Structural and spectroscopic studies of lithium tetraborate glass co-doped with Sm and Cu

The local structure and spectroscopic properties of Cu-Sm co-doped Li₂B₄O₇ glass have been studied using XRD, EPR, UV-Vis-NIR absorption and luminescence methods and compared with the corresponding results for Sm-doped and Cu-doped Li₂B₄O₇ glasses. The EPR spectrum reveals an axially-symmetric signal with a characteristic hyperfine structure belonging to Cu²⁺ ions. The Cu²⁺/Cu_total ratio was determined. The optical absorption spectrum shows a broad, intense band attributed to Cu²⁺ ions and several weaker narrow bands attributed to Sm³⁺ ions. Photoluminescence (PL) and photoluminescence excitation (PLE) spectra as well as PL decay kinetics of Sm³⁺ and Cu⁺ ions were registered, analyzed, and discussed. The PL spectra show a broad band assigned to the 3d⁹4s¹ → 3d¹⁰ transition of Cu⁺ ions and four narrow bands assigned to the ⁴G_5/2 → ⁶H_J (J = 5/2-11/2) transitions of Sm³⁺ ions. The observed changes in PL intensity, shortening of the Sm³⁺ lifetime and prolongation of the Cu⁺ lifetime were explained by energy transfer from Sm³⁺ → Cu⁺ and re-absorption of the Sm³⁺ emission by Cu²⁺ ions.