Nonlinear Optical Properties and Passive Q-Switch Performance of Co~(2+) : MgAl_2O_4 Crystal

摘要

The differential absorption spectra of the tetrahedral Co~(2+) ion in MgAl_2O_4 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 ~4T_1(~4P) -> ~4A_2 transition was observed. The excited-state absorption bands were assigned to the ~4T_2 -> ~4T_1(~4P) transition and the transitions from the thermally populated ~2E(~2G) state to the energetically higher doublet levels arising from the ~2F free-ion level of the tetrahedral Co~(2+) ion. Intensity-dependent transmission measurements were carried out at wavelengths of 1.34-#mu#m and 1.54-#mu#m. Passive Q-switching of a Nd~(3+): YAlO_3 (1.34-#mu#m) and Er~(3+): glass (1.54-#mu#m) laser with the Co~(2+): MgAl_2O_4 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.