Quantum cutting through downconversion in rare-earth compounds by crystal-field control

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In accordance with the concept of quantum cutting through downconversion for the rare-earth ion couple, -for example Gd^<3+>-Eu^<3+>, visible quantum cutting with a quantum efficiency of higher than 100% is theoretically possible for VUV excitation in rare earth based fluorides with an optimal rare-earth ion concentration. In this research the results obtained are summarized as follows.(1)Upon VUV excitation of Gd^<3+>, visible quantum cutting (the emission of two photons of visible light per absorbed photon) through downconversion is observed for the Gd^<3+>-Eu^<3+> couple in KGd_3F_<10> : Eu^<3+>, KGd_2F_7: Eu^<3+>CsGd_2F_7 : Eu^<3+>, and KLiGdF_5 : Eu^<3+>. This can result in visible emission due to the ^5D_0 -^7 F_J transition of Eu^<3+> with a quantum efficiency higher than 100%. The dependence of the efficiency of cross-relaxation required for visible quantum cutting on the Eu^<3+> concentration and crystal structure is and evaluated and discussed in terms of the probability of e … More nergy transfer as a function of the rare-earth interatomic distance. KGd_3F_<10> : Eu^<3+> exhibits the highest efficiency in the cross-relaxation step of these four fluorides. The highest efficiency of the cross-relaxation step, 0.65, is obtained for the KGd_3F_<10> doped with 2 at.% Eu^<3+>. If it is assumed that there are no non-radiative losses due to energy migration to quenching centers, the quantum efficiency is 165%. The highest cross-relaxation efficiency for KLiGdF_5 : Eu^<3+>, CsGd_2F_7 : Eu^<3+>, and KGd_2F_7 : Eu^<3+>, CsGd_2F_7 : Eu^<3+> are 0.40, 0.15, 0.05, respectively. From the critical concentration quenching of the ^5D_0 emission of Eu^<3+> and number of nearest-neighbor rare-earth ion, it was evaluated that in KGd_3F_<10> : Eu^<3+>, KGd_2F_7 : Eu^<3+>, CsGd_2F_7 : Eu^<3+>, and KLiGdF_5 : Eu^<3+> an excited Gd^<3+> ion can transfer its energy to third-nearest Eu^<3+> neighbors.(2)We have examined dynamical behavior of quantum cutting in KGd_3F_<10> : Eu^<3+> KLiGdF_5 : Eu^<3+> and CsGd_2F_7 : Eu^<3+> crystals. Time resolved spectra of emissions Eu^<3+> for KGd_3F_<10> : Eu^<3+>, KLiGdF_5 : Eu^<3+>, and CsGd_2F_7 : Eu^<3+> have been studied upon VUV excitation of Gd^<3+> using an ArF excimer laser. The ^5Do emission has an earlier rise in the decay profiles, while the ^5D_1 emission has a later rise in the profiles. It was clarified from the these results that upon VUV excitation in the ^6G_J levels of Gd^<3+> quantum cutting occurs through a two-step energy transfer (downconversion) from Gd^<3+> to Eu^<3+> by cross-relaxation and sequential the remaing excitation energy migration via the Gd^<3+> sublattice.(3)UV or visible excitation for newly discovered Ce^<3+>-doped melilite crystals Ca_2Al_2SiO_7 melilite and Eu^<3+>-doped Ba_2SiO_4 and Ba_3SiO_5 crystals produces long-lasting phosphorescence in the blue/green and yellow regions from Ce^<3+> or Eu^<2+> ions in addition to the intrinsic luminescence. The results of the temperature dependence of the long-lasting phosphorescence intensities integrated in a time domain suggest that electrons and holes produced by UV or visible excitation move back to Ce^<3+> or Eu^<2+> sites in the crystals though thermal hopping and/or tunneling and recombine radiatively at Ce^<3+> or Eu^<2+>. Less