Nonlinear optical properties and passive Q-switch performance of Co2+:MgAl2O4 crystal

The differential absorption spectra of the tetrahedral Co2+ ion in MgAl2O4 crystal were studied under picosecond excitation. The difference spectra of stimulated emission and excited-state absorption were derived from the differential absorption spectra. The stimulated emission around 660 nm related to the T-4(1)(P-4) --> (4)A(2) transition was observed. The excited-state absorption bands were assigned to the T-4(2) --> T-4(1)(P-4) transition and the transitions from the thermally populated E-2((2)G) state to the energetically higher doublet levels arising from the F-2 free-ion level of the tetrahedral Co2+ ion. Intensity-dependent transmission measurements were carried out at wavelengths of 1.34-mu m and 1.54-mu m. Passive e-switching of a Nd3+:YAlO3 (1.34-mu m) and Er3+:glass (1.54-mu m) laser with the Co2+:MgAl2O4 crystal as saturable absorber was demonstrated. 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 mu m (1.34 mu m) were obtained.