 crystal

The differential absorption spectra of Co2+: MgAl2O4 (MALO) crystal were studied with a picosecond pump-probe technique under excitation of the (4)A(2) --> T-4(1)(P-4) and (4)A(2) --> T-4(1)(F-4) transitions of the tetrahedral Co2+ ion. The difference spectra of stimulated emission (SE) and excited state absorption (ESA) were derived from the measured differential absorption spectra. The T-4(1)((4)p) --> (4)A(2) SE band around 660 nm was observed. The ESA bands were assigned to the T-4(2) --> T-4(1)(P-4) transition and to transitions from the thermally populated E-2((2)G) excited state to doublet levels arising from the F-2 free-ion level of the tetrahedrally coordinated Co2+ ion. Absorption saturation measurements were performed at 1.34 mu m and 1.54 mu m. Passive Q-switching of 1.34-mu m Nd3+:YAlO3 and 1.54-mu m Er3+:glass lasers was realized using the Co2+:MALO crystal as saturable absorber. The Q-switched laser pulses of 38 ns (110 ns) in duration and up to 2.7 mJ (10 mJ) in energy at 1.54 Ccm (1.34 Gem) were obtained.