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 4A2→4T1(4P) and 4A2→4T1(4F) 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 4T1(4P)→4A2 SE band around 660 nm was observed. The ESA bands were assigned to the 4T2→4T1(4P) transition and to transitions from the thermally populated 2E(2G) excited state to doublet levels arising from the 2F free-ion level of the tetrahedrally coordinated Co2+ ion. Absorption saturation measurements were performed at 1.34 μm and 1.54 μm. Passive Q-switching of 1.34-μm Nd3+:YAlO3 and 1.54-μm 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 μm (1.34 μm) were obtained.