Tm3+-doped (Lu0.25Y0.25Sc0.25Gd0.25)2O3 high-entropy sesquioxide crystal fibers with doping concentrations of 2, 4, and 8 at.% were successfully grown using the laser-heated pedestal growth (LHPG) method. X-ray diffraction (XRD) confirmed that the crystal fibers possess a cubic structure. A comprehensive spectroscopic characterization was performed, including absorption and fluorescence spectra, as well as fluorescence decay curves. The JuddOfelt theory was applied to determine the spontaneous emission probabilities and radiative lifetime. For the 2.0 at.% doped fiber, the absorption cross-section at 793 nm was measured to be 2.76 & times; 10-21 cm2. The emission cross-section corresponding to the 3F4 -> 3H6 transition in the 2.0 at.% doped fiber reached 0.94 & times; 10-20 cm2 with an FWHM of 81.89 nm. The fluorescence lifetime of the 3F4 level decreased with increasing Tm3+ concentration, from 1.52ms at 2.0 at.% to 0.27 ms at 8.0 at.%. These results demonstrate that Tm3+-doped (Lu,Y,Sc, Gd)2O3 high-entropy crystal fibers exhibit outstanding spectral properties, making them promising gain media for tunable and ultrafast lasers in the near-infrared region.

Ho3+-doped YGdO3 single crystal fibers with doping concentrations of 0.2, 0.5, and 0.7 at.% were successfully grown via the laser-heated pedestal growth (LHPG) method. Comprehensive structural and spectroscopic characterizations, including Judd-Ofelt analysis, were performed. The crystals exhibited broad mid-infrared (∼2.85 μm) emission from the 5I6 → 5I7 transition, with a full width at half maximum exceeding 75 nm. The maximum emission cross-sections were determined to be 0.72×10-20, 0.66×10-20, and 0.75×10-20 cm2 for the 0.2, 0.5, and 0.7 at.% doped samples, respectively. The fluorescence lifetimes of the 5I6 level and 5I7 level were measured, with variations attributed to concentration quenching effects. The combination of a large emission cross-section, broad bandwidth, and favorable gain properties demonstrates the strong potential of Ho: YGdO3 crystals for applications in efficient and broadly tunable ultrafast mid-infrared lasers.

Pr:LaLuO3 crystal fibers with doping concentrations of 0.15, 0.3 and 0.5 at.% were successfully grown by the laser-heated pedestal growth (LHPG) method. The absorption spectra, fluorescence spectra and fluorescence decay curves were measured at room temperature, followed by Judd-Ofelt (J-O) theory analysis. The 0.5 at.% Pr: LaLuO3 crystal fiber exhibits a strong, broad absorption band centered at 452 nm, with an absorption cross section of 2.54 x 10- 20 cm2 and a full width at half maximum (FWHM) of 18.4 nm. The J-O intensity parameters were determined as S22 = 2.68 x 10-20 cm2, S24 = 0.88 x 10-20 cm2, and S26 = 2.96 x 10-20 cm2, respectively. Notably, the 3P0-*3F2 transition at 659 nm yields the largest emission cross section of 2.86 x 10-19 cm2 with a FWHM of 5.82 nm. The influence of doping concentration on the fluorescence lifetime was also investigated. These results demonstrated that Pr:LaLuO3 is a promising gain medium for visible laser operation.

Undoped and titanium-doped sapphire (Ti:Al2O3) single-crystal rods were grown using the micro-pulling-down (mu-PD) technique to investigate the effect of Ti incorporation on bubble defect formation. Crystals with Ti concentrations of 0, 500, and 1000 ppm were obtained with uniform geometry and good optical quality. Microstructural analysis revealed a strong dependence of bubble distribution and morphology on the Ti dopant. Undoped sapphire exhibited only a thin peripheral layer of small spherical bubbles (0-20 mu m), whereas Ti-doped crystals showed a significant increase in bubble layer thickness (up to similar to 370 mu m), accompanied by the appearance of elongated and irregular bubble shapes. A quasi-steady-state numerical model, including heat transfer, melt convection, and thermocapillary (Marangoni) effects, was developed to clarify the governing mechanisms. Simulations indicate that increasing Ti concentration enhances the meniscus height and intensifies the Marangoni convection, with melt velocities rising from similar to 5.6 to 20 mm s-1. The resulting flow structure promotes bubble transport toward the crystal periphery and contributes to their deformation under strong shear near the meniscus. These results provide a consistent interpretation of the relationship between titanium doping, melt flow, and bubble distribution during mu-PD sapphire growth.

We demonstrate the potential of magnesium (Mg2+) co-doping in Ce3+-doped gadolinium aluminum gallium garnet Gd-3(Ga3Al2)O-12 (GAGG:Ce) scintillating crystals to obtain a fast decay time performance and the feasibility of mu-PD technique for growing GAGG:Ce,Mg with high dopant concentrations. We demonstrate that Mg2+ co-doping can enhance scintillation kinetics through compositional engineering. An ultra-fast effective decay time of similar to 1 ns was achieved with 2500 ppm Ce and 500 ppm Mg, suggesting that Mg2+ co-doping modulates energy transfer pathways through Ce3+/Ce4+ activator ions effectively. These results pave the way for the development of ultrafast scintillator materials.

Er3+-doped (Lu,Y,Sc)2O3 mixed sesquioxide crystal fiber, along with Lu2O3, Y2O3, and Sc2O3 crystal fibers were fabricated using the laser-heated pedestal growth (LHPG) method. XRD characterization revealed that Er: (Lu,Y, Sc)2O3 crystal fiber maintains a single-phase cubic structure with the Ia-3 space group. The absorption and emission spectra, along with fluorescence lifetimes, were systematically investigated. Judd-Ofelt (JO) analysis was employed to determine the spontaneous emission probabilities, branching ratios, and radiative lifetimes. The material exhibits an absorption cross-section of 2.12 x 10-21 cm2 at 983 nm, with emission cross-sections reaching 0.90 x 10-20 cm2 at 2742 nm (4I11/2-4I13/2 transition) and 0.93 x 10-20 cm2 at 1558 nm (4I13/2-4I15/2 transition). The measured fluorescence lifetimes were 1.04 ms for the 4I11/2 upper laser level and 5.76 ms for the 4I13/2 lower level. These results highlight the promising potential of Er: (Lu,Y,Sc)2O3 crystal fibers for 2.7 mu m laser applications.

This study integrates experimental and numerical approaches to reveal the mechanisms controlling bubble transport, distribution, and entrapment in sapphire single crystals grown by the micro-pulling-down (mu -PD) technique. Sapphire rods (diameter: 3 mm) were grown from a molybdenum crucible under argon atmosphere using pulling rates of 0.25-2.5 mm/min. A global finite element model, incorporating precise furnace geometry, crucible, and after heater, was developed and validated experimentally with excellent agreement. A systematic analysis of pulling rate, meniscus height (hm), and crystal-to-die diameter ratio (Drod/Ddie) demonstrates that bubble incorporation is governed by meniscus geometry and melt convection rather than pulling rate alone. When the rod diameter matches exactly the die (D-rod=D-die), the meniscus is nearly cylindrical with low height and curvature, decreasing the Marangoni convections at the periphery and the forced convection in the core. This configuration yields at low pulling rates (e.g., 0.25 mm/min), bubble-free crystals. Conversely, D-rod

Undoped sapphire plates were grown by the micro-pulling down technique from Mo crucibles, and the use of graphite and alumina ceramics thermal insulation was evaluated. Through the use of these two thermal insulations, the bubbles encapsulated in the plates were analyzed both qualitatively and in terms of their distribution. By comparing with plates grown from Ir crucibles, it is shown that growing sapphire plates from Mo crucibles in graphite thermal insulation results in the best quality, including a more favorable bubble distribution and fewer particle inclusions. It is evaluated that the critical factor is to avoid any source of oxygen in the chamber, as this highly influences the state of the Mo crucible and thereby the sapphire crystals.

Ce:Al2O3-YAG binary and Ce:Al2O3-YAG-ZrO2 ternary eutectic ceramics were synthesized by the micro-pullingdown (mu-PD) method to evaluate the influence of ZrO2 addition on their microstructure, residual stress, and luminescent characteristics. Increasing Ce3+ concentration resulted in progressively deeper surface coloration, consistent with its chromophoric behavior. The microstructural analysis of SEM revealed a typical "Chinese script" morphology in the binary eutectic, whereas the ternary system exhibited a colony-like architecture. X-ray diffraction confirmed the formation of binary and ternary eutectic structures without secondary phases over the investigated Ce3+ concentration range. The binary system exhibited lower residual stress compared with the ternary counterpart under identical processing conditions. Photoluminescence spectra demonstrated efficient visible emission in both systems, although concentration quenching was evident at higher Ce3+ levels. A finite element simulation incorporating induction heating, convection, and radiation was conducted to model thermal transport and melt flow during mu-PD growth. The results indicated that the highest crucible temperature was located near the coil center, and that melt flow was primarily governed by Marangoni convection-more pronounced in the ternary system due to its reduced viscosity and enhanced thermocapillary forces.

This work presents a comprehensive experimental and numerical investigation of bubble formation, motion, and entrapment in titanium-doped sapphire (Ti:Al2O3) crystals from micro to large-scale. Crystals of 5 mm, 30 mm, and 260 mm diameters were grown using distinct furnace configurations, enabling systematic comparison of bubble morphology and density across crystal scale. In all cases, the seed orientation was (11-20) and the average titanium concentration was around 300 ppm. A global finite element model was developed to simulate coupled heat and mass transfer in the growth systems, including heat conduction, buoyant and thermocapillary (Marangoni) convection, internal and surface radiation, and forced convection induced by pulling or rotation. In mu-PD sapphire rods (D approximate to 5 mm), the strong thermal gradients (270-680 K cm-1), forced convection induced by rod pulling and Marangoni convection close to the meniscus, promote bubble accumulation and entrapment. Optical microscopy revealed bubbles ranging from 10 to 70 mu m, with different morphology and size evolving with the pulling rate and Ti concentration. In contrast, Cz-grown crystals (D approximate to 30 mm) exhibited lower temperature gradients (150-300 K cm-1) and efficient bubble removal due to enhanced upward flow near the crucible sidewall and large free surface area, resulting in a nearly bubble-free central zone. For large-diameter crystals (D approximate to 260 mm), the highly convex interface and small upward buoyant flow with a small free surface area (compared to the crucible cross section) limit gas evacuation, leading to localized peripheral bubble zones. The strong correlation between simulations and experiments demonstrates that bubble dynamics in Ti:sapphire growth are governed by the interaction of Marangoni, buoyant, and forced convection and titanium concentration. These insights provide key guidelines for optimizing furnace design and growth parameters to minimize gas inclusions and improve optical quality in high-performance laser crystals.

Pr:CaREAl3O7 (RE = Y, Gd, La) crystal fibers with varying Pr3+ doping concentrations were successfully grown using laser-heated pedestal growth (LHPG) method. The structure of the as-grown crystal fibers was determined by X-ray diffraction (XRD) and Raman spectroscopy. The fundamental spectroscopic properties, including absorption and emission cross sections, fluorescence lifetimes, and Judd-Ofelt (J-O) analysis, were systematically investigated. The absorption spectra are dominated by intense bands centered at 443 nm, which correspond well to the emission wavelengths of commercial InGaN laser diodes. The maximum emission cross-sections for all Pr:CaREAl3O7 crystal fibers occur at 646 nm, attributed to the 3P0 -> 3F2 transition, with values of 10.84 & times; 10-20 cm2, 7.92 & times; 10-20 cm2, and 10.94 & times; 10-20 cm2 for RE = Y, Gd, and La, respectively. The effect of Pr3+ doping concentration on the fluorescence lifetime was also examined. The results demonstrate the potential of Pr:CaREAl3O7 crystal fibers as promising gain media for visible laser applications.

Al2O3-YAG binary (BE) and Al2O3-YAG-ZrO2 ternary (TE) eutectic ceramic rods were successfully grown using the micro-pulling-down (& micro;-PD) technique. This work integrates experimental observations with finite element simulations to elucidate the solidification behavior of both eutectic systems throughout the growth process. Numerical modeling was carried out using the exact furnace geometry, accounting for variations in meniscus height, rod diameter and length, as well as key process parameters including input power, initial powder mass, and pulling rate. A constant pulling rate of 0.3 mm.min-1 was applied for both systems, while the electromagnetic power was set to 35% for BE and 29% for TE growth. Three distinct growth stages were identified for both eutectics. During the initial stage, the rod diameter exceeded 3 mm of the capillary die diameter, with this effect being more pronounced in the TE system. The second stage represented steady-state growth, characterized by a uniform rod diameter matching the die size. In the final stage, the diameter decreased and exhibited significant fluctuations, particularly for TE, attributed to its lower melt viscosity, larger meniscus height, and higher temperature gradient. As the melt height decreased, especially upon reaching the crucible's conical region, growth stability deteriorated, leading to diameter oscillations confirmed by post-growth rod measurements.

Single crystals (SCs) of Tm:Y2O3 and Tm,Ho:Y2O3 were successfully fabricated using the laser heated pedestal growth (LHPG) technique. X-ray diffraction analysis confirmed the high crystallinity of the as-grown crystals. Systematic spectroscopic characterization, including absorption and fluorescence measurements was performed, with particular emphasis on the dopant-concentration dependence of fluorescence lifetimes. Using Judd-Ofelt analysis, the spontaneous emission probabilities, branching ratios, and radiative lifetimes were determined. All crystals demonstrated broad absorption bands, with calculated absorption cross-sections at 801 nm 4.54 & times; 10-21 cm2 for 3% Tm:Y2O3, 5.06 & times; 10-21 cm2 for 3% Tm, 0.25% Ho:Y2O3 and 4.74 & times; 10-21 cm2 for 3% Tm, 0.5% Ho:Y2O3, respectively. The stimulated emission cross-section of the peak 1936 nm corresponding to the transition 3F4 -> 3H6 were calculated to be 3.0 & times; 10-21 cm2 with the FWHM 124.45 nm. The corresponding fluorescence lifetimes were measured to be 3.03 ms, 2.21 ms, and 3.40 ms, respectively. These results demonstrate optimized SCs are promising gain media for 2 mu m diode-pumped solid-state lasers.

Due to their beautiful color and exceptional mechanical strength and optical properties, ruby crystals are highly valued in the luxury, watchmaking, and jewelry industries. Similar to other types of doped and undoped sapphire, it is suffering from the presence of a specific defect, namely bubbles, that are affecting the yield and degrading the mechanical properties. In this paper, we present single-crystalline ruby fibers with a diameter of around 1 mm, grown by the micro-pulling down (& micro;-PD) technique. The coloration and bubble distribution are evaluated and discussed, and it is revealed that the red/pink coloration increases with increasing nominal Cr content and pulling rate (growth rate). However, bubble entrapment is also increasing with those two parameters, making it necessary to compromise depending on the specific application. The bubble trend is similar to that of undoped sapphire but shifted to lower pulling rates. Finally, a defect-coloration correlation is identified, and the potential mechanisms are discussed.

Nd:LuxY3-xAl5O12 (x = 0, 1, 1.5, 2, 3) crystal fibers doped with 0.2 at.% Nd3+ were successfully grown using the laser-heated pedestal growth (LHPG) method. The effects of the Lu3+/Y3+ ratio on the structural, absorption, and emission properties were systematically studied. Absorption spectra show that Nd:LuxY3-xAl5O12 (Nd:LuYAG) crystal fibers combine the high absorption cross section of Nd:YAG and the broad absorption bandwidth of Nd:LuAG. Judd-Ofelt analysis was employed to evaluate the optical transition parameters. Under 808 nm excitation, the fluorescence lifetime of the F-4(3/2) level gradually decreased with increasing Lu3+ content. These results indicate that adjusting the Lu3+/Y3+ ratio in Nd:LuYAG crystal fibers allows optimization for high-performance laser applications, presenting a potential alternative to conventional Nd:YAG gain media.

Shaped Co-doped Al2O3-YAG and Al2O3-YAG-ZrO2 blue-colored eutectic ceramic rods with a diameter of 3 mm were solidified from the melt using the micro-pulling down (mu-PD) technique. The blue color contrast of the rods depends on the Co concentration and the type of eutectic system. Regardless of the Co dopant concentration, mainly two textured phases (Al2O3-YAG) were observed in the binary system, and three phases (Al2O3-YAGZrO2) with a Chinese script microstructure were observed in the ternary system. The Co concentration in the binary Al2O3-YAG system does not affect the solidification process or morphology, unlike in the ternary Al2O3YAG-ZrO2 system. In both eutectic systems, the solidified phases are well oriented along specific directions. In the ternary system, only the YAG phase does not strongly deviate from its preferential growth direction (100). The hydrostatic stress in the binary eutectic system is lower than in the ternary eutectic system with the same Co dopant concentration due to the distribution of ZrO2 particles in the microstructure, which affects residual stresses. The luminescence properties of the Co-doped binary and ternary eutectic systems were investigated.

2 at.% Yb:(LuxY1-x)3Al5O12 (x = 0, 0.25, 0.5, 0.75, 1) garnet crystal rods were successfully grown using the micro-pulling-down (μ-PD) method. The effect of different ratios of Lu3+ and Y3+ ions on the crystal structure and the luminescence of Yb3+ ions in (LuxY1-x)3Al5O12 crystals was systematically studied. Optical microscopy revealed a clear composition-dependent increase in crystal defects with increasing Lu3+ content. Yb:YAG exhibited the highest optical quality, containing only a few small spherical bubbles (<10 µm) and a thin bubble-rich peripheral layer (∼10 µm). In contrast, Yb:LuAG showed larger and more elongated bubbles (10–30 µm), thicker bubble-rich regions (150–200 µm), and frequent crack-like defects. Bubble elongation along the growth direction was also observed. The absorption spectra show that with the increasing of Lu3+ concentration, the main absorption peak gradually blue shifts, and the absorption cross-section gradually decreases. As the Lu3+ content increases, the emission cross-section of the crystal at 1030 nm increases from 2.27 × 10−20 cm2 to 2.61 × 10−20 cm2. The fluorescence lifetime of the Yb:LuYAG crystal falls between those of Yb:YAG and Yb:LuAG crystals. These results demonstrate that the optical properties of Yb:LuYAG crystals strike a balance between the characteristics of YAG and LuAG, providing more options for near-infrared laser applications.

The laser-heated pedestal growth (LHPG) method was used to grow Tm3+-doped calciumniobiumgalliumgarnet (CNGG) crystal fibers with different Tm3+ concentrations. Room temperature absorption spectra, emission spectra, and fluorescence decay curves were recorded. For the 4 at.% Tm:CNGG crystal fiber, the absorption cross-section at 785 nm was determined to be 1.29 × 10-20 cm2, with a full width at half maximum (FWHM) of 17.3 nm. Fluorescence characteristics were evaluated using Judd-Ofelt theory, from which the intensity parameters (Ω2, Ω4, Ω6), radiative transition rates, branching ratios, and radiative lifetime were derived. The Tm:CNGG crystal fiber exhibited strong emission centered at 1865 nm, with an emission cross-section of 0.42 × 10−20 cm2 and a broad FWHM of 102.8 nm. The measured fluorescence lifetimes of the 3F4 level were 5.08 ms, 5.06 ms, 3.98 ms, and 1.58 ms for Tm3+ concentrations of 0.5 at.%, 1.0 at.%, 2.0 at.%, and 4.0 at.%, respectively. The results indicate that Tm:CNGG crystal fibers are promising gain media for ultrashort pulse laser systems.

ABSTRACT A novel series of Er 3+ ,Yb 3+ co‐doped Al 2 O 3 ‐YAG composite ceramics was synthesized via high‐temperature solid‐state reaction for optical thermometry applications. The ceramics exhibit stable up‐conversion luminescence under 980 nm excitation, with green emissions originating from the thermally coupled levels ( 2 H 11/2 and 4 S 3/2 ) of Er 3+ . The fluorescence intensity ratio (FIR) of these transitions follows Boltzmann distribution, enabling temperature sensing from 293 to 493 K. Maximum relative and absolute sensitivities reached 0.9% K −1 at 293 K and 0.2% K −1 at 493 K, respectively. The results demonstrate that Er 3+ ,Yb 3+ :Al 2 O 3 ‐YAG ceramics are promising candidates for high‐temperature optical thermometry due to their stability, sensitivity, and robust physicochemical properties.