Two types of spectral holes in Y2O3:Pr3+ crystalline systems

In Y2O3:Pr3+, we found a crossover of the hole-burning mechanism between particular temperature ranges; narrow holes due to the optical pumping were burned at low temperature (4.2 K), while broader holes with higher thermal stability (stable hole) were burned at higher temperatures (11-20 K). We can understand this temperature behavior by using a simple 3-level model for the optical pumping process. On the contrary, in Y2O3-MO (M(2+) = Mg2+, Ca2+, or Ba2+):Pr3+ systems, the stable broad hole was burned at all temperatures below 40 K.