The mechanism of photobleaching in Sm²⁺-doped alkaline-earth fluorohalides

The fluorescence intensity of divalent samarium (Sm²⁺) ions doped in alkaline-earth fluorohalides were found to decrease under the optical excitation from a blue laser, a green laser or even a red laser. This photobleaching effect has a mysterious and unknown mechanism. After the bleaching effect happened, no Sm³⁺ ions generation, no relative spectral change in Sm²⁺ emissions and no lifetime decrease for the levels of Sm²⁺ ions could be observed. Under blue- or green-laser irradiation, the decrease of Sm²⁺ fluorescence showed a perfect biexponential function; however, when the exciting photon energy was lower than that of ⁵D₀→⁷F₀ transition, the photobleaching effect could not be observed anymore. An identical mechanism in the photobleaching with that in the spectral hole burning was proposed.