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LMA (Large Mode Area) optical fibers are presently under active investigation to explore their potential for generating laser action or broadband emission directly within the optical fiber structure. Additionally, a wide mode profile significantly reduces the power distribution density in the fiber cross-section, minimizing the power density, photodegradation, or thermal damage. Multi-stage deposition in the MCVD-CDT system was used to obtain the structural doping profile of the LMA fiber multi-ring core doped with Tm 3+ and Tm 3+ /Ho 3+ layer profiles. The low alumina content (Al 2 O 3 : 0.03wt%) results in low refractive index modification. The maximum concentrations of the lanthanide oxides were Tm 2 O 3 : 0.18wt % and Ho 2 O 3 : 0.15wt%. The double-clad construction of optical fiber with emission spectra in the eye-safe spectral range of (1.55-2.10 µm). The calculated LP 01 Mode Field Diameter (MFD) was 69.7 µm (@ 2000 nm, and 1/e of maximum intensity), which confirms LMA fundamental mode guiding conditions. The FWHM and λmax vs. fiber length are presented and analyzed as a luminescence profile modification. The proposed structured optical fiber with a ring core can be used in new broadband optical radiation source designs.

The near-infrared emission in fabricated low-phonon energy, gallo-germanate glass, and double-core optical fiber has been investigated. Broadband amplified spontaneous emission (ASE) was obtained in optical fiber with cores doped with: 1 st - 0.2Er 2 O 3 and 2 nd - 0.5Yb 2 O 3 /0.4Tm 2 O 3 /0.05Ho 2 O 3 as a result of the superposition of emission bands from both cores corresponding to the Er 3+ : 4 I 13/2 → 4 I 15/2 (1 st core) and Tm 3+ : 3 F 4 → 3 H 6 /Ho 3+ : 5 I 7 → 5 I 8 (2 nd core) transitions. The effect of fiber length and pump wavelength on the near-infrared amplified spontaneous emission (ASE) properties has been analyzed for 1 m and 5 m optical fiber. The widest emission bandwidth (355 nm - 3 dB level) was obtained for a 5 m length optical fiber pumped by a 940 nm laser.

Double-clad optical fiber with a multi-ring core profile doped with thulium and holmium fabricated by Modified Chemical Vapor Deposition Chelate Doping Technology (MCVD-CDT) is presented. The measured Tm 2 O 3 and Ho 2 O 3 complexes' weight concentrations were 0.5% and 0.2% respectively. Numerical analyses show weakly guiding conditions and 42.2 µm of MFD LP 01 at 2000 nm. The low NA numerical aperture (NA = 0.054) was obtained for the 20/250 µm core/cladding ratio optical fiber construction. The emission spectra in the range of 1.6-2.1 µm vs. the fiber length are presented. The full width at half maximum (FWHM) decreases from 318 to 270 nm for fiber lengths from 2 to 10 m. The presented fiber design is of interest for the development of new construction of optical fibers operating in the eye-safe spectral range., (© 2023. Springer Nature Limited.)

In the current work, germanate phosphors Li 2 MgGeO 4 :Ln 3+ (Ln = Pr, Tm) have been synthesized and then investigated using luminescence spectroscopy. The X-ray diffraction analysis demonstrate that ceramic compounds Li 2 MgGeO 4 containing Pr 3+ and Tm 3+ ions crystallize in a monoclinic crystal lattice. Luminescence properties of Pr 3+ and Tm 3+ ions have been examined under different excitation wavelengths. The most intense blue emission band related to the 1 D 2 → 3 F 4 transition of Tm 3+ is overlaps well with broad band located near 500 nm, which is assigned to F-type centers. These effects are not evident for Pr 3+ ions. Ceramic phosphors Li 2 MgGeO 4 :Ln 3+ (Ln = Pr, Tm) are characterized based on measurements of the excitation/emission spectra and their decays. The experimental results indicate that germanate ceramics Li 2 MgGeO 4 doped with trivalent rare earth ions can be applied as inorganic phosphors emitting orange (Pr 3+ ) or blue (Tm 3+ ) light., Competing Interests: There are no conflicts to declare., (This journal is © The Royal Society of Chemistry.)

An investigation of fluoroindate glass and fiber co-doped with Yb 3+ /Er 3+ ions as a potential temperature sensor was assessed using the fluorescence intensity ratio (FIR) technique. Analysis of thermally coupled levels (TCLs- 2 H 11/2 and 4 S 3/2 ), non-thermally coupled levels (non-TCLs- 4 F 7/2 and 4 F 9/2 ), and their combination were examined. Additionally, the luminescent stability of the samples under constant NIR excitation using different density power at three different temperatures was carried out. The obtained values of absolute sensitivity (0.003 K -1 -glass, 0.0019 K -1 -glass fiber 2 H 11/2 → 4 S 3/2 transition) and relative sensitivity (2.05% K -1 -glass, 1.64% K -1 -glass fiber 4 F 7/2 → 4 F 9/2 transition), as well as high repeatability of the signal, indicate that this material could be used in temperature sensing applications.

Nanophosphors are widely used, especially in biological applications in the first and second biological windows. Currently, nanophosphors doped with lanthanide ions (Ln 3+ ) are attracting much attention. However, doping the matrix with lanthanide ions is associated with a narrow luminescence bandwidth. This paper describes the structural and luminescence properties of co-doped LaPO 4 nanophosphors, fabricated by the co-precipitation method. X-ray structural analysis, scanning electron microscope measurements with EDS analysis, and luminescence measurements (excitation 395 nm) of LaPO 4 :Eu 3+ /Nd 3+ and LaPO 4 :Eu 3+ /Nd 3+ /Yb 3+ nanophosphors were made and energy transfer between rare-earth ions was investigated. Tests performed confirmed the crystal structure of the produced phosphors and deposition of rare-earth ions in the structure of LaPO 4 nanocrystals. In the range of the first biological window (650-950 nm), strong luminescence bands at the wavelengths of 687 nm and 698 nm ( 5 D 0 → 7 F 4 :Eu 3+ ) and 867 nm, 873 nm, 889 nm, 896 nm, and 907 nm ( 4 F 3/2 → 4 I 9/2 :Nd 3+ ) were observed. At 980 nm, 991 nm, 1033 nm ( 2 F 5/2 → 2 F 7/2 :Yb 3+ ) and 1048 nm, 1060 nm, 1073 nm, and 1080 nm ( 4 F 3/2 → 4 I 9/2 :Nd 3+ ), strong bands of luminescence were visible in the 950 nm-1100 nm range, demonstrating that energy transfer took place.

Modified barium gallo-germanate glass hosts are still worthy of attention in studying structure-property relationships. In this work, two different series of glass systems based on (60-x)GeO 2 -xTiO 2 -30BaO-10Ga 2 O 3 and (60-x)GeO 2 -xB 2 O 3 -30BaO-10Ga 2 O 3 (x = 10, 30, 50 mol%) were synthesized, and their properties were studied using spectroscopic techniques. X-ray diffraction (XRD) patterns revealed that all fabricated glasses were fully amorphous material. The absorption edge shifted toward the longer wavelengths with a gradual substitution of GeO 2 . The spectroscopic assignments of titanium ions were performed with excitation and emission spectra compared to the additional sample containing an extremely low content of TiO 2 (0.005 mol%). On the basis of Raman and FT-IR investigations, it was found that increasing the TiO 2 content caused a destructive effect on the GeO 4 and GeO 6 structural units. The Raman spectra of a sample containing a predominantly TiO 2 (50 mol%) proved that the band was located near 650 cm -1 , which corresponded to the stretching vibration of Ti-O in TiO 6 unit. The deconvoluted IR results showed that the germanate glass network consisted of the coexistence of two BO 3 and BO 4 structural groups. Based on the experimental investigations, we concluded that the developed materials are a promising candidate for use as novel glass host matrices for doping rare-earth and/or transition metal ions.

In this paper, the effect of the GeO 2 :TiO 2 molar ratio in glass composition on the spectroscopic properties of germanate glasses was systematically investigated. The visible luminescence bands associated with characteristic 1 D 2 → 3 H 4 (red), 5 S 2 , 5 F 4 → 5 I 8 (green), and 1 D 2 → 3 F 4 (blue) transitions of Pr 3+ , Ho 3+ , and Tm 3+ ions in systems modified by TiO 2 were well observed, respectively. It was found that the luminescence intensity of glasses containing Pr 3+ and Ho 3+ ions increases, whereas, for Tm 3+ -doped systems, luminescence quenching with increasing content of TiO 2 was observed. Based on Commission Internationale de I'Eclairage (CIE) chromaticity coordinates (x, y) analysis, it was demonstrated that the value of chromaticity coordinates for all glasses depends on the GeO 2 :TiO 2 molar ratio. The addition of TiO 2 to system compositions doped with Tm 3+ ions shifts the (x, y) to the center of the CIE diagram. However, chromaticity coordinates evaluated for glasses containing Pr 3+ ions move to a purer red color. Our results confirm that the spectroscopic properties of the studied glasses strongly depend on TiO 2 content. Moreover, it can be stated that germanate-based glass systems modified by TiO 2 can be used for optoelectronics in RGB technology as red (Pr 3+ ), green (Ho 3+ ), and blue (Tm 3+ ) emitters.

In this work, the synthesis and characterization of Li 2 MgGeO 4 :Ho 3+ ceramics were reported. The X-ray diffraction measurements revealed that the studied ceramics belong to the monoclinic Li 2 MgGeO 4 . Luminescence properties were analyzed in the visible spectral range. Green and red emission bands correspondent to the 5 F 4 , 5 S 2 → 5 I 8 and 5 F 5 → 5 I 8 transitions of Ho 3+ were observed, and their intensities were significantly dependent on activator concentration. Luminescence spectra were also measured under direct excitation of holmium ions or ceramic matrix. Holmium ions were inserted in crystal lattice Li 2 MgGeO 4 , giving broad blue emission and characteristic 4f-4f luminescent transitions of rare earths under the selective excitation of the ceramic matrix. The presence of the energy transfer process between the host lattice and Ho 3+ ions was suggested.

The effect of BaF 2 , MgF 2 , and AlF 3 on the structural and luminescent properties of gallo-germanate glass (BGG) doped with erbium ions was investigated. A detailed analysis of infrared and Raman spectra shows that the local environment of erbium ions in the glass was influenced mainly by [GeO] 4 and [GeO] 6 units. Moreover, the highest number of non-bridging oxygens was found in the network of the BGG glass modified by MgF 2 . The 27 Al MAS NMR spectrum of BGG glass with AlF 3 suggests the presence of aluminum in tetra-, penta-, and octahedral coordination geometry. Therefore, the probability of the 4 I 13/2 → 4 I 15/2 transition of Er 3+ ions increases in the BGG + MgF 2 glass system. On the other hand, the luminescence spectra showed that the fluoride modifiers lead to an enhancement in the emission of each analyzed transition when different excitation sources are employed (808 nm and 980 nm). The analysis of energy transfer mechanisms shows that the fluoride compounds promote the emission intensity in different channels. These results represent a strong base for designing glasses with unique luminescent properties.

Up-conversion nanoparticles have garnered lots of attention due to their ability to transform low energy light (near-infrared) into high-energy (visible) light, enabling their potential use as remote visible light nano-transducers. However, their low efficiency restricts their full potential. To overcome this disadvantage, fluoroindate glasses (InF 3 ) doped at different molar concentrations of Yb 3+ and Er 3+ were obtained using the melting-quenching technique, reaching the highest green emission at 1.4Yb and 1.75Er (mol%), which corresponds to the 4 S 3/2 → 4 I 15/2 (540-552 nm) transition. The particles possess the amorphous nature of the glass and have a high thermostability, as corroborated by thermogravimetric assay. Furthermore, the spectral decay curve analysis showed efficient energy transfer as the rare-earth ions varied. This was corroborated with the absolute quantum yield (QY) obtained (85%) upon excitation at 385 nm with QYEr = 17% and QYYb = 68%. Additionally, InF 3 -1.4Yb-1.75Er was milled and functionalized using poly(ethylene glycol) to impart biocompatibility, which is essential for biomedical applications. Such functionalization was verified using FTIR, TG/DSC, and XRD., Competing Interests: There are no conflicts to declare., (This journal is © The Royal Society of Chemistry.)

Glass-ceramic is semi-novel material with many applications, but it is still problematic in obtaining fibers. This paper aims to develop a new glass-ceramic material that is a compromise between crystallization, thermal stability, and optical properties required for optical fiber technology. This compromise is made possible by an alternative method with a controlled crystallization process and a suitable choice of the chemical composition of the core material. In this way, the annealing process is eliminated, and the core material adopts a glass-ceramic character with high transparency directly in the drawing process. In the experiment, low phonon antimony-germanate-silicate glass (SGS) doped with Eu 3+ ions and different concentrations of P 2 O 5 were fabricated. The glass material crystallized during the cooling process under conditions similar to the drawing processes'. Thermal stability (DSC), X-ray photo analysis (XRD), and spectroscopic were measured. Eu 3+ ions were used as spectral probes to determine the effect of P 2 O 5 on the asymmetry ratio for the selected transitions ( 5 D 0 → 7 F 1 and 5 D 0 → 7 F 2 ). From the measurements, it was observed that the material produced exhibited amorphous or glass-ceramic properties, strongly dependent on the nucleator concentration. In addition, the conducted study confirmed that europium ions co-form the EuPO 4 structure during the cooling process from 730 °C to room temperature. Moreover, the asymmetry ratio was changed from over 4 to under 1. The result obtained confirms that the developed material has properties typical of transparent glass-ceramic while maintaining high thermal stability, which will enable the fabrication of fibers with the glass-ceramic core.

Inorganic glasses co-doped with rare-earth ions have a key potential application value in the field of optical communications. In this paper, we have fabricated and then characterized multicomponent TiO 2 -modified germanate glasses co-doped with Yb 3+ /Ln 3+ (Ln = Pr, Er, Tm, Ho) with excellent spectroscopic properties. Glass systems were directly excited at 980 nm (the 2 F 7/2 → 2 F 5/2 transition of Yb 3+ ). We demonstrated that the introduction of TiO 2 is a promising option to significantly enhance the main near-infrared luminescence bands located at the optical telecommunication window at 1.3 μm (Pr 3+ : 1 G 4 → 3 H 5 ), 1.5 μm (Er 3+ : 4 I 13/2 → 4 I 15/2 ), 1.8 μm (Tm 3+ : 3 F 4 → 3 H 6 ) and 2.0 μm (Ho 3+ : 5 I 7 → 7 I 8 ). Based on the lifetime values, the energy transfer efficiencies (η ET ) were estimated. The values of η ET are changed from 31% for Yb 3+ /Ho 3+ glass to nearly 53% for Yb 3+ /Pr 3+ glass. The investigations show that obtained titanate-germanate glass is an interesting type of special glasses integrating luminescence properties and spectroscopic parameters, which may be a promising candidate for application in laser sources emitting radiation and broadband tunable amplifiers operating in the near-infrared range.

Spectral properties of lead-free glasses doped with Dy 3+ ions in function of glass formers and glass modifiers were studied. The glass systems in a function of concentration GeO 2 and CaO/SrO/BaO which was partially or totally replaced by CaF 2 /SrF 2 /BaF 2 were synthesized. The visible luminescence spectra for obtained glasses were registered and value of parameters such as ratio of yellow-to-blue luminescence and the Commission Internationale de I'Eclairage (CIE) chromaticity coordinates (x, y) were analyzed in detail. The Y/B increase from 2.97 to 3.8 in systems with increasing of concentration glass former (GeO 2 ), whereas ratio Y/B slightly decreases when the of content fluoride glass modifiers increases. The obtained results confirm that impact on visible luminescence of lead-free borate glasses doped with Dy 3+ ions is greater for glass former than glass modifier. Moreover, studied systems can be used for optoelectronic as yellowish emitters., Competing Interests: 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., (Copyright © 2021 Elsevier B.V. All rights reserved.)

The results presented in this communication concern visible and near-IR emission of Pr 3+ ions in selected inorganic glasses, i.e., borate-based glass with Ga 2 O 3 and BaO, lead-phosphate glass with Ga 2 O 3 , gallo-germanate glass modified by BaO/BaF 2 , and multicomponent fluoride glass based on InF 3 . Glasses present several emission bands at blue, reddish orange, and near-infrared spectral ranges, which correspond to 4f-4f electronic transitions of Pr 3+ . The profiles of emission bands and their relative intensity ratios depend strongly on glass-host. Visible emission of Pr 3+ ions is tuned from red/orange for borate-based glass to nearly white light for multicomponent fluoride glass based on InF 3 . The positions and spectral linewidths for near-infrared luminescence bands at the optical telecommunication window corresponding to the 1 G 4 → 3 H 5 , 1 D 2 → 1 G 4 , and 3 H 4 → 3 F 3 , 3 F 4 transitions of Pr 3+ are dependent on glass-host matrices and excitation wavelengths. Low-phonon fluoride glasses based on InF 3 and gallo-germanate glasses with BaO/BaF 2 are excellent candidates for broadband near-infrared optical amplifiers. Spectroscopic properties of Pr 3+ -doped glasses are compared and discussed in relation to potential optical applications.

This study presented an analysis of the TeO 2 /GeO 2 molar ratio in an oxide glass system. A family of melt-quenched glasses with the range of 0-35 mol% of GeO 2 has been characterized by using DSC, Raman, MIR, refractive index, PLE, PL spectra, and time-resolved spectral measurements. The increase in the content of germanium oxide caused an increase in the transition temperature but a decrease in the refractive index. The photoluminescence spectra of europium ions were examined under the excitation of 465 nm, corresponding to 7 F 0 → 5 D 2 transition. The PSB (phonon sidebands) analysis was carried out to determine the phonon energy of the glass hosts. It was reported that the red ( 5 D 0 → 7 F 2 ) to orange ( 5 D 0 → 7 F 1 ) fluorescence intensity ratio for Eu 3+ ions decreased from 4.49 (Te0Ge) to 3.33 (Te15Ge) and showed a constant increase from 4.58 (Te20Ge) to 4.88 (Te35Ge). These optical features were explained in structural studies, especially changes in the coordination of [4] Ge to [6] Ge. The most extended lifetime was reported for the Eu 3+ doped glass with the highest content of GeO 2 . This glass was successfully used for the drawing of optical fiber.

Glasses containing two different network-forming components and doped with optically active ions exhibit interesting properties. In this work, glass systems based on germanium dioxide and boron trioxide singly doped with lanthanides (Eu 3+ ) and transition metals (Cr 3+ ) ions are research subjects. Optical spectroscopy was the major research tool used to record excitation and emission spectra in a wide spectral range for studied systems. The emitted radiation of glasses doped with Cr 3+ ions is dominated by broadband luminescence centered at 770 nm and 1050 nm ( 4 T 2 → 4 A 2 ). Interestingly, the increase of concentration of one of the oxides contributed to the detectable changes of the R-line ( 2 E → 4 A 2 ) of Cr 3+ ions. Moreover, EPR spectroscopy confirmed the paramagnetic properties of the obtained glasses. The influence of molar ratio GeO 2 :B 2 O 3 on spectroscopic properties for Eu 3+ ions is discussed. The intensity of luminescence bands due to transitions of trivalent europium ions as well as the ratio R/O decrease with the increase of B 2 O 3 . On the other hand, the increase in concentration B 2 O 3 influences the increasing tendency of luminescence lifetimes for the 5 D 0 state of Eu 3+ ions. The results will contribute to a better understanding of the role of the glass host and thus the prospects for new optical materials.

An investigation of the structural and luminescent properties of the transparent germanate glass-ceramics co-doped with Ni 2+ /Er 3+ for near-infrared optical fiber applications was presented. Modification of germanate glasses with 10-20 ZnO (mol.%) was focused to propose the additional heat treatment process controlled at 650 °C to obtain transparent glass-ceramics. The formation of 11 nm ZnGa 2 O 4 nanocrystals was confirmed by the X-ray diffraction (XRD) method. It followed the glass network changes analyzed in detail (MIR-Mid Infrared spectroscopy) with an increasing heating time of precursor glass. The broadband 1000-1650 nm luminescence (λ exc = 808 nm) was obtained as a result of Ni 2+ : 3 T 2 ( 3 F) → 3 A 2 ( 3 F) octahedral Ni 2+ ions and Er 3+ : 4 I 13/2 → 4 I 15/2 radiative transitions and energy transfer from Ni 2+ to Er 3+ with the efficiency of 19%. Elaborated glass-nanocrystalline material is a very promising candidate for use as a core of broadband luminescence optical fibers.

This work reports on the fabrication and analysis of near-infrared and mid-infrared luminescence spectra and their decays in fluoroindate glasses co-doped with Yb 3+ /Ho 3+ . The attention has been paid to the analysis of the Yb 3+ → Ho 3+ energy transfer processed ions in fluoroindate glasses pumped by 976 nm laser diode. The most effective sensitization for 2 μm luminescence has been obtained in glass co-doped with 0.8YbF 3 /1.6HoF 3 . Further study in the mid-infrared spectral range (2.85 μm) showed that the maximum emission intensity has been obtained in fluoroindate glass co-doped with 0.1YbF 3 /1.4HoF 3 . The obtained efficiency of Yb 3+ → Ho 3+ energy transfer was calculated to be up to 61% (0.8YbF 3 /1.6HoF 3 ), which confirms the possibility of obtaining an efficient glass or glass fiber infrared source for a MID-infrared (MID-IR) sensing application.

Fluoroindate glasses co-doped with Pr 3+ /Er 3+ ions were synthesized and their near-infrared luminescence properties have been examined under selective excitation wavelengths. For the Pr 3+ /Er 3+ co-doped glass samples several radiative and nonradiative relaxation channels and their mechanisms are proposed under direct excitation of Pr 3+ and/or Er 3+ . The energy transfer processes between Pr 3+ and Er 3+ ions in fluoroindate glasses were identified. In particular, broadband near-infrared luminescence (FWHM = 278 nm) associated to the 1 G 4  →  3 H 5  (Pr 3+ ), 1 D 2  →  1 G 4  (Pr 3+ ) and 4 I 13/2  →  4 I 15/2  (Er 3+ ) transitions of rare earth ions in fluoroindate glass is successfully observed under direct excitation at 483 nm. Near-infrared luminescence spectra and their decays for glass samples co-doped with Pr 3+ /Er 3+ are compared to the experimental results obtained for fluoroindate glasses singly doped with rare earth ions.

In this paper, some series of lead borate glasses and glass ceramics singly doped with Dy 3+ ions were prepared and then studied using spectroscopic techniques. Our research includes mainly studies of the luminescence properties of received materials for white light. The luminescence bands associated with the characteristic 4 F 9/2 → 6 H 15/2 (blue), 4 F 9/2 → 6 H 13/2 (yellow) and 4 F 9/2 → 6 H 11/2 (red) transitions of trivalent dysprosium in lead borate systems are well observed. In particular, the Commission Internationale de I'Eclairage (CIE) chromaticity coordinates (x, y) were calculated in relation to potential applications for white light-emitting diodes (W-LEDs). Their values depend on the relative B 2 O 3 /PbO ratios and PbX 2 contents (where X = Cl, F, Br) in glass composition. For glass-ceramics, the chromaticity coordinates are changed significantly under different excitation wavelengths.

The effect of oxide modifiers on multiple properties (structural and spectroscopic) of phosphate glasses with molar composition 60P 2 O 5 -(10-x)Ga 2 O 3 -30MO-xEu 2 O 3 and 60P 2 O 5 -(10-y)Ga 2 O 3 -30MO-yEr 2 O 3 (where M = Ca, Sr, Ba; x = 0, 0.5; y = 0, 1) were systematically examined and discussed. The local structure of systems was evidenced by the infrared (IR-ATR) and Raman spectroscopic techniques. The spectroscopic behaviors of the studied glass systems were determined based on analysis of recorded spectra (excitation and emission) as well as luminescence decay curves. Intense red and near-infrared emissions (1.5 μm) were observed for samples doped with Eu 3+ and Er 3+ ions, respectively. It was found that the value of fluorescence intensity ratio R/O related to 5 D 0 → 7 F 2 (red) and 5 D 0 → 7 F 1 (orange) transition of Eu 3+ ions depends on the oxide modifiers MO in the glass host. However, no clear influence of glass modifiers on the luminescence linewidth (FWHM) was observed for phosphate systems doped with Er 3+ ions. Moreover, the 5 D 0 and 4 I 13/2 luminescence lifetimes of Eu 3+ and Er 3+ ions increase with the increasing ionic radius of M 2+ (M = Ca, Sr, Ba) in the host matrix. The obtained results suggest the applicability of the phosphate glasses with oxide modifiers as potential red and near-infrared photoluminescent materials in photonic devices.

Novel multicomponent titanate-germanate glasses singly doped with transition metal (Cr 3+ ) and rare earth ions (Eu 3+ ) were synthesized and the glass transition temperatures and thermal stability parameters were determined using DSC measurements. X-ray diffraction analysis confirmed fully amorphous nature of the received samples. Their structural and optical properties were compared with germanate glasses without TiO 2 . Correlation between local structure and optical properties in titanate-germanate glasses is well evidenced by FT-IR, Raman, EPR, and luminescence spectroscopy. In particular, luminescence spectra and their decays are examined for glass samples, where GeO 2 was partially substituted by TiO 2 ., Competing Interests: The authors declare no conflict of interest.

Glass and ceramic materials doped with rare earth (RE) ions have gained wide interest in photonics as active materials for lasers, optical amplifiers, and luminescent sensors. The emission properties of RE-doped glasses depend on their chemical composition, but they can also be tailored by modifying the surrounding active ions. Typically, this is achieved through heat treatment (including continuous-wave and pulsed lasers) after establishing the ordering mechanisms in the particular glass-RE system. Within the known systems, silicate glasses predominate, while much less work relates to materials with lower energy phonons, which allow more efficient radiation sources to be constructed for photonic applications. In the present work, the luminescent and structural properties of germanate glasses modified with phosphate oxide doped with Eu 3+ ions were investigated. Europium dopant was used as a "spectroscopic probe" in order to analyze the luminescence spectra, which characterizes the changes in the local site symmetries of Eu 3+ ions. Based on the spectroscopic results, a strong influence of P 2 O 5 content was observed on the excitation and luminescence spectra. The luminescence study of the most intense 5 D 0 → 7 F 2 (electric dipole) transition revealed that the increase in the P 2 O 5 content leads to the linewidth reduction (from 15 nm to 10 nm) and the blue shift (~2 nm) of the emission peak. According to the crystal field theory, the introduction of P 2 O 5 into the glass structure changes the splitting number of sublevels of the 5 D 0 → 7 F 1 (magnetic dipole) transition, confirming the higher polymerization of fabricated glass. The slightly different local environment of Eu 3+ centers the results in a number of sites and causes inhomogeneous broadening of spectral lines. It was found that the local asymmetry ratio estimated by the relation of ( 5 D 0 → 7 F 2 )/( 5 D 0 → 7 F 1 ) transitions also confirms greater changes in local symmetry around Eu 3+ ions. Our results indicate that modification of germanate glass by P 2 O 5 allows control of their structural properties in order to functionalize the emissions for application as luminescent light sources and sensors.

The ErF 3 -doped oxyfluoride phospho-tellurite glasses in the (40-x) TeO 2 -10P 2 O 5 -45 (BaF 2 -ZnF 2 ) -5Na 2 O-xErF 3 system (where x = 0.25, 0.50, 0.75, 1.00, and 1.25 mol%) have been prepared by the conventional melt-quenching method. The effect of erbium trifluoride addition on thermal, structure, and spectroscopic properties of oxyfluoride phospho-tellurite precursor glass was studied by differential scanning calorimetry (DSC), Fourier-transform infrared (FTIR), and Raman spectroscopy as well as emission measurements, respectively. The DSC curves were used to investigate characteristic temperatures and thermal stability of the precursor glass doped with varying content of ErF 3 . FTIR and Raman spectra were introduced to characterize the evolution of structure and phonon energy of the glasses. It was found that the addition of ErF 3 up to 1.25 mol% into the chemical composition of phospho-tellurite precursor glass enhanced 2.7 µm emission and upconversion. By controlled heat-treatment process of the host glass doped with the highest content of erbium trifluoride (1.25 mol%), transparent erbium-doped phospho-tellurite glass-ceramic (GC) was obtained. X-ray diffraction analysis confirmed the presence of BaF 2 nanocrystals with the average 16 nm diameter in a glass matrix. Moreover, MIR, NIR, and UC emissions of the glass-ceramic were discussed in detail and compared to the spectroscopic properties of the glass doped with 1.25 mol% of ErF 3 (the base glass).