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In the framework of luminescent rare-earth-doped glasses for near-infrared applications, TiO 2 -containing inorganic glasses have been recently demonstrated to be a promising alternative to commercially used high-phonon SiO 2 -based glasses. This study investigates the effect of TiO 2 concentration on the near-infrared spectroscopic properties of Yb 3+ ions in multicomponent titanate-germanate glasses. A series of glass samples in the xTiO 2 -(60-x)GeO 2 -BaO-Ga 2 O 3 -Yb 2 O 3 system (x ranging from 0 to 50 mol%) were synthesized using the melt-quenching technique. X-ray diffraction analysis confirmed the fully amorphous nature of the fabricated titanate-germanate samples. Fundamental spectroscopic properties of Yb 3+ -doped titanate-germanate system consisting of absorption spectra, near-IR emission spectra, and luminescence decay curves have been determined based on measurement using optical spectroscopy. The intensity of the emission band at 1 µm due to the 2 F 5/2 → 2 F 7/2 laser transition of Yb 3+ ions increases by over 2.3-fold (TiO 2 as the network former) compared to a barium gallo-germanate sample without TiO 2 . Our previous studies indicate that Yb 3+ -doped titanate-germanate glass is a promising optical material and could be successfully applied to laser technology.

In this work, we report the results of our investigations on the structural and luminescence properties of SiO 2 -LaF 3 :Pr 3+ nano-glass-ceramics synthesized using the sol-gel method. Based on XRD, microscopic (TEM), and ATR-IR measurements, the crystallization of LaF 3 nanocrystals favorably occupied by Pr 3+ ions and overall transformations within the silicate sol-gel hosts dependent on heat-treatment conditions of the as-prepared amorphous xerogels were characterized. The fabricated oxyfluoride nano-glass-ceramics revealed the emissions within the greenish-blue ( 3 P 0,1 → 3 H 4 , 3 P 0,1 → 3 H 5 ), reddish-orange ( 3 P 0,1 → 3 H 6 , 1 D 2 → 3 H 4 , 3 P 0 → 3 F 2,3 ), and NIR spectral scopes ( 1 D 2 → 3 F 4 , 1 G 4 , 1 G 4 → 3 H 5 , 3 F 3,4 → 3 H 4 ). Based on the luminescence spectra in the VIS range, the CIE chromaticity coordinates, correlated color temperatures (CCT), and color purities (CP) were calculated. The obtained results clearly indicate that the prepared Pr 3+ -doped sol-gel nano-glass-ceramics exhibit warm or neutral white light emissions with CCT values in the range from 2567 K to 3962 K. The lowest CP value was estimated at 12.8%, indicating that the fabricated samples are able to emit bright white light. Additionally, the NIR emissions cover E, S, C, and L bands, which are important for devices applicable in telecommunication technologies. For further characterization, the τ ( 3 P 0 ) and τ ( 1 D 2 ) decay times were estimated. It was established that the emissions from the 3 P 0 and the 1 D 2 excited states of Pr 3+ ions, as well as the participation of cross-relaxation (CR) processes, are dependent on the size of crystallized LaF 3 phase, distribution of optically active Pr 3+ ions between amorphous and crystalline phase (determining the Pr 3+ -Pr 3+ inter-ionic distances), and relative content of OH groups in the prepared sol-gel hosts.

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.)

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 series of Dy 3+ -doped silicate xerogels were synthesized by sol-gel technique and further processed at 350 °C into SiO 2 -LaF 3 :Dy 3+ nano-glass-ceramic materials. The X-ray diffraction (XRD) measurements, along with the thermal analysis, indicated that heat-treatment triggered the decomposition of La(TFA) 3 inside amorphous sol-gel hosts, resulting in the formation of hexagonal LaF 3 phase with average crystal size at about ~10 nm. Based on the photoluminescence results, it was proven that the intensities of blue ( 4 F 9/2 → 6 H 15/2 ), yellow ( 4 F 9/2 → 6 H 13/2 ), and red ( 4 F 9/2 → 6 H 11/2 ) emissions, as well as the calculated yellow-to-blue (Y/B) ratios, are dependent on the nature of fabricated materials, and from fixed La 3+ :Dy 3+ molar ratios. For xerogels, the emission was gradually increased, and the τ( 4 F 9/2 ) lifetimes were elongated to 42.7 ± 0.3 μs (La 3+ :Dy 3+ = 0.82:0.18), however, for the sample with the lowest La 3+ :Dy 3+ molar ratio (0.70:0.30), the concentration quenching was observed. For SiO 2 -LaF 3 :Dy 3+ nano-glass-ceramics, the concentration quenching effect was more visible than for xerogels and started from the sample with the highest La 3+ :Dy 3+ molar ratio (0.988:0.012), thus the τ( 4 F 9/2 ) lifetimes became shorter from 1731.5 ± 5.7 up to 119.8 ± 0.4 μs. The optical results suggest, along with an interpretation of XRD data, that Dy 3+ ions were partially entered inside LaF 3 phase, resulting in the shortening of Dy 3+ -Dy 3+ inter-ionic distances.

Borogermanate glasses singly doped with Dy 3+ ions were synthesized and then studied using the absorption and luminescence spectra. Spectroscopic changes of Dy 3+ ions have been examined for compositional-dependent glasses with various molar ratios GeO 2 :B 2 O 3 . In this work, several spectroscopic parameters of Dy 3+ ions were obtained experimentally and compared to the calculated values from the Judd-Ofelt theory. Luminescence spectra measured for borogermanate glasses consist of blue, yellow and red bands, which correspond to 4 F 9/2 → 6 H 15/2 , 4 F 9/2 → 6 H 13/2 and 4 F 9/2 → 6 H 11/2 transitions of Dy 3+ , respectively. Luminescence lifetimes for the 4 F 9/2 excited state are reduced, whereas the stimulated emission cross-sections for the most intense 4 F 9/2 → 6 H 13/2 yellow transition of Dy 3+ increase with increasing GeO 2 and decreasing B 2 O 3 concentrations in glass-hosts. Quantum efficiency of the 4 F 9/2 (Dy 3+ ) excited state is nearly independent on molar ratios GeO 2 :B 2 O 3 . Attractive spectroscopic properties related to the 4 F 9/2 → 6 H 13/2 transition of Dy 3+ ions are found for borogermanate glasses implying their potential utility for yellow laser action and solid-state lighting technology.

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.

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.

The synthesis and characterization of multicolor light-emitting nanomaterials based on rare earths (RE 3+ ) are of great importance due to their possible use in optoelectronic devices, such as LEDs or displays. In the present work, oxyfluoride glass-ceramics containing BaF 2 nanocrystals co-doped with Tb 3+ , Eu 3+ ions were fabricated from amorphous xerogels at 350 °C. The analysis of the thermal behavior of fabricated xerogels was performed using TG/DSC measurements (thermogravimetry (TG), differential scanning calorimetry (DSC)). The crystallization of BaF 2 phase at the nanoscale was confirmed by X-ray diffraction (XRD) measurements and transmission electron microscopy (TEM), and the changes in silicate sol-gel host were determined by attenuated total reflectance infrared (ATR-IR) spectroscopy. The luminescent characterization of prepared sol-gel materials was carried out by excitation and emission spectra along with decay analysis from the 5 D 4 level of Tb 3+ . As a result, the visible light according to the electronic transitions of Tb 3+ ( 5 D 4 → 7 F J (J = 6-3)) and Eu 3+ ( 5 D 0 → 7 F J (J = 0-4)) was recorded. It was also observed that co-doping with Eu 3+ caused the shortening in decay times of the 5 D 4 state from 1.11 ms to 0.88 ms (for xerogels) and from 6.56 ms to 4.06 ms (for glass-ceramics). Thus, based on lifetime values, the Tb 3+ /Eu 3+ energy transfer (ET) efficiencies were estimated to be almost 21% for xerogels and 38% for nano-glass-ceramics. Therefore, such materials could be successfully predisposed for laser technologies, spectral converters, and three-dimensional displays.

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 paper deals with broadband near-infrared luminescence properties of lead germanate glass triply doped with Yb 3+ /Er 3+ /Tm 3+ . Samples were excited at 800 nm and 975 nm. Their emission intensities and lifetimes depend significantly on Er 3+ and Tm 3+ concentrations. For samples excited at 800 nm, broadband emissions corresponding to the overlapped 3 H 4 → 3 F 4 (Tm 3+ ) and 4 I 13/2 → 4 I 15/2 (Er 3+ ) transitions centered at 1.45 µm and 1.5 µm was identified. Measurements of decay curves confirm reduction of 3 H 4 (Tm 3+ ), 2 F 5/2 (Yb 3+ ) and 4 I 13/2 (Er 3+ ) luminescence lifetimes and the presence of energy-transfer processes. The maximal spectral bandwidth equal to 269 nm for the 3 F 4 → 3 H 6 transition of Tm 3+ suggests that our glass co-doped with Yb 3+ /Er 3+ /Tm 3+ is a good candidate for broadband near-infrared emission. The energy transfer from 4 I 13/2 (Er 3+ ) to 3 F 4 (Tm 3+ ) and cross-relaxation processes are responsible for the enhancement of broadband luminescence near 1.8 µm attributed to the 3 F 4 → 3 H 6 transition of thulium ions in lead germanate glass under excitation of Yb 3+ ions at 975 nm.

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.

In this work, the series of Tb 3+ /Eu 3+ co-doped xerogels and derivative glass-ceramics containing CaF 2 nanocrystals were prepared and characterized. The in situ formation of fluoride crystals was verified by an X-ray diffraction technique (XRD) and transmission electron microscopy (TEM). The studies of the Tb 3+ /Eu 3+ energy transfer (ET) process were performed based on excitation and emission spectra along with luminescence decay analysis. According to emission spectra recorded under near-ultraviolet (NUV) excitation (351 nm, 7 F 6 → 5 L 9 transition of Tb 3+ ), the mutual coexistence of the 5 D 4 → 7 F J (J = 6-3) (Tb 3+ ) and the 5 D 0 → 7 F J (J = 0-4) (Eu 3+ ) luminescence bands was clearly observed. The co-doping also resulted in gradual shortening of a lifetime from the 5 D 4 state of Tb 3+ ions, and the ET efficiencies were varied from η ET = 11.9% (Tb 3+ :Eu 3+ = 1:0.5) to η ET = 22.9% (Tb 3+ :Eu 3+ = 1:2) for xerogels, and from η ET = 25.7% (Tb 3+ :Eu 3+ = 1:0.5) up to η ET = 67.4% (Tb 3+ :Eu 3+ = 1:2) for glass-ceramics. Performed decay analysis from the 5 D 0 (Eu 3+ ) and the 5 D 4 (Tb 3+ ) state revealed a correlation with the change in Tb 3+ -Eu 3+ and Eu 3+ -Eu 3+ interionic distances resulting from both the variable Tb 3+ :Eu 3+ molar ratio and their partial segregation in CaF 2 nanophase.

In this work, we performed a systematic analysis of the impact of selected chemical reagents used in sol-gel synthesis (i.e., N,N-dimethylformamide) and different catalyst agents (i.e., CH 3 COOH, HNO 3 ) on the formation and luminescence of Eu 3+ -doped SiO 2 -LaF 3 nano-glass-ceramics. Due to the characteristic nature of intra-configurational electronic transitions of Eu 3+ ions within the 4f 6 manifold ( 5 D 0 → 7 F J , J = 0-4), they are frequently used as a spectral probe. Thus, the changes in the photoluminescence profile of Eu 3+ ions could identify the general tendency of rare earth materials to segregate inside low-phonon energy fluoride nanocrystals, which allows us to assess their application potential in optoelectronics. Fabricated sol-gel materials, from sols to gels and xerogels to nano-glass-ceramics, were examined using several experimental techniques: X-ray diffraction (XRD), transmission electron microscopy (TEM), infrared spectroscopy (IR), and luminescence measurements. It was found that the distribution of Eu 3+ ions between the amorphous silicate sol-gel host and LaF 3 nanocrystals is strictly dependent on the initial composition of the obtained sols, and the lack of N,N-dimethylformamide significantly promotes the segregation of Eu 3+ ions inside LaF 3 nanocrystals. As a result, we detected long-lived luminescence from the 5 D 0 excited state equal to 6.21 ms, which predisposes the obtained glass-ceramic material for use as an optical element in reddish-orange emitting devices.

In this work, spectroscopic properties of europium and erbium ions in heavy metal oxide glasses have been studied. The phonon energy of the glass host was determined based on Eu 3+ excitation spectra measurements. Near-IR emission spectra at 1550 nm related to 4 I 13/2 → 4 I 15/2 transition of erbium in heavy metal glasses were examined with special regards to luminescence bandwidth and measured lifetime. In particular, correlation between phonon energy and the measured lifetime 4 I 13/2 (Er 3+ ) was proposed. The luminescence lifetime for the 4 I 13/2 upper laser state of erbium decreases with increasing phonon energy in glass matrices. Completely different results were obtained glass samples with europium ions, where the 5 D 0 lifetime increases with increasing phonon energy. Our investigations suggest that the values of measured 5 D 0 lifetime equal to radiative lifetimes for all heavy metal oxide glasses.

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.

In the present work, the Tb 3+ /Eu 3+ co-activated sol-gel glass-ceramic materials (GCs) containing MF 3 (M = Y, La) nanocrystals were fabricated during controlled heat-treatment of silicate xerogels at 350 °C. The studies of Tb 3+ → Eu 3+ energy transfer process (ET) were performed by excitation and emission spectra along with luminescence decay analysis. The co-activated xerogels and GCs exhibit multicolor emission originated from 4f n -4f n optical transitions of Tb 3+ ( 5 D 4 → 7 F J , J = 6-3) as well as Eu 3+ ions ( 5 D 0 → 7 F J , J = 0-4). Based on recorded decay curves, it was found that there is a significant prolongation in luminescence lifetimes of the 5 D 4 (Tb 3+ ) and the 5 D 0 (Eu 3+ ) levels after the controlled heat-treatment of xerogels. Moreover, for both types of prepared GCs, an increase in ET efficiency was also observed (from η ET ≈ 16% for xerogels up to η ET = 37.3% for SiO 2 -YF 3 GCs and η ET = 60.8% for SiO 2 -LaF 3 GCs). The changes in photoluminescence behavior of rare-earth (RE 3+ ) dopants clearly evidenced their partial segregation inside low-phonon energy fluoride environment. The obtained results suggest that prepared SiO 2 -MF 3 :Tb 3+ , Eu 3+ GC materials could be considered for use as optical elements in RGB-lighting optoelectronic devices operating under near-ultraviolet (NUV) excitation.

Nanocrystalline transparent BaF 2 :Eu 3+ glass-ceramic materials emitting reddish-orange light were fabricated using a low-temperature sol-gel method. Several experimental techniques were used to verify structural transformation from precursor xerogels to sol-gel glass-ceramic materials containing fluoride nanocrystals. Thermal degradation of xerogels was analyzed by thermogravimetric analysis (TG) and differential scanning calorimetry method (DSC). The presence of BaF 2 nanocrystals dispersed in sol-gel materials was confirmed by the X-ray diffraction (XRD) analysis and transmission electron microscopy (TEM). In order to detect structural changes in silica network during annealing process, the infrared spectroscopy (IR-ATR) was carried out. In particular, luminescence spectra of Eu 3+ and their decays were examined in detail. Some spectroscopic parameters of Eu 3+ ions in glass-ceramics containing BaF 2 nanocrystals were determined and compared to the values obtained for precursor xerogels. It was observed, that the intensities of two main red and orange emission bands corresponding to the 5 D 0 → 7 F 2 electric-dipole transition (ED) and the 5 D 0 → 7 F 1 magnetic-dipole (MD) transition are changed significantly during transformation from xerogels to nanocrystalline BaF 2 :Eu 3+ glass-ceramic materials. The luminescence decay analysis clearly indicates that the measured lifetime 5 D 0 (Eu 3+ ) considerably enhanced in nanocrystalline BaF 2 :Eu 3+ glass-ceramic materials compared to precursor xerogels. The evident changes in luminescence spectra and their decays suggest the successful migration of Eu 3+ ions from amorphous silica network to low-phonon BaF 2 nanocrystals.

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).

In the paper analysis of structural and luminescent properties of antimony-germanate-borate glasses and glass fiber co-doped with 0.6AgNO 3 /0.2Eu 2 O 3 are presented. Heat treatment of the fabricated glass and optical fiber (400 °C, 12 h) enabled to obtain Ag nanoparticles (NPs) with average size 30-50 nm on their surface. It has been proofed that silver ions migrate to the glass surface, where they are reduced to Ag 0 nanoparticles. Simultaneously, FTIR analysis showed that heat treatment of the glass and optical fiber increases the local symmetry of the Eu 3+ site., (Copyright © 2018 Elsevier B.V. All rights reserved.)

In this paper structural and optical properties of Rare Earth doped (RE) gallo-germanate glasses modified with various amount of Al 2 O 3 have been investigated. Glasses doped with Yb 3+ , Tm 3+, and Ho 3+ ions were synthesized to study Al 2 O 3 additive influence on their structural and emission properties in the visible spectral region. MIR spectra indicated that the structure of prepared glasses tends to order and its polymerization along with the aluminum content increase. Glass samples consisting of the low molar content of Al 2 O 3 are characterized by a significant contribution of Tm 3+ ions in light emission while Ho 3+ ions luminescence dominates in samples consisting of the higher molar content of Al 2 O 3 . Additionally, investigation of light emission in visible range showed that samples consisting of the low molar content of Al 2 O 3 are characterized by greenish blue light emission whereas light emitted by samples consisting of 15-20 mol% is much closer to the white colour., (Copyright © 2018 Elsevier B.V. All rights reserved.)

Near-infrared luminescence properties of Nd 3+ and Ho 3+ ions in barium gallo-germanate glasses have been reported. Several spectroscopic parameters for Nd 3+ and Ho 3+ ions have been determined from the Judd-Ofelt analysis and absorption/luminescence measurements. Quite large luminescence lifetime, quantum efficiency and stimulated emission cross-section have been obtained for the main 4 F 3/2  →  4 I 11/2 (Nd 3+ ) and 5 I 7  →  5 I 8 (Ho 3+ ) laser transitions of rare earths in barium gallo-germanate glasses. It suggests that barium gallo-germanate glass is promising for near-infrared laser application at emission wavelengths 1064 nm (Nd 3+ ) and 2020 nm (Ho 3+ )., (Copyright © 2018 Elsevier B.V. All rights reserved.)

Lead borate glasses singly and doubly doped with Tm 3+ and Dy 3+ were prepared by traditional melt-quenching technique. The emission spectra of rare earths in studied glass systems were registered under different excitation wavelengths. The observed emission bands are located in the visible spectral region. They correspond to 1 D 2 → 3 F 4 (blue) and 1 G 4 → 3 H 6 (blue) transitions of Tm 3+ as well as 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 Dy 3+ . Moreover, the energy transfer process from Tm 3+ to Dy 3+ was observed. The luminescence bands originating to characteristic transitions of thulium and dysprosium ions are present on emission spectra under direct excitation of Tm 3+ . Luminescence lifetimes for the excited states of Tm 3+ and Dy 3+ ions in lead borate glass were also determined based on decay measurements. The luminescence intensities and lifetimes depend significantly on the relative concentrations of the optically active dopants., (Copyright © 2017 Elsevier B.V. All rights reserved.)

Technology of active glass-ceramics (GC) is an important part of luminescent materials engineering. The classic method to obtain GC is based on annealing of parent glass in proper temperature and different time periods. Generally, only the bulk materials are investigated as a starting host for further applications. However, the effect of an additional heat-treatment process on emission and structural properties during GC processing is omitted. Here, we focus on the possibility of obtaining transparent glass-ceramic doped with europium ions directly with a melt-quenching method. The influence of phosphate concentration (up to 10 mol %) on the inversion symmetry of local environment of Eu 3+ ions in antimony-germanate-silicate (SGS) glass has been investigated. The Stark splitting of luminescence spectra and the local asymmetry ratio estimated by relation of (⁵D₀→⁷F₂)/(⁵D₀→⁷F₁) transitions in fabricated glass confirms higher local symmetry around Eu 3+ ions. Based on XRD and SEM/EDX measurements, the EuPO₄ nano- and microcrystals with monoclinic geometry were determined. Therefore, in our experiment, we confirmed possibility of one-step approach to fabricate crystalline structures (glass-ceramic) in Eu-doped SGS glass without additional annealing process., Competing Interests: The authors declare no conflict of interest.

In this paper, upconversion (UC) luminescence processes in a GeO 2 -Ga 2 O 3 -BaO glass system codoped with 0.7Yb 2 O 3 /(0.07-0.7)Tb 2 O 3 (mol.%) and double-clad optical fiber codoped with 0.7Yb 2 O 3 /0.7Tb 2 O 3 (mol.%) were investigated. The highest emission intensity (energy transfer efficiency equals 12.92%) was obtained for 0.7Yb 2 O 3 /0.7Tb 2 O 3 codoped glass. Comparative analysis showed significant differences in the shape of luminescence of fabricated germanate glass and optical fiber. Due to dominant transition from D 4 5 sublevel the main green UC peak (Tb 3+ :  D 4 5→F 5 7) of fabricated double-clad optical fiber is shifted by 4 nm toward longer wavelengths.

Thermal stability and broadband NIR luminescence of Pr(3+) doped gallo-germanate glasses with BaF2 have been studied. The thermal factors are larger for glass samples with low BaF2 content exhibiting good thermal stability against devitrification. Luminescence due to (1)D2 → (1)G4 transition of Pr(3+) was measured under 450 nm excitation. The (1)D2 measured lifetimes depend critically on activator concentration, but remain nearly unchanged with BaF2 content. The emission linewidth, the emission cross-section, the figure of merit (FOM) and the σem x FWHM product are relatively large, suggesting that Pr(3+)-doped gallo-germanate glasses with presence of BaF2 are promising as gain media for broadband near-infrared amplifiers.

Spectroscopic properties of rare earths in lead-free oxyfluoride germanate glasses were studied. The absorption and luminescence spectra of Eu(3+), Pr(3+) and Er(3+) ions were examined as a function of BaF₂ concentration and several spectroscopic parameters for rare earths were determined. The ratio of integrated luminescence intensity of the (5)D₀→(7)F₂ transition to that of the (5)D₀→(7)F₁ transition of Eu(3+) decrease significantly with increasing BaF₂ content. The absorption (Er(3+)) and emission (Pr(3+)) 'hypersensitive transitions' of rare earths are shifted in direction to shorter wavelengths with increasing BaF₂ content in glass composition. Emission spectra and their decays corresponding to the main (4)I₁₃/₂→(4)I₁₅/₂ laser transition of Er(3+) were also analyzed. Quite long-lived NIR luminescence of Er(3+) is observed for lead-free glass samples with low BaF₂ concentration., (Copyright © 2014 Elsevier B.V. All rights reserved.)

Lead borate glasses doubly doped with Dy(3)(+)-Tb(3+) and Tb(3+)-Eu(3+) were investigated using optical spectroscopy. Luminescence spectra of rare earths were detected under various excitation wavelengths. The main green emission band due to (5)D4→(7)F5 transition of Tb(3+) is observed under excitation of Dy(3+), whereas the main red emission band related to (5)D0→(7)F2 transition of Eu(3+) is successfully observed under direct excitation of Tb(3+). In both cases, the energy transfer processes from Dy(3+) to Tb(3+) and from Tb(3+) to Eu(3+) in lead borate glasses occur through a nonradiative processes with efficiencies up to 16% and 18%, respectively. The presence of energy transfer process was also confirmed by excitation spectra measurements., (Copyright © 2014 Elsevier B.V. All rights reserved.)

GdF3 nanocrystals doped with Eu3+ ions in oxyfluoride glass ceramics were prepared by a solgel method. The structural properties were examined by x-ray diffraction measurements. The effects of gadolinium codoping on europium emission in the prepared solgel glasses and glass ceramics have been studied. The emission bands originating from the 5D0 state of Eu3+ ions are enhanced under excitation of Gd3+ ions by 273 nm line. The electric dipole 5D0→7F2 transitions were dominant in the samples before heat treatment, whereas magnetic dipole 5D0→7F1 transitions had a higher probability in the samples after annealing. The luminescence lifetime for the 5D0 level of Eu3+ ions in the samples after excitation at 273 nm is long lived in comparison to excitation at 393 nm and increased to 190%. Energy transfer from Gd3+ to Eu3+ was observed.

Correlation between structure and optical properties of rare earth ions in lead borate glasses and glass-ceramics was evidenced by X-ray-diffraction, Raman, FT-IR and luminescence spectroscopy. The rare earths were limited to Eu(3+) and Er(3+) ions. The observed BO(3)↔BO(4) conversion strongly depends on the relative PbO/B(2)O(3) ratios in glass composition, giving important contribution to the luminescence intensities associated to (5)D(0)-(7)F(2) and (5)D(0)-(7)F(1) transitions of Eu(3+). The near-infrared luminescence and up-conversion spectra for Er(3+) ions in lead borate glasses before and after heat treatment were measured. The more intense and narrowing luminescence lines suggest partial incorporation of Er(3+) ions into the orthorhombic PbF(2) crystalline phase, which was identified using X-ray diffraction analysis., (Copyright © 2010 Elsevier B.V. All rights reserved.)

Visible luminescence of Dy(3+) ions in oxyhalide lead borate glasses was examined. Luminescence spectra show two intense bands at 480 nm and 573 nm due to (4)F(9/2)→(6)H(15/2) (blue) and (4)F(9/2)→(6)H(13/2) (yellow) transitions of Dy(3+). Luminescence decays from (4)F(9/2) state and yellow-to-blue luminescence intensity ratios (Y/B) were analysed with PbX(2) (X=F, Cl) content. An introduction of PbX(2) to the borate glass results in the increasing of (4)F(9/2) lifetime and the decreasing of yellow-to-blue luminescence intensity ratio, which is due to reduction of covalency between Dy(3+) and O(2-)/X(-) ions., (Copyright © 2010 Elsevier B.V. All rights reserved.)

Lead phosphate glasses containing Eu(3+) and Dy(3+) have been studied. Local structure was verified using Fourier transform (FT)-IR spectroscopy. Emission bands of Eu(3+) and Dy(3+) ions in lead phosphate glasses are observed in the visible spectral range, which correspond to 5D0→7F(J) (J=0,1,2,4) and 4F(9/2)→6H(J/2) (J=15,13,11) transitions, respectively. Shorter luminescence decays from excited states of Eu(3+) and Dy(3+0 are due to the presence of PbO in phosphate glass.

Upconversion tri-color luminescence of Er(3+)/Tm(3+)/Yb(3+) doped BaTiO3 nanocrystals is observed under the excitation of a 980 nm laser diode. Especially, Er(3+) emission has a considerable contribution to the blue portion of the UC spectra, different from the ever-reported results, in which blue emission originates only from Tm(3+). This realization is beneficial to lower the color separation between blue and green (or red) emissions, in fluorescent labeling. The analysis of excitation power dependence and decay time revealed that blue emission of Er(3+) ion is induced by a dual energy transfer upconversion, while Tm(3+) plays a role of both emitter and activator.

Lead borate based glasses have been analyzed using Raman and infrared spectroscopy. The formation of different borate groups and the direction of BO3 <--> BO4 conversion strongly depends on the PbO- and/or PbF2-to-B2O3 ratio in chemical composition. PbF2-PbO-B2O3 based glasses containing Er3+ ions have been studied after annealing. The orthorhombic PbF2 crystallites are formed during thermal treatment, which was evidenced by X-ray diffraction analysis. Near-infrared luminescence at 1530 nm and green up-conversion at 545 nm have been registered for samples before and after annealing. The luminescence bands correspond to 4I13/2-4I15/2 and 4S3/2-4I15/2 transitions of Er3+ ions, respectively. In comparison to the precursor glasses, the luminescence intensities are higher in the studied transparent oxyfluoride glass ceramics. Simultaneously, the half-width of the luminescence lines slightly decreases. It can be the evidence that a small amount of the Er3+ ions is incorporated into the orthorhombic PbF2 phase.