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We report on a detailed spectroscopic study of Eu 3+ ions in the monoclinic KYb(WO 4 ) 2 crystal. The polarized room and low-temperature absorption spectra are measured. The maximum σ abs corresponding to the 7 F 1 → 5 D 1 transition is 1.32×10 –20 cm 2 at 534.2 nm with a bandwidth of 0.7 nm (for E || N m ). The Stark sub-levels of the excited mulitplets are determined. A Judd-Ofelt analysis is applied to the Eu 3+ :KYb(WO 4 ) 2 crystal to determine the probability of spontaneous transitions, radiative lifetimes and luminescence branching ratios. Within the strong configuration interaction (SCI) approximation, the intensity parameters are Ω 2 =4.757, Ω 4 =2.295, Ω 6 =1.644 [10 –20 cm 2 ] and Δ f =50160 cm –1 . The radiative lifetime of the 5 D 0 state is 351 µs. The maximum stimulated-emission cross-section corresponding to the 7 F 1 → 5 D 1 transition is 1.44×10 –20 cm 2 at 703.2 nm (for E || N m ). Under UV excitation, the Eu 3+ :KYb(WO 4 ) 2 crystal provides intense red photoluminescence with CIE coordinates, x=0.675, y=0.325. [ABSTRACT FROM AUTHOR]

In the present work, we report on a comprehensive study of crystallization, structure and optical spectroscopy of transparent glass–ceramics with (Eu 3+ ,Yb 3+ ):YNbO 4 nanocrystals synthesized by secondary heat-treatments of glass of the Li 2 O–Al 2 O 3 –SiO 2 system, for the first time, to the best of our knowledge. Heat treatments result in volume crystallization of RENbO 4 with the sizes of 4–15 nm. Crystals of rare-earth niobates with disordered fluorite structure (T′) appear during heat-treatment at 720–740 °C for 6 h, crystals with tetragonal structure (T) appear at higher temperatures or longer durations of heat-treatment, and in samples heat-treated at 1000 °C, the monoclinic form (M) is detected additionally. Rare-earth niobates act as nucleating agents for bulk crystallization of β -quartz solid solutions, the main crystalline phase of the glass–ceramics, which ensures their good thermo-mechanical properties. Optical spectroscopy confirms entering of both Eu 3+ and Yb 3+ ions into the RENbO 4 nanophase and their specroscopic properties variation according to the T′→T→M phase transformations. Under UV excitation, glass–ceramics heat-treated at 900 °C provide intense red emission with the color coordinates x =0.665, y =0.335 (CIE 1931). In the sample, heat-treated at 1000 °C, a partial reduction of Eu 3+ to Eu 2+ is observed which allows for tuning the color properties of emission. When excited in the near-IR by an InGaAs diode, the initial glass and glass–ceramics show red cooperative upconversion due to the 2Yb 3+ →Eu 3+ energy transfer. The efficiency of the latter is ~10%. The developed materials due to the good emission and thermo-mechanical properties are promising for the development of color-tunable red phosphors. [ABSTRACT FROM AUTHOR]

We report on growth and detailed spectroscopic study of Eu 3+ -doped tetragonal sodium gadolinium double tungstate, Eu:NaGd(WO 4 ) 2 , a new promising crystal for deep-red lasers. Large-volume crystal doped with 4.9 at.% Eu is grown by Czochralski method along the [001] crystallographic direction. Absorption of Eu 3+ ions is studied at room temperature (RT) and at 6 K. For the absorption band related to the 7 F 1 → 5 D 1 transition suitable for pumping of Eu:NaGd(WO 4 ) 2 , the maximum cross-section is σ abs = 1.2 × 10 −21 cm 2 at 535.5 nm with the full width at half maximum (FWHM) of 3.1 nm (at RT, for E || a polarization). For the 5 D 0 → 7 F 4 transition, the maximum stimulated-emission cross-section is σ SE = 1.6 × 10 −21 cm 2 at 698.3 nm (RT, E || c polarization). Lifetime of the 5 D 0 state is 490 ± 10 μs (at RT). Under UV excitation, Eu:NaGd(WO 4 ) 2 provides intense red emission with CIE coordinates ( x = 0.671, y = 0.329). [ABSTRACT FROM AUTHOR]

Abstract: Novel oxyfluoride glass doped with YbF3 and Eu2O3 is synthesized in the SiO2–PbO–PbF2–CdF2 system by melt-quenching technique. Its physical properties, as well as the optical absorption of Eu3+ and Yb3+ ions are studied. Spectroscopic properties of Eu3+ ions are modeled within modified Judd–Ofelt theory, yielding absorption oscillator strength, luminescence branching ratios and radiative lifetime of 5D0 state. Intrinsic emission of Eu3+ ions under direct excitation in the UV, as well as up-conversion luminescence under near-IR excitation via cooperative energy transfer, 2Yb3+ →Eu3+, are studied. In the latter case, the glass provides intense reddish-orange emission with CIE coordinates x =0.636, y =0.363. The asymmetry parameter R for Eu3+ emission in the studied glass is about 8. The efficiency of this transfer mechanism is estimated from shortening of 2F5/2(Yb3+) lifetime to be 10±1%. [Copyright &y& Elsevier]