Rare-Earth Laser & Luminescent Materials

Oxyfluoride glass-ceramics (GCs) containing LaF₃ or α-NaLuF₄ nanocrystals, co-doped with 2 mol% Yb³⁺ and 0.5 mol% Er³⁺, are considered as core materials for the implementation of optical fiber photoluminescence (PL) thermometers. A dual-channel ratiometric thermometry approach combining green (UC, ²H_11/2 → ⁴I_15/2 vs ⁴S_3/2 → ⁴I_15/2) and infrared emissions (IR, ⁴I_13/2(m´) → ⁴I_15/2 vs ⁴I_13/2(m) → ⁴I_15/2) allows the extension of the sensing temperature range above the UC luminescence quenching (at ≈ 650 K) by over 100 K (LaF₃) or 150 K (α-NaLuF₄). The IR channel, working near λ= 1.55 mm, operates at the wavelength of minimum propagation losses of optical fibers, facilitating long-distance propagation of luminescence signals. The UC channel in LaF₃-GC shows a maximum absolute sensitivity S_A = 102 × 10^-4 K^-1 at 602 K in glass and S_A = 71 × 10^-4 K^-1 at 591 K in GC with thermal resolution δ = 1.5–3 K. The α-NaLuF₄-glass and -GC reach UC S_A = 90 × 10^-4 K^-1 at 698 K. The IR channel in both GCs, based on PL intensity ratios at 1498 nm and 1610 nm, exhibits S_A ≈ 30-10 × 10^-4 K^-1 for the 300-800 K range.

Published in Advanced Optical Materials vol 14, pg e01720, 2026

Lanthanide-doped Ca₃(NbGa)₅O₁₂-type disordered single crystals are promising media for high power lasers with ultrashort pulse durations. In this work, planar optical waveguides in lithium-modified Yb:Ca₃(NbGa)₅O₁₂ single crystals have been fabricated by O or F room temperature irradiation using 20 MeV of energy and a fluence of 1×10¹⁵ at/cm². The produced waveguides show 7 dark modes  in the O  case and 6 dark modes in the F one, monitored at λ= 632.8 nm. Light confinement is induced by a ≈ 4% drop of the refractive index at ≈ 4-4.5 µm of depth which is primary associated to electronic damage for both irradiations. The irradiation induces some crystal coloration that is largely eliminated by air annealing in the 150-300 ºC temperature range. A post implantation thermal annealing reduces the waveguide propagation losses at λ= 632.8 nm of F-implanted samples up to 1.8 dB/cm.

The cationic charge increase on the tetrahedral garnet site shifts the ytterbium level energy towards higher values.

Crystallographic information on the cationic interactions that may broaden the Yb³⁺ optical bands in garnets for sustaining shorter femtosecond laser pulses is provided. It is shown that the energy of the Yb^{3+ 2}F_7/2(0)↔²F_5/2(0´)  optical absorption (0↔0´ OA) measured at a cryogenic temperature (T= 6 K) in Ca₃(NbGa□)₅O₁₂ (CNGG) type disordered single crystal garnets is sensitive to the electric charge of cations incorporated in the 24d tetrahedral site while very little sensitive to the substitutions made on the 16a octahedral one.

Li⁺ and Ge⁴⁺ fill exclusively tetrahedral garnet site with a strong reduction of the cationic vacancy density monitored in the Li case by positron annihilation lifetime spectroscopy (PALS). Ti⁴⁺ sits exclusively in the octahedral garnet site with no effect on the crystal cationic vacancy density, while Mg²⁺ is distributed in the three garnet sites with preference for the tetrahedral one and a moderate reduction of the crystal vacancy density.

Yb³⁺ 0↔0´ OA bands observed at λ = 973 nm, 971.9 nm, 971.3 nm, 971.1 nm and 969.6 nm are correlated with the presence in the nearest to Yb³⁺ tetrahedral garnet sites of a vacancy (electric charge Q=0), Li⁺, Mg²⁺, Ga³⁺ and Ge⁴⁺, respectively. However, the incorporation of Ti⁴⁺ in the octahedral site substituting mainly Ga³⁺ has not observable effect on the Yb³⁺ 0↔0´ OA.

CDs derived from eco-friendly precursors (xylose and biomass-derived hemicellulose) are combined with GdVO₄:Tb,Eu nanoparticles via two distinct approaches: (i) hydrothermal deposition onto the lanthanide-doped particles and (ii) physical mixing of prefabricated components. Notably, the spectroscopic properties of the resulting composites depend on the fabrication route, see our work published in Adv. Mater. Interfaces 2025, e00410, doi: 10.1002/admi.202500410. While method (i) enables competitive energy transfers from the vanadate charge transfer band (CTB) to Eu³⁺ and CDs, method (ii), which involves lower concentrations of the emissive components, promotes a cooperative mechanism wherein CDs sensitize the CTB, enhancing the Eu³⁺ red emission. In both scenarios, Tb³⁺ is believed to serve as an intermediary, aiding the repopulation of the Eu^{3+ 5}D_j energy levels. The dual-emissive nature of the composites results in violet-magenta chromaticity, reflecting intermediate behavior between the blue CDs and red-emitting Ln-NPs, and supporting their use in tunable optical applications. The described mechanisms also influence the composites’ performance as ratiometric nanothermometers. Upon evaluating their thermal response from 298 K to 358 K, distinct behaviors emerge, with relative thermal sensitivities ranging from 0.84 % K⁻¹ (298 K) to 5.6 % K⁻¹ (358 K)—the latter being among the highest reported for similar material

 

This study shows the potential improvement of current biomedical imaging techniques on the basis of the combined information provided by magnetic resonance imaging (MRI) contrast and high-penetration near infrared (NIR) imaging from a single nanoprobe. For this, our work in J. Mater. Chem. B, 2025, 13, 9642 , doi:10.1039/d5tb00548e, presents the design of core-multishell lanthanide-doped nanostructures with a unique quasi-bidimensional morphology, and the demonstration of their behaviour as biosafe multifunctional nanoprobes integrating dual MRI contrast and NIR imaging, as well as high sensitivity contactless temperature sensing adequate for biological systems. Key advances of this study are the enhancement of the T1 MRI contrast, associated with the large surface-to-volume ratio inherent to the distinctive planar shape of developed nanoprobes, the versatility of these nanoprobes as T1, T2 and dual T1/T2 MRI contrast agents depending on the magnetic field, and the high sensitivity for temperature sensing in the second biological window, surpassing that of commonly used fluoride nanoprobes.

The REALM Groups contributes to the World Conference on Carbon -CARBON 2025-, Saint-Malo, France, 29th June-4th July,  with the communication Preparation and characterization of GdVO4:Tb,Eu/carbon dot composites through hydrothermal deposition synthesis, J. J. Carvajal, R. Sisó-Moliné, C. Cascales, C. Zaldo, M. Méndez, M. C. Pujol.

This work was possible through the Grant PID2021-128090OB-C21 funded by MICIN/AEI/10.13039/501100011033 and by ‘ERDF A way of making Europe’.

REALM group contributes to the 4th Conference on Properties, Design, and Applications of Upconversion Nanomaterials -UPCON24 Conference-, Montréal, Canada, with the communication «Surface functionalization of upconverting nanoparticles for their transfer in aqueous media» . https://upcon.community/upcon2024/.

This work was possible through the Grant PID2021-128090OB-C21 funded by MICIN/AEI/10.13039/501100011033 and by ‘ERDF A way of making Europe’.

REALM group participates in the 4th Conference on Properties, Design, and Applications of Upconversion Nanomaterials -UPCON24 Conference-, Montréal, Canada, with the oral presentation “Upconverting nanoparticles in aqueous media: Not an end road. Avoiding degradation by using hydrophobic polymeric shells”. https://upcon.community/upcon2024/.

The work was possible through the Grant PID2021-128090OB-C21, funded by MICIN/AEI/10.13039/501100011033 and by ‘ERDF A way of making Europe’.

REALM group participates in the 6^th International Conference on Rare Earth Materials, Vilnius, Lithuania, with the invited presentation “Preparation and characterization of GdVO₄:Tb,Eu/carbon dot composites through hydrothermal deposition synthesis”. https://www.remat2024.chgf.vu.lt/.

This work was possible through the Grant PID2021-128090OB-C21, funded by MICIN/AEI/10.13039/501100011033 and by ‘ERDF A way of making Europe’.

Modelocked femtosecond lasers require of luminescent ions with large emission bandwidth and tunability. This requirement is difficult to fulfill with lanthanides due to the inner nature of the 4f orbital. The concept of «disordered single crystals» which  is based on multiple (isovalent or heterovalent) cationic substitutions on the same lattice site has gained attention to solve the problem but results are still limited. In our recent publication in Journal of Alloys and Compounds 961 (2023) 171092 (doi 10.1016/j.jallcom.2023.171092) we show that the random orientation of the Bi3+ non-bonded 6s2 lone pair induces a large band broadening of the Yb3+ emission in monoclinic KY1-x-yBixYby(WO4)2 single crystals. The  growth of these crystals and their laser capabilities are demonstrated within the activity of our THINLAS project.