Site-dependent excited state absorption in Ce-doped CaGa2S4 single crystal

Excited state absorption (ESA) by Ce3+ ions in the 5d(1) lowest excited state in Ce-doped CaGa2S4 single crystal was investigated by nanosecond two-pulse pump-CW probe spectroscopy. The first pump pulse of 425 nm excited the 4f ground state electron to the 5d(1) first excited state and the second pump pulse of 589 nm was introduced at 13 ns after the first pump in order to selectively excite the 5d(1) state. Transient intensity change of the CW probe of a diode laser of 780 nm was observed. Temporal behavior of photoluminescence intensity by the 5d(1) to 4f state transition was also observed under excitation of the same two-pulse pump. The experimental results can be explained by the model of site-dependent excited state absorption. The photoluminescence is generated from the 5d1 states of Ce3+ at two sites with different decay time, whereas the excited state absorption takes place mainly from Ce3+ at one site as well as metastable Ce2+. The model predicts that optical gain by the 5d(1) to 4f transition can be achieved by Ce3+ at the other site if the populations at both the 5d(1) state of Ce3+ at the site resulting in ESA and Ce2+ can be bleached by simultaneous secondary optical pumping with an IR (e.g. Nd: YAG) laser.