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We report on a modification of the Judd-Ofelt theory accounting for thermal populations of Stark sub-levels of the multiplets involved in transitions in absorption and emission. The calculations require the data on the crystal-field splitting, the irreducible representations of Stark sub-levels and the selection rules for electric-dipole transitions. Simple formulas for absorption oscillator strengths and probabilities of radiative spontaneous transitions are obtained. The application of the proposed model allowed us to remove the contradiction between the transition intensities in absorption and emission (3H 6 ↔ 3F 4) for Tm3+-doped crystals. It was also used to explain the experimental temperature dependences of luminescence lifetimes of the 3F 4 Tm3+ manifold in two widely used laser crystals, LiYF 4 and Y 3 Al 5 O 12. The case of Yb3+ ions featuring a small number of crystal-field sub-levels is also considered. • Judd-Ofelt theory accounting for thermal populations of Stark sub-levels. • Simple formulas for absorption strengths and radiative transition probabilities. • The contradiction between Tm3+ absorption and emission transition intensities is resolved. • Temperature dependence of luminescence lifetimes of the 3F 4 Tm3+ manifold. • The case of Yb3+ ions featuring a small number of crystal-field sub-levels is considered. [ABSTRACT FROM AUTHOR]

Single-crystals of potassium gadolinium double tungstate KGd(WO 4) 2 doped with Sm3+ ions up to 20 at.% were grown from the flux using K 2 W 2 O 7 as a solvent and a detailed polarization-resolved spectroscopic study of Sm3+ ions was performed with the goal of developing novel materials emitting in the visible spectral range. Polarized absorption spectra were measured and 4f-4f transition intensities of Sm3+ ions were calculated using a modified Judd-Ofelt theory yielding the intensity parameters of Ω 2 = 8.027, Ω 4 = 7.210 and Ω 6 = 2.322 [10−20 cm2] and α = −0.017 [10−4 cm]. Polarized stimulated-emission properties of Sm3+ ions were studied. For the 4G 5/2 → 6H 9/2 transition in the red, the maximum σ SE is 0.55 × 10−20 cm2 at 649.0 nm for light polarization E || N p (luminescence branching ratio: 40.2 %). The radiative lifetime of the 4G 5/2 state is 734 μs. The luminescence dynamics from this manifold was analyzed using the Inokuti-Hirayama model. The possible role of 4f – 4f excited-state absorption from the 4G 5/2 level in preventing laser operation in the orange and deep red is analyzed. A 0.8 at.% Sm:KGd(WO 4) 2 laser generated an output power of 37.8 mW at 649 nm, with a laser threshold of 87 mW and a slope efficiency of 17.6 %. • Top-seeded solution growth of monoclinic KGd(WO 4) 2 crystals doped with Sm3+ ions. • Polarization-resolved study of absorption and stimulated-emission properties. • Transition intensities of Sm3+ ions calculated using a modified Judd-Ofelt theory. • Analysis of 4f - 4f excited-state absorption from the 4G 5/2 level in the visible. • Red Sm:KGd(WO 4) 2 laser generated 37.8 mW at 649 nm with a laser threshold of 87 mW. [ABSTRACT FROM AUTHOR]

A monoclinic 0.86 at.% Ho3+:MgWO4 crystal is grown by the Top-Seeded-Solution Growth method. Its spectroscopic properties are studied with polarized light for E || a, b, c. The Ho3+ ion transition probabilities are determined within the modified Judd-Ofelt theory (mJ-O) accounting for the configuration interaction. The intensity parameters are Ω2 = 21.09, Ω4 = 4.42, Ω6 = 2.28 [10–20 cm2] and α = 0.053 [10-4 cm]. The calculated radiative lifetime of the 5I7 state is 6.18 ms. The Stark splitting of the 5I7 and 5I8 multiplets is determined with low-temperature spectroscopy. The absorption, stimulated-emission (SE) and gain cross-sections for the 5I8 ↔ 5I7 transition are derived. Ho3+:MgWO4 features a large Stark splitting of the ground-state (380 cm-1), high maximum σSE of 1.82 × 10–20 cm2 at 2.083 μm, broad gain spectra and high luminescence quantum yield making it suitable for efficient continuous-wave and mode-locked lasers at ∼2.1 μm. First laser operation of Ho3+:MgWO4 crystal is demonstrated at 2.104 μm reaching a slope efficiency of 72%. [ABSTRACT FROM AUTHOR]

We report on the crystal growth, structure determination and the vibronic and spectroscopic properties of a novel red-emitting anisotropic crystal featured by a layered structure, the Eu 3+ -doped orthorhombic potassium yttrium double molybdate crystal, KY(MoO 4 ) 2 . The transition probabilities of Eu 3+ ions are determined within the Judd-Ofelt theory. The stimulated-emission cross-sections are calculated for the 5 D 0 → 7 F J transitions of Eu 3+ with polarized light. The maximum σ SE reaches 11.4 × 10 −20  cm 2  at 613.9 nm for E || c . The strong anisotropy of the spectroscopic properties of Eu 3+ is attributed to the layered structure of KY(MoO 4 ) 2 . The lifetime of the 5 D 0 state is 0.65 ms. Under UV excitation, Eu:KY(MoO 4 ) 2 provides red emission ( CIE 1931 coordinates, x  = 0.665 and y  = 0.335) with almost unity color purity. Eu 3+ ions are studied as structural probes, revealing a single type of site (C 2 symmetry). The polarization-anisotropy of the purely magnetic-dipole 5 D 0 → 7 F 1 transition is discussed. Eu:KY(MoO 4 ) 2 is promising for red phosphors and deep-red lasers. [ABSTRACT FROM AUTHOR]

The spectroscopic properties of Tb 3+ ions in monoclinic KLu(WO 4 ) 2 double tungstate crystal are studied with polarized light. The absorption spectra in the visible, near- and mid-IR including the transitions to all lower-lying 7 F J ( J  = 0 … 5) excited states are measured. The maximum absorption cross-section for the 7 F 6  →  5 D 4 transition is 3.42 × 10 −21  cm 2  at 486.7 nm for light polarization E || N m . The transition probabilities for Tb 3+ ions are calculated within the Judd-Ofelt theory modified for the case of an intermediate configuration interaction (ICI). The radiative lifetime of the 5 D 4 state is 450 μs and the luminescence quantum yield is >90%. The polarized stimulated-emission cross-section spectra for all 5 D 4  →  7 F J ( J  = 0 … 6) emission channels are evaluated. The maximum σ SE is 11.4 × 10 −21  cm 2  at 549.4 nm (for E || N m ). Tb 3+ :KLu(WO 4 ) 2 features high transition cross-sections for polarized light being promising for color-tunable visible lasers and imaging. [ABSTRACT FROM AUTHOR]

"Mixed" calcium rare-earth aluminate laser host crystals Ca(Gd,Lu)AlO 4 (CALGLO) with up to 10.8 at.% Lu codoped with Tm3+ and Ho3+ ions are grown by the Czochralski method along the [001] direction. The segregation of rare-earth ions is studied. The crystal structure is refined by the Rietveld method. Tm,Ho:Ca(Gd,Lu)AlO 4 crystallizes in the tetragonal system (sp. gr. I 4/ mmm) exhibiting a K 2 NiF 4 type structure. The lattice constants are a = 3.6585(6) Å and c = 11.9660(9) Å for a crystal with a composition of CaGd 0.8947 Lu 0.0551 Tm 0.0448 Ho 0.0054 AlO 4. The stability of Ca(Gd,Lu)AlO 4 solid-solutions is discussed. The polarized Raman spectra are measured, revealing a most intense mode at 311 cm−1 and a maximum phonon frequency of ∼650 cm−1. The polarized absorption spectra are measured. The transition intensities for the Ho3+ ion are analyzed using the modified Judd-Ofelt theory accounting for configuration interaction. • Tm3+,Ho3+-codoped "mixed" disordered Ca(Ga,Lu)AlO 4 aluminate crystal. • Rietveld refinement of the crystal structure and discussion of the crystal disorder. • The stability of CaGd 1-x Lu x AlO 4 solid-solutions is discussed. • Polarized Raman spectra are measured, revealing the most intense mode at 311 cm−1. • Transition intensities for Ho3+ ions are analyzed using the modified Judd-Ofelt theory. [ABSTRACT FROM AUTHOR]

Thulium ions (Tm3+) are known for their emissions in the near- and mid-infrared spectral ranges suitable for efficient laser operation. We systematically study the stimulated-emission cross-sections σ SE for the 3H 4 → 3F 4 (at ~1.5 μm) and 3H 4 → 3H 5 (at ~2.3 μm) Tm3+ transitions in five low-phonon energy fluoride single-crystals, namely, cubic Tm:CaF 2 and Tm:KY 3 F 10 , tetragonal Tm:LiYF 4 and Tm:LiLuF 4 and monoclinic Tm:BaY 2 F 8. A promising material is Tm:BaY 2 F 8 which offers broad and intense polarized emission spectra in the mid-infrared (2251–2452 nm); the maximum σ SE is 0.39 × 10−20 cm2 at 2289 nm (for E ||Z) and the emission bandwidth is exceeding 100 nm (for E ||X). We also revise the transition probabilities for Tm3+ ions in LiLuF 4 and BaY 2 F 8 crystals using the Judd-Ofelt formalism and accounting for an intermediate configuration interaction (ICI). • Stimulated-emission cross-sections for 3H 4.→3F 4 and 3H 4 →3H 5 thulium transitions; • Tm:CaF 2 , Tm:KY 3 F 10 , Tm:LiYF 4 , Tm:LiLuF 4 and Tm:BaY 2 F 8 fluoride crystals. • Tm:BaY 2 F 8 offers broadband and intense emission in the mid-infrared (2251–2452 nm). • In monoclinic Tm:BaY 2 F 8 , the maximum σ SE is 0.39 × 10−20 cm2 at 2289 nm. • Judd-Ofelt theory accounting for an intermediate configuration interaction (ICI). [ABSTRACT FROM AUTHOR]