Research ArticleThermal stability and photoluminescence properties of RE-doped (RE = Ho, Er, Tm) alumina nanoparticles in bulk and fiber-optic silica glass
Introduction
Fiber lasers and amplifiers represent a great success of modern photonics that has rapidly found applications in telecommunications, metrology, material processing technology, etc. [1]. Rare-earth (RE) doped glass is one of the most frequently used materials in the optical fiber lasers and amplifiers. Namely, Er3+, Ho3+, and Tm3+ ions are amongst the most perspective luminophores with various applications in photonics thanks to their emission in the 3rd telecommunication window at 1.5 μm in the case of Er3+ ions or in the “eye-safe” spectral region around 2 μm in the case of Ho3+ and Tm3+ ions [[2], [3], [4]]. The output power of recent fiber lasers has achieved several hundreds of kilowatts, reaching the limits determined by the material durability [5]. Achieving such power places high demands on the lasing efficiency and material properties of the applied materials. To improve the lasing efficiency, intensive material research has been focused on novel low-phonon energy glass with improved solubility of RE ions, such as germanate, chalcogenide or fluoride glass [[6], [7], [8]]. However, the glass transition temperature of these glass systems usually does not exceed 500 °C; their thermal stability and chemical resistance are quite low, limiting their application in high-power devices [9]. Therefore, alumina-doped silica glass remains one of the most perspective materials for the construction of high-power fiber lasers thanks to the unmatched thermal and mechanical properties of silica glass. The incorporation of alumina into silica glass breaks up the silica network, increasing the solubility limits of RE ions and forming a beneficial low-phonon environment suppressing the multiphonon relaxation [10,11].
Nanoparticle doping is a common way to incorporate alumina into the silica optical fibers [[11], [12], [13], [14], [15]]. The fiber core formation can be considered as a solid-state reaction of the Al2O3 nanoparticles and aggregated SiO2 soot. The structural and chemical properties in the Al2O3-SiO2 phase system are well investigated because this system provides a set of traditional ceramics with high industrial impact. The stoichiometric 3Al2O3·2SiO2 composition results in the formation of pure mullite phase [16], which has been recently shown as a promising RE-doped phosphor [17,18]. The low symmetry of the mullite crystal lattice and the presence of the oxygen vacancies can explain the increased solubility of RE ions up to 5 mol. % in terms of the RE3+/Al ratio, with no observation of concentration quenching. In the excess of SiO2, the solid-state reaction between Al2O3 and SiO2 matrix may produce a mixture of mullite and amorphous or crystalline silica, or lead to a complete break-up of the nanoparticles, and the incorporation of RE ions, the structural arrangements in their vicinity and the dependence of photoluminescence properties on these factors become questioned [[19], [20], [21], [22]].
Moreover, the extreme thermal and mechanical conditions during the optical fiber processing and drawing may cause a significant deviation in the local arrangement of RE ions in optical fibers and their photoluminescence properties compared to bulk samples, despite the fundamental solubility limits remaining the same [12,20]. It has been previously observed that photoluminescence lifetimes of RE ions in optical fibers are significantly shorter than values reported for bulk preform samples [23]. Therefore, the clarification of the incorporation of RE ions into optical fiber matrix remains one of the most pressing challenges of fiber-optic material research. The knowledge of the chemical and structural relations in the optical fiber matrix is necessary for a precise tailoring of photoluminescence properties and further progress in the field of high-power fiber lasers.
In this paper, we demonstrate the thermal stability and the photoluminescence properties of RE-doped (RE = Ho, Er, Tm) alumina nanoparticles in the phase system Al2O3-SiO2 with respect to the chemical composition and thermal processing conditions applied in the fiber-optic technology. We studied the photoluminescence properties of RE-doped Al2O3-SiO2 system across a wide range of Al2O3 concentrations, processing temperatures and cooling rates. We use the obtained data to evaluate the local arrangement of RE ions in the final bulk material and we compare the structural properties of bulk samples with the matrix structure occurring in optical fibers. The results bring a fundamental information about the thermal stability and photoluminescence properties of RE-doped alumina nanoparticles in the silica matrix, and they can be used to improve the photoluminescence efficiency of active optical fibers for high-power applications.
Section snippets
Preparation of the samples
To simulate the conditions occurring during the optical fiber processing, a set of samples was prepared by the solid-state reaction of alumina nanoparticles with amorphous silica soot. The stoichiometric ratio 3Al2O3·2SiO2 was chosen as a boundary composition that should result in the formation of pure mullite phase. Furthermore, the silica soot was added in excess to regularly reduce the concentration of Al2O3 to the values of 50, 40, 30, 20, 10 and 5 mol. %. The samples were co-doped by Ho3+
XRD structural analysis
In bulk ceramics, the Al2O3 nanoparticles and silica are converted into mullite for the stoichiometric ratio of Al2O3:SiO2 equal to 3:2 [16]. To confirm the chemical processes and the solid-state reactions of the nanoparticles in the system of a stoichiometric composition 3Al2O3·2SiO2, we studied the thermal stability of Al2O3 in the silica soot using the XRD. Regardless of the kind of RE, all samples exhibited very similar thermal behavior. The diffraction patterns demonstrating the
Conclusions
We have demonstrated the thermal stability and the photoluminescence properties of RE-doped alumina nanoparticles (RE = Ho, Er, Tm) in the Al2O3-SiO2 phase system with respect to the chemical composition and the thermal processing conditions applied in the fiber-optic technology. The alumina nanoparticles and silica soot reacted together to form mullite at the concentration of Al2O3 higher than 5 mol. % regardless of the cooling rate. For the concentration of approx. 5 mol. % Al2O3, amorphous
CRediT authorship contribution statement
P. Vařák: Investigation, Formal analysis, Methodology, Data curation, Writing – original draft. J. Mrázek: Conceptualization, Investigation, Methodology, Supervision, Writing – review & editing. A.A. Jasim: Investigation. S. Bysakh: Investigation. A. Dhar: Investigation. M. Kamrádek: Investigation. O. Podrazký: Investigation. I. Kašík: Conceptualization, Resources, Funding acquisition. I. Bartoň: Investigation. P. Nekvindová: Supervision, Investigation.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This work was supported by the Czech Science Foundation (GACR), project number GAP19−03141S.
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