Optical energy storage-type tunable long-lasting phosphorescence of low valent rare-earth ions using crystal-field distributions

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Low valent rare-earth ions(Eu^<2+>,Ce^<3+>,Sm^<2+>) in crystals with crystal-field distributions are expected to produce long-lasting phosphorescence in visible region and to show novel optical phenomena. We have prepared new melilite-type aluminosilactes with disordered stuructures, Ba silicates, and fluondes with superstructures. We have examined the optical properties, the local structures, and the mechanism of long-lasting phosphorescence and photoreaction produced by the incoherent UV light or near IR femtosecond laser excitaion. The results of this study are summarized as follows.1. UVexcitation for new Eu^<2+>-doped melilite Me_2A_3O_7(Me=Ca,Sr,Y,Gd;B=Al,Si) produces long-lasting phpsphorescence from Eu<2+> ions in the blue and bitter orange regions. The temperature dependence is derived from decay rates including both radiative and non-radiative processes. We proposed the following model of the phosphorescence of Eu^<2+> in the melilite crystals.(1) Pairs of electrons and holes … More are produced at Eu^<2+> ions by the UV excitation, move to stable trapped centers.(2) The electrons or holes again back to Eu^<2+> through thermal hopping ana tunneling, and recombine radiatively.2. Long-lasting phosphorescence in the blue and bitter orange regions has been observed for newly discoverd Eu^<2+>-doped Ba-silicates(Ba_2SiO_4, BaSi_2O_5,Ba_3SiO_5). The decay is due to the recombination of the electron-hole pairs through tunneling.3. Trivalent Eu^<3+> -doped Ca_2Al_2SiO_7 melilite single crystals were grown. Infrared femto-second laser-induced bright and long-lasting phosphorescence in the red region was observed. This is first obsevation of long-lasting phosphorescence of trivalent Eu^<3+> in the red region by the infrared laser excitation.4. UV excitation of Ce^<3+>-doped melilite crystals Ca_2Al_2SiO_7 and CaYAl_3O_7 produces long-lasting phosphorescence besides intrinsic Ce^<3+> luminescence. The phosphorescence is explained by a model of the recombination of distant paires of trapped electrons and holes. The self -trapped holes, Al^<4+>, produced by the UV excitation move back to Ce^<3+> sites throug tunneling and thermal hopping, and the trapped holes of Ce^<4+> recombine radiatively the trapped electrons in the form of F^+ centers through tunnling. It is strongly related to hopping between the adiacent self-trapped staes of holes speparated by a barrier with the activetion energy.5. New phosphors, Ce^<3+> -and Sm^<3+>Sm^<2+>-doped BaMgF_4 and Ce^<3+>-doped CaNaYF6 were grown. Ce^<3+>-doped BaMgF_4 crystal exmoits a commensurate supser siruciurre. After the irradiation by the femtosecond laser, a part of Sm^<3+> in BaMgF_4 was converted to Sm^<2+>. The BaMgF_4:Sm^<3+>/Sm^<2+> crystal nas a potential for applications in opucai siorage. the effects of disorder in the CaNaF_6 crystal on the inhomogeneous brodening of the Ce^<3+> luminescence was analyzed. The Ce^<3+>-doped CaNaYF_6 crystal has a potential as a new material capable of lasing with wider tunability from the near UV to green regions. Less