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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]

Terbium (Tb 3+ ) ions are promising for the development of visible lasers. We report on the spectroscopy of Tb 3+ in the monoclinic KYb(WO 4 ) 2 (KYbW) crystal, including the Judd-Ofelt (J-O) modelling of the transition probabilities, evaluation of the absorption and stimulated-emission (SE) cross-sections with polarized light, and emission lifetime study. Within the strong configuration interaction (SCI) approximation of the J-O theory, the intensity parameters of Tb 3+ are Ω 2 =0.864, Ω 4 =1.657, Ω 6 =1.975 [10 -20 cm 2 ] and Δ=38810 cm –1 . The maximum σ abs for the 7 F 6 → 5 D 4 transition reaches 2.3×10 -21 cm 2 at 486.7 nm and the maximum σ SE for the green emission according to the 5 D 4 → 7 F 5 transition is 2.3×10 -21 cm 2 at 549.3 nm (both for E || N m ). The measured lifetime of the 5 D 4 state for a 1 at% Tb 3+ doping is 395±5 μs. Under blue excitation, Tb: KYbW exhibits intense green emission with the CIE 1931 coordinates of (0.312; 0.644). [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]

Tm 3+ -doped monoclinic magnesium monotungstate (MgWO 4 ) is a promising material for lasers emitting at above 2 μm. Stark splitting of 3 H 6 to 1 G 4 Tm 3+ multiplets in MgWO 4 is determined using low-temperature spectroscopy, calculated within crystal-field theory modified for the case of an intermediate configuration interaction (ICI) and analyzed in terms of the barycenter plot. Tm 3+ :MgWO 4 features a large Stark splitting of the ground-state ( 3 H 6 ), 633 cm −1 , and high transition cross-sections for polarized light due to its low-symmetry structure. A unique feature of Tm 3+ :MgWO 4 to provide a naturally polarized emission at >2 μm, as well as its suitability for broadly tunable and mode-locked lasers in this spectral range are argued. The first tunable and vibronic Tm 3+ :MgWO 4 lasers are demonstrated, yielding a continuous tuning from 1897 to 2062 nm in the former case. A vibronic laser operated at even longer wavelengths up to 2093 nm due to electron-phonon coupling with low-energy phonons of the crystalline host. Moreover, the optical indicatrix axes of high-refractive-index biaxial MgWO 4 are assigned. [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]

A transparent "mixed" 7 at.% Er:(Lu,Sc) 2 O 3 sesquioxide ceramic was fabricated by hot isostatic pressing of commercial sesquioxide powders at 1750 °C/200 MPa in argon atmosphere. It exhibited a cubic bixbyite-type structure (a = 10.198 Å), a mean grain size of 5.7 μm and high low-signal transmission of 79.1 % in the near-IR. The spectroscopic properties of Er3+ ions were studied. For the 4I 11/2 → 4I 13/2 transition, the stimulated-emission cross-section σ SE is 1.30 × 10−20 cm2 at 2719 nm, the corresponding luminescence branching ratio B (JJ') is 19.9 % and the radiative lifetime of the 4I 11/2 state is 4.80 ms, as determined using a Judd-Ofelt theory accounting for intermediate configuration interaction. The crystal-field splitting of Er3+ multiplets was studied at 12 K. The ceramic exhibits a significant inhomogeneous broadening of spectral bands. An Er:(Lu,Sc) 2 O 3 ceramic laser generated 342 mW at 2.71 and 2.85 μm with a high slope efficiency of 41.7 % (exceeding the Stokes limit) and a laser threshold of 125 mW. • Transparent "mixed" 7 at.% Er:(Lu,Sc)2O3 sesquioxide ceramic. • Fabrication by hot isostatic pressing at 1750 °C / 200 MPa in argon atmosphere. • The stimulated-emission cross-section for Er3+ ions is 1.30 × 10-20 cm2 at 2719 nm. • The ceramic exhibits a significant inhomogeneous broadening of the spectral bands. • The ceramic laser generated 342 mW at 2.71&2.85 μm with a slope efficiency of 41.7 %. [ABSTRACT FROM AUTHOR]

Thulium-doped calcium fluoride (Tm:CaF 2) single-crystalline films are grown on (100)-oriented undoped bulk CaF 2 substrates by Liquid Phase Epitaxy (LPE) using LiF as a solvent. Their spectroscopic properties are studied in details. For ~2 at.% Tm-doped films, the active ions are predominantly isolated and are located in oxygen-assisted trigonal sites, C 3v (T 2). With increasing the Tm doping level, ion clustering is promoted but it is slowed down as compared to bulk crystals. For ~6 at.% Tm-doped films, the majority of active ions form clusters resulting in a "glassy-like" spectroscopic behavior (smooth and broad spectral bands). For such layers, the maximum stimulated-emission cross-section for the 3F 4 → 3H 6 transition is 0.14 × 10−20 cm2 at 1856 nm, the luminescence lifetime of the 3F 4 state is 21.70 ms and the emission bandwidth exceeds 180 nm. The transition probabilities of Tm3+ ions are determined using the Judd-Ofelt theory. The waveguiding properties of the Tm:CaF 2 films are confirmed. Highly-doped Tm:CaF 2 epitaxial films are promising for waveguide lasers at ~2 μm. • Tm:CaF 2 waveguiding films are grown on CaF 2 substrates by Liquid Phase Epitaxy. • Spectroscopic evidence of oxygen-assisted trigonal sites C 3v (T 2) for Tm3+ ions. • Tm3+ ion clustering is slowed down in epitaxial films as compared to single-crystals. • For highly-doped films, the luminescence lifetime of the 3F 4 state reaches 21.70 ms. • The transition probabilities of Tm3+ ions are determined using the Judd-Ofelt theory. [ABSTRACT FROM AUTHOR]

We report on the growth, structure, polarized spectroscopy and first laser operation of a double molybdate compound featuring a layered structure – the monoclinic Nd:CsGd(MoO 4) 2. Single-crystals of 3 at.% Nd:CsGd(MoO 4) 2 are grown from the flux using cesium trimolybdate (Cs 2 Mo 3 O 10) as a solvent. They crystallize in the monoclinic system (sp. gr. P 2/ c) with lattice constants a = 9.5239(4) Å, b = 5.0752(5) Å, c = 8.0580(7) Å and β = 91.157(9)°. Nd:CsGd(MoO 4) 2 exhibits a perfect cleavage along the (100) plane. Polarized absorption and luminescence spectra of the Nd3+ dopant ion are measured. The maximum stimulated-emission cross-section is 29.0 × 10−20 cm2 at 1065.9 nm for light polarization E || b . The luminescence lifetime is 145 μs. The transition probabilities are determined using the modified Judd-Ofelt theory yielding the intensity parameters Ω 2 = 26.736, Ω 4 = 12.736 and Ω 6 = 17.771 [10−20 cm2] and α = 0.122 [10−4 cm]. Continuous-wave laser operation is achieved in cleaved single-crystal plates of Nd:CsGd(MoO 4) 2 yielding a maximum output power of 0.54 W at 1066 nm with a laser threshold of 70 mW, a slope efficiency η of 60.4% and a linearly polarized laser output ( E || b ). • Single-crystals of Nd:CsGd(MoO 4) 2 are grown from the flux using the Cs 2 Mo 3 O 10 solvent. • The crystal structure (monoclinic, sp. gr. P 2/ c) and thermal expansion are studied. • The transition probabilities of Nd3+ ions are calculated within the modified Judd-Ofelt theory. • The maximum stimulated-emission cross-section is 29.0 × 10−20 cm2 at 1065.9 nm for E .|| b ; • The Nd:CsGd(MoO 4) 2 laser generated 0.54 W at 1066 nm with a slope efficiency of 60.4%. [ABSTRACT FROM AUTHOR]

We report on the growth, structure refinement, optical spectroscopy, continuous wave and femtosecond mode-locked laser operation of a Tm3+,Ho3+-codoped disordered calcium niobium gallium garnet (CNGG) crystal. The 2.64 at.% Tm, 0.55 at.% Ho:CNGG is grown by the Czochralski method. Its cubic structure, sp. gr. I a 3 ¯ d - O10 h , a = 12.4952(1) Å, is refined by the Rietveld method revealing a random distribution of Ga3+ and Nb5+ cations over octahedral and tetrahedral sites. The Ho3+ transition probabilities are determined within the Judd-Ofelt theory accounting for an intermediate configuration interaction (ICI). For the 5I 7 → 5I 8 Ho3+ transition, the maximum stimulated-emission cross-section σ SE is 0.47 × 10−20 cm2 at 2080.7 nm. The gain bandwidth of Tm,Ho:CNGG at ∼2 μm is > 150 nm and the thermal equilibrium decay time - 6.80 ms. The Tm3+ ↔ Ho3+ energy transfer parameters are determined. A diode-pumped Tm,Ho:CNGG microchip laser generated 413 mW at 2088.4 nm with a slope efficiency of 15.9%. A continuous wavelength tuning between 1940.3 and 2144.6 nm is demonstrated. Ultrashort pulses as short as 73 fs are achieved at 2061 nm from a Tm,Ho:CNGG laser mode-locked by a GaSb semiconductor saturable absorber mirror at a repetition rate of 89.3 MHz. • Tm3+,Ho3+-codoped disordered calcium niobium gallium garnet (CNGG) crystal. • Cubic structure, sp. gr. I a 3 ¯ d - O10 h , a = 12.4952(1) Å, is refined by the Rietveld method. • Transition probabilities, emission cross-section and Tm3+↔Ho3+ energy-transfer. • Continuous-wave Tm,Ho:CNGG laser is tuned between 1940.3 and 2144.6 nm. • Tm,Ho:CNGG laser is mode-locked by a GaSb-based SESAM generating 73 fs pulses. [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]