U6+ in SrWO4 as a two-level system: a study of the thermal activation of spectral hole refilling

The U6+ ion occupies a double-well potential in SrWO4. Spectral hole burning takes place when the ion changes from one well to the other during the process of optical excitation and relaxation. The temperature dependence of hole refilling has been studied in order to determine the nature of the double-well potential and the hole refilling mechanism. Analysis reveals that the refill rate follows a simple Arrhenius equation with a single barrier of 0.013 eV. The high-temperature extrapolated refill rate is extremely low at only 200 s−1. Possible explanations and the significance of this result to studies of amorphous systems are discussed.