Optical methods, using sub-ps laser pulses, were used to study the kinetics of defect creation by intense electronic excitation. An intense pulse was first used to create a high density of electron-hole pairs. A weaker second pulse was then used to probe the state of the material after various delays. Transient absorption measurements could be used to monitor the appearance of point defects following the trapping of free carriers. It was shown that, unlike the situation in other oxides (Al2O3, MgO), the trapping process here was extremely fast (150fs) and occurred at all temperatures in the triplet state of the self-trapped exciton. Permanent absorption was shown to appear only at room temperature, and resulted from the thermal conversion of self-trapped excitons into color centers.

G.Petite, P.Daguzan, S.Guizard, P.Martin: Journal de Physique III, 1996, 6[12], 1647-76