Effects of electron irradiation and subsequent annealing on the optical absorption and photoluminescence of CaF2 single crystals

The colorless transparent CaF2 single crystals turned blue after 1.7 MeV electron irradiation at fluences of 4 × 1016 and 1 × 1017 cm−2 at room temperature. The irradiated samples were annealed for 1 h at 70, 150, 200 and 300 °C in ambient atmosphere. Optical absorption and photoluminescence (PL) spectra were used to characterize the as-received, irradiated, and annealed crystals. Five obvious absorption bands peaked at 224, 339, 397, 579, and 800 nm, respectively, appear in the optical absorption spectra of the as irradiated samples. The absorption bands at 224, 339, and 800 nm disappear after annealing at 150 °C and the other two bands at 397 and 579 nm can still be observed even after annealing at 300 °C. A PL emission band at 422 nm is observed in the as-received crystals. The PL intensities of irradiated crystals gradually increase up to the level of the as-received samples with the increasing annealing temperature up to 300 °C.