Effect of Yb3+ on the Structural and Visible to Near-Infrared Wavelength Photoluminescence Properties in Sm3+-Yb3+-Codoped Barium Fluorotellurite Glasses

We report on the Sm3+ and Sm3+:Yb3+-doped barium fluorotellurite glasses prepared using the conventional melting and quenching method. The spectroscopic characterisations were investigated with Raman and FTIR to evaluate the glasses’ structural and hydroxyl (-OH) content. The Raman analysis revealed a structural modification in the glass network upon adding and increasing the Yb3+ concentration from a TeO3 trigonal pyramid to a TeO4 trigonal bi-pyramid polyhedral. At the same time, the FTIR measurements showed the existence of -OH groups in the glass. Thus, under the current experimental conditions and nominal composition, the -OH group contents are too large to enable an effective removal. The near-infrared region of the absorption spectra is employed to determine the nephelauxetic ratio and bonding parameters. The average nephelauxetic ratio decreases, and the bonding parameter increases with the increasing Yb3+ content in the glasses. A room temperature visible and near-infrared photoluminescence ranging from 500 to 1500 nm in wavelength and decay properties were investigated for glasses doped with Sm3+ and Sm3+-Yb3+ by exciting them with 450 and 980 nm laser sources. Exciting the Sm3+- and Sm3+-Yb3+-doped glasses by 450 nm excitation reveals a new series of photoluminescence emissions at 1200, 1293, and 1418 nm, corresponding to the 6F11/2 state to the 6HJ (J = 7/2, 9/2, 11/2) transitions. Under the 976 nm laser excitation, a broad photoluminescence emission from 980 to 1200 nm was detected. A decay lifetime decreased from ~244 to ~43 μs by increasing the Yb3+ content, ascribing to concentration quenching and the OH content.


Introduction
During the last decades, various rare-earth (RE 3+ ) ions doped in different host-glass matrices have been investigated for applications such as laser sources for optical telecommunications, optical amplifiers, multicolour displays, and medical diagnostics (sensing and imaging in the anatomical environment) [1][2][3][4]. Tellurium oxide (TeO 2 )-based glasses have numerous advantages over the borate, phosphate, and silicate host glasses [1,2,5]. This is due to their unique high linear and nonlinear refractive indices; outstanding characteristics, including high density, thermal stability, crystallisation, chemical durability, and low melting temperature; and high optical transparency in the visible to IR region up to 0.35-5 µm [5]. In addition, TeO 2 -based glasses have a high stimulated emission cross-section, a low-phonon-energy range from 560 to 750 cm −1 depending on the lattice modifier, and a high RE 3+ -ion concentration solubility [6][7][8][9]. However, a pure TeO 2 has a low glass-forming ability due to its high quenching rate [10][11][12][13]. Consequently, this requires lattice modifiers such as alkali oxides, alkaline earth oxides, and transition metal oxides and halides to vitrify with a moderate quenching rate [13]. These lattice modifiers enable the TeO 4 and TeO 3 structural units to break down and decrease the average Te-O coordination [14], thus enhancing the glass-forming ability. For instance, adding a barium fluoride (BaF 2 ) modifier to a TeO 2 -based glass network tends to decrease the phonon energy remarkably as compared to the phonon energy of the TeO 2 -based (Te-O~750 cm −1 ) [15,16]. Thus, reducing the nonradiative and multiphonon relaxation mechanism promotes optical transitions in RE 3+ ions.
In this article, we have fabricated Sm 3+ -and Yb 3+ :Sm 3+ -doped barium fluorotellurite zinc glasses by varying the Yb 3+ concentrations. We performed a comprehensive analysis to investigate the influence of the Yb 3+ content on the barium fluorotellurite zinc glasses' structure and -OH content via Raman spectroscopy and Fourier transform infrared (FTIR) transmittance and absorption spectra. The Yb 3+ concentration effect on the glass refractive index and nephelauxetic ratios are also determined. Furthermore, the visible and NIR photoluminescence emission and decay lifetimes are examined to understand the energy transfer process between the Yb 3+ and Sm 3+ ions by increasing the Yb 3+ concentration under 450 and 980 nm pumping schemes.

Materials and Method
Barium fluorotellurite glasses doped/codoped with Sm 3+ and Sm 3+ : Yb 3+ ions with molar composition of (97.5 − x)TeO 2 -10ZnO-10BaF 2 -0.5Sm 2 O 3 -(x = 0-1.0Yb 2 O 3 ) were fabricated using conventional melting and quenching technique. The starting materials comprise high-purity TeO 2 (≥99.99%), ZnO (99.99%), BaF 2 (99.99%), Yb 2 O 3 (99.99%), and Sm 2 O 3 (99.99%), which were purchased from Alfa Aesar. A 30 g batch of Sm 3+ -and Sm 3+ : Yb 3+ -doped barium fluorotellurite glasses were synthesised by weighing the appropriate amount of each powder raw material. A homogenous mixture of the powder samples was obtained using a mortar and a pestle to mix thoroughly for about 20 min. The mixture was transferred into a gold crucible and melted at 750-800 • C for about 3 h by employing an electrical furnace (Elite Thermal Systems Limited, Market Harborough, UK). During the glass melting, the furnace atmosphere was maintained under a dry oxygen atmosphere by purging a high-purity oxygen gas to control moisture and OH-ion ingress in the glass melt. The homogeneous molten glass was poured into a preheated brass mould at 300 • C for 3 h and then annealed at 300 • C for 4 h to remove thermal stress.
Finally, the annealing furnace was allowed to cool down to room temperature overnight and polished for optical measurements. A prism coupler (Metricon model 2010/M) and Thermo Pycnomatic ATC Helium Pycnometer were also utilised to measure the glass's linear refractive index and density. The molar composition, density, and refractive index of the achieved glasses are depicted in Table 1. Raman spectroscopy measurements were performed utilising a Renishaw via Raman spectrometer with a green Ar + laser (λ = 514.5 nm) excitation source (Renishaw, Wotton-under-Edge, UK) to determine the molecular structure of as-prepared glasses within the spectral range of 100-1000 cm −1 . Similar, infrared absorbance spectroscopy was conducted to investigate the effect of the single-doped local structures compared to codoped glasses. The infrared absorbance spectra were collected in a Vertex 70 FITR spectrometer (Brucker, Coventry, UK) over 4 cm −1 resolution between 12,000 and 400 cm −1 . The visible and NIR photoluminescence properties were investigated at room temperature with an FS920 spectrometer (Edinburgh Instruments, Livingston, UK) by exciting with 450 and 980 nm laser sources in the wavelength range of 500-750 and 850-1500 nm. The visible and NIR photoluminescence emission measurements were performed with a 495 nm short-pass filter cut-off wavelength.  Table 1 shows the measured densities and the refractive index as a function of the Y 3+ ion concentration. The average density values were determined using the Thermo Pycnomatic ATC Helium Pycnometer instrument range from 5.632 to 5.618 g/cm 3 . The refractive index increases slightly as the content of the Yb 3+ increases, which could be attributed to its dependence on the polarisability of the ions or electron density [29] in the glass.
Raman spectroscopy is a well-known technique for determining the host materials' vibrational, structural, and functional groups and the phonon energy for the optical applications. Hence, the Raman spectra of the SBT glass series with different Yb 3+ concentrations were monitored within a wavenumber spectral range of 0-1000 cm −1 , as depicted in Figure 1. The Boson peak of the tellurite glass due to the disordered structure of the amorphous state occurring between 60 and 100 cm −1 is observed in these glass samples.  Table 1 shows the measured densities and the refractive index as a function of the Y 3+ ion concentration. The average density values were determined using the Thermo Pycnomatic ATC Helium Pycnometer instrument range from 5.632 to 5.618 g/cm 3 . The refractive index increases slightly as the content of the Yb 3+ increases, which could be attributed to its dependence on the polarisability of the ions or electron density [29] in the glass Raman spectroscopy is a well-known technique for determining the host materials' vibrational, structural, and functional groups and the phonon energy for the optical applications. Hence, the Raman spectra of the SBT glass series with different Yb 3+ concentrations were monitored within a wavenumber spectral range of 0-1000 cm −1 , as depicted in Figure 1. The Boson peak of the tellurite glass due to the disordered structure of the amorphous state occurring between 60 and 100 cm −1 is observed in these glass samples. It can be noticed that the Raman spectra of all the glasses prepared exhibit a broad band between 0 and 200 cm −1 and a strong peak centred at ~65 cm −1 , which confirms the presence of the Boson peak in the SBT series' glassy structure. This apparent characteristic peak can be attributed to acoustic phonon and rotational molecular modes [29]. The Raman spectra of all the glass samples prepared were characterised by deconvoluting three prominent broad bands ranging from 220 to 370, 370 to 546, and 546 to 900 cm −1 , as shown in Figure 1a-d, using origin software and fitted with Gaussian sub-band curves. The weak Raman intensity observed at 295 cm −1 can be attributed to the Ag and Fg Raman active modes of Sm-O [30,31]. The vibration band that peaked at 450 cm −1 is assigned to the bend- It can be noticed that the Raman spectra of all the glasses prepared exhibit a broad band between 0 and 200 cm −1 and a strong peak centred at~65 cm −1 , which confirms the presence of the Boson peak in the SBT series' glassy structure. This apparent characteristic peak can be attributed to acoustic phonon and rotational molecular modes [29]. The Raman spectra of all the glass samples prepared were characterised by deconvoluting three prominent broad bands ranging from 220 to 370, 370 to 546, and 546 to 900 cm −1 , as shown in Figure 1a-d, using origin software and fitted with Gaussian sub-band curves. The weak Raman intensity observed at 295 cm −1 can be attributed to the Ag and Fg Raman active modes of Sm-O [30,31]. The vibration band that peaked at 450 cm −1 is assigned to the bending and stretching vibrations of the Te-O-Te or O-Te-O linkage bonds, increasing the amplitude by increasing the Yb 3+ content. On the other hand, enhancing this prominent peak could be attributed to the formation of Te-O-Ba/Yb sharing a vertex with TeO 4 , TeO 3+δ , and TeO 3 [32][33][34]. In addition, the vibration bands centred at 664 and 734 cm −1 show a vigorous Raman intensity and are ascribed to the structural fragment of the TeO 4 trigonal bi-pyramid polyhedral (tbps), TeO 3 trigonal pyramid (tp), and intermediate TeO 3+δ polyhedron units, respectively [2,31,33,34]. The TeO 4 (tbps) vibration peak is accredited to the asymmetric stretching vibrations of the Te-O-Te bridges which connect the TeO 3 tbps at the vertices, whereas the TeO 3 (tp) vibration band represents symmetric and asymmetric stretching modes due to the formation of nonbridge oxygen. It can be noticed that the relative intensity of a Sm 3+ -doped barium fluorotellurite (sample SBT1) Raman peak centred at 734 cm −1 (TeO 3 ) is more intense as compared to the vibration band that occurred at 664 cm −1 (TeO 4 ). The vibrational band centred at 781 cm −1 is ascribed to the stretching mode of the nonbridging oxygen TeO 3+1 unit. Nevertheless, upon doping and increasing the Yb 3+ concentration, the amplitude of the vibration band at 664 cm −1 progressively increases and predominates over the 734 cm −1 peak, as shown in Figure 2a. This demonstrates that the TeO 3 (tps) unit has undergone a structural transformation by reducing the network connectivity to the TeO 4 unit in the glass network. Figure 2b illustrates the intensity ratio of the bridge oxygen to nonbridge oxygen atoms I 664 I 735 as a function of the Yb 2 O 3 concentration (mol%). It was observed that an increase in the Yb 2 O 3 concentration shows a slight enhancement in the TeO 4 (tbps) vibration band amplitude without any shift in the wavenumber. According to Maaoui et al. [35], such behaviour could be due to F − (149 pm) substituting O 2− (152 pm) because of the similarities in the ionic radius. For instance, the network form can be broken down by replacing two F-with one oxygen atom or interchanging F-with Ba in the TeO3 (tps) to become Te(O, F)3 someplace within the glass network. ing and stretching vibrations of the Te-O-Te or O-Te-O linkage bonds, increasing the amplitude by increasing the Yb 3+ content. On the other hand, enhancing this prominent peak could be attributed to the formation of Te-O-Ba/Yb sharing a vertex with TeO4, TeO3+δ, and TeO3 [32][33][34]. In addition, the vibration bands centred at 664 and 734 cm −1 show a vigorous Raman intensity and are ascribed to the structural fragment of the TeO4 trigonal bi-pyramid polyhedral (tbps), TeO3 trigonal pyramid (tp), and intermediate TeO3+δ polyhedron units, respectively [2,31,33,34]. The TeO4 (tbps) vibration peak is accredited to the asymmetric stretching vibrations of the Te-O-Te bridges which connect the TeO3 tbps at the vertices, whereas the TeO3 (tp) vibration band represents symmetric and asymmetric stretching modes due to the formation of nonbridge oxygen. It can be noticed that the relative intensity of a Sm 3+ -doped barium fluorotellurite (sample SBT1) Raman peak centred at 734 cm −1 (TeO3) is more intense as compared to the vibration band that occurred at 664 cm −1 (TeO4). The vibrational band centred at 781 cm −1 is ascribed to the stretching mode of the nonbridging oxygen TeO3+1 unit. Nevertheless, upon doping and increasing the Yb 3+ concentration, the amplitude of the vibration band at 664 cm −1 progressively increases and predominates over the 734 cm −1 peak, as shown in Figure 2a. This demonstrates that the TeO3 (tps) unit has undergone a structural transformation by reducing the network connectivity to the TeO4 unit in the glass network. Figure 2b illustrates the intensity ratio of the bridge oxygen to nonbridge oxygen atoms as a function of the Yb2O3 concentration (mol%). It was observed that an increase in the Yb2O3 concentration shows a slight enhancement in the TeO4 (tbps) vibration band amplitude without any shift in the wavenumber. According to Maaoui et al. [35], such behaviour could be due to F − (149 pm) substituting O 2− (152 pm) because of the similarities in the ionic radius. For instance, the network form can be broken down by replacing two F-with one oxygen atom or interchanging F-with Ba in the TeO3 (tps) to become Te(O, F)3 someplace within the glass network.  Figure 3a illustrates a mid-infrared transmittance spectra of samples SBT1, SBT2, SBT3, and SBT4 with corresponding thicknesses of 4.45, 4.58, 4.56, and 4.48 mm recorded using a Brucker Vertex 70 FTIR spectrometer. Sample SBT1 at 4.45 mm thick exhibits a high average transmittance of about 52% within the spectral range of 2.5 to 3.0 μm and longer mid-infrared cut-off edges near 6.25 μm. The transmission of sample SBT1 revealed two broad bands dipping around 3.3 μm (3000 cm −1 ) and 4.38 μm (2304 cm −1 ), which are assigned to the -OH groups connected in the glass network [36][37][38]. The absorption band that occurred at 3000 cm −1 is attributed to the free -OH groups coupled to cations in the glass network, with a typical example being tellurium ion (Te-OH) and weakly bonded with hydrogen (H-OH) [36]. The absorption peak centred at 2304 cm −1 is due to the -OH absorption strongly connected with hydrogen (H-OH) and multiphonon absorption.  Figure 3a illustrates a mid-infrared transmittance spectra of samples SBT1, SBT2, SBT3, and SBT4 with corresponding thicknesses of 4.45, 4.58, 4.56, and 4.48 mm recorded using a Brucker Vertex 70 FTIR spectrometer. Sample SBT1 at 4.45 mm thick exhibits a high average transmittance of about 52% within the spectral range of 2.5 to 3.0 µm and longer mid-infrared cut-off edges near 6.25 µm. The transmission of sample SBT1 revealed two broad bands dipping around 3.3 µm (3000 cm −1 ) and 4.38 µm (2304 cm −1 ), which are assigned to the -OH groups connected in the glass network [36][37][38]. The absorption band that occurred at 3000 cm −1 is attributed to the free -OH groups coupled to cations in the glass network, with a typical example being tellurium ion (Te-OH) and weakly bonded with hydrogen (H-OH) [36]. The absorption peak centred at 2304 cm −1 is due to the -OH absorption strongly connected with hydrogen (H-OH) and multiphonon absorption.

Physical and Structural Properties-Density, Refractive Index, Raman and FTIR Spectroscopies
However, no noticeable changes are observed upon doping with various concentrations of Yb 3+ with sample STBT1; nevertheless, transmission decreases with decreasing Yb 3+ content in the glass, as shown in Figure 3a. However, no noticeable changes are observed upon doping with various concentrations of Yb 3+ with sample STBT1; nevertheless, transmission decreases with decreasing Yb 3+ content in the glass, as shown in Figure 3a.
Meanwhile, the corresponding concentration (ions/cm ) is obtained using where T(%) is the maximum transmittance in percentage at 3000 and 2304 cm −1 , L is the glass thickness (cm), A is the absorbance, ε is the -OH group molar absorptivity in the tellurite glass (4.91 × 10 4 cm 2 mol −1 ) [38], and NAVO represents Avogadro's constant (6.02 × 10 23 mol −1 ). The absorption coefficient spectra were determined using the transmittance spectra and Equation (1), as depicted in Figure 3b. The maximum absorption peaks centred at 3.30 and 4.38 μm in Figure 3b were employed to estimate the -OH concentration in all the glasses prepared, which is summarised in Table 2. It is observed that the (cm −1 ) and (ions/cm ) increase with increasing Yb 3+ content in the glass series and these values correlate with oxyfluoride tellurite glasses reported elsewhere, respectively [35]. Therefore, high hydroxide absorption in the glasses can be reduced by either increasing the BaF2 content or substituting a portion of the ZnO with ZnF2.  The absorption coefficient of -OH, α OH cm −1 in the fluorotellurite glass was estimated by employing the Beer-Lambert law equation, which is expressed as [36]: Meanwhile, the corresponding concentration N OH − ions/cm −3 is obtained using Equation (2) [37] where T(%) is the maximum transmittance in percentage at 3000 and 2304 cm −1 , L is the glass thickness (cm), A is the absorbance, ε is the -OH group molar absorptivity in the tellurite glass (4.91 × 10 4 cm 2 mol −1 ) [38], and N AVO represents Avogadro's constant (6.02 × 10 23 mol −1 ). The absorption coefficient spectra were determined using the transmittance spectra and Equation (1), as depicted in Figure 3b. The maximum absorption peaks centred at 3.30 and 4.38 µm in Figure 3b were employed to estimate the -OH concentration in all the glasses prepared, which is summarised in Table 2. It is observed that the α OH (cm −1 ) and N OH − ions/cm −3 increase with increasing Yb 3+ content in the glass series and these values correlate with oxyfluoride tellurite glasses reported elsewhere, respectively [35]. Therefore, high hydroxide absorption in the glasses can be reduced by either increasing the BaF 2 content or substituting a portion of the ZnO with ZnF 2 .  Figure 4a represents the absorption coefficient spectra of the SBT glass series measured at room temperature using a FITIR spectrometer in the wavenumber region of 4000 to 12,000 cm −1 . The absorption bands of the Yb 3+ and various Sm 3+ transitions are labelled in Figure 4a,b. About seven of the 4f 5 -4f 5 electronic transitions of the Sm 3+ from the 5 H 5/2 ground state to different excited states are observed in the hypersensitive frequency, as shown in Figure 4b. The Sm 3+ -hypersensitive absorption bands occur as a result of the transition from the 5 H 5/2 ground state to excited states such as 6 F 11/2 , 6 F 9/2 , 6 F 7/2 , 6 F 5/2 , 6 F 3/2 , 6 H 15/2 , and 6 F 1/2 , which correspond to wavenumbers of 10,333 cm −1 (968 nm), 9238 cm −1 (1082 nm), 811 cm −1 (1233 nm), 7244 cm −1 (1381 nm), 6731 cm −1 (1486 nm), 6513 cm −1 (1536 nm), and 6292 cm −1 (1589 nm), respectively. In addition, the intense and broad absorption band in the wavenumber range of 11,200-9601 cm −1 are attributed to the overlap transitions between Sm 3+ ( 5 H 5/2 → 6 F 11/2 ) and Yb 3+ ( 2 F 7/2 → 2 F 5/2 ). The NIR absorption transitions are sensitive to an environmental change around the ions, thus leading to broad absorption ranging from 7542 to 9560 cm −1 as the Yb 3+ content increases to 1 mol%. The broad absorption band in the SBT4 glass could be accredited to the presence of a -OH second overtone in the glass. In addition, most of the NIR transitions from 5 H 5/2 → 6 F 9/2 , 6 F 7/2 , 6 F 5/2 , 6 F 3/2 , and 6 F 1/2 in Figure 4 obey the selection rule with ∆J ≤ 6 inducing electric-dipole interactions, thus exhibiting sharp and more intense absorption bands.  Figure 4a represents the absorption coefficient spectra of the SBT glass series measured at room temperature using a FITIR spectrometer in the wavenumber region of 4000 to 12,000 cm −1 . The absorption bands of the Yb 3+ and various Sm 3+ transitions are labelled in Figure 4a,b. About seven of the 4f 5 -4f 5 electronic transitions of the Sm 3+ from the 5 H5/2 ground state to different excited states are observed in the hypersensitive frequency, as shown in Figure 4b. The Sm 3+ -hypersensitive absorption bands occur as a result of the transition from the 5 H5/2 ground state to excited states such as 6 F11/2, 6 F9/2, 6 F7/2, 6 F5/2, 6 F3/2, 6 H15/2, and 6 F1/2, which correspond to wavenumbers of 10,333 cm −1 (968 nm), 9238 cm −1 (1082 nm), 811 cm −1 (1233 nm), 7244 cm −1 (1381 nm), 6731 cm −1 (1486 nm), 6513 cm −1 (1536 nm), and 6292 cm −1 (1589 nm), respectively. In addition, the intense and broad absorption band in the wavenumber range of 11,200-9601 cm −1 are attributed to the overlap transitions between Sm 3+ ( 5 H5/2 → 6 F11/2) and Yb 3+ ( 2 F7/2 → 2 F5/2). The NIR absorption transitions are sensitive to an environmental change around the ions, thus leading to broad absorption ranging from 7542 to 9560 cm −1 as the Yb 3+ content increases to 1 mol%. The broad absorption band in the SBT4 glass could be accredited to the presence of a -OH second overtone in the glass. In addition, most of the NIR transitions from 5 H5/2 → 6 F9/2, 6 F7/2, 6 F5/2, 6 F3/2, and 6 F1/2 in Figure 4 obey the selection rule with ∆ ≤ 6 inducing electric-dipole interactions, thus exhibiting sharp and more intense absorption bands.  Furthermore, the absorption spectra of the as-prepared glasses were employed to investigate the significant nature of the bonding between Sm 3+ and their surrounding oxygen atoms in the host glasses. The slight change in the absorption peak position at the NIR transitions of the glasses is attributed to the variation in the surrounding environment of Sm 3+ and the structure complexity. These were evaluated using the nephelauxetic ratio (β) and bonding parameters (δ). The nephelauxetic effect arises from the expansion of partially filled 4f-4f-shells owing to different glass compositions with a charge transfer from the ligand to the core of the Sm 3+ and Yb 3+ [39,40]. The nephelauxetic ratio, ( ), is expressed as the ratio of a particular RE 3+ ion-absorption transition in wavenumber (cm  Furthermore, the absorption spectra of the as-prepared glasses were employed to investigate the significant nature of the bonding between Sm 3+ and their surrounding oxygen atoms in the host glasses. The slight change in the absorption peak position at the NIR transitions of the glasses is attributed to the variation in the surrounding environment of Sm 3+ and the structure complexity. These were evaluated using the nephelauxetic ratio (β) and bonding parameters (δ). The nephelauxetic effect arises from the expansion of partially filled 4f-4f-shells owing to different glass compositions with a charge transfer from the ligand to the core of the Sm 3+ and Yb 3+ [39,40]. The nephelauxetic ratio, (β), is expressed as the ratio of a particular RE 3+ ion-absorption transition in wavenumber (cm −1 ), v gl , in the host glass, to the corresponding transition of the aquo-ion in wavenumber (cm −1 ), (v a ) [5,41,42]: The average value of the nephelauxetic ratio, β = β N , (N is the total amount of observed absorption in the glass matrix) was utilised to determine the bonding parameter, (σ), from the following equation [17,41,42]:

Absorption Spectra, Bonding between Sm 3+ and Surrounding Oxygen Atoms
The σ can be either positive or negative, which signifies the covalent or ionic nature of the RE 3+ -ligand bond depending on the surrounding environment. All absorption coefficient spectra of the glass samples were fitted with multiple Gaussian function to determine the peak positions of each NIR transition of the Sm 3+ ions. This provides an accurately centred wavenumber for each transition and enables accuracy in assessing the nephelauxetic ratio (β) and bonding parameters. The nephelauxetic effect is caused by the shortening of the metal-ligand distance bond formation due to the expansion of the electron cloud, which leads to a decrease in the coordination number. Table 3 shows β and σ values obtained from Equations (3) and (4) for the SBT glass series. The negative values of σ confirm the ionic character of the Sm 3+ -and Sm 3+ :Yb 3+ -doped and -codoped barium fluorotellurite glasses. The ionicity, σ, values increased with the increasing Yb 3+ content leading to a structural modification. Such structural alterations have been observed in various glasses, as shown in Figure 2, leading to an increase in the TeO 4 bridge oxygen peak, which accounts for increasing bonding parameters with increasing Yb 3+ .

Conclusions
The Sm 3+ -and Sm 3+ :Yb 3+ -doped barium fluoride-substituted zinc tellurite glasses were fabricated and characterised for their numerous properties such as density, refractive index, structural, -OH content, and photoluminescence. Adding the appropriate amount of Yb 3+ ions into the glass network composition leads to a structural reformation, and the -OH content increases significantly. The photoluminescence emission and decay dynamics of the RE 3+ ion single-doped and codoped barium fluorotellurite glasses were investigated under 450 and 976 nm band excitations. The photoluminescence emission spectra were recorded using a 450 nm laser diode excitation to monitor the glasses' visible and NIR emission spectra. This measurement revealed four intense emission peaks at the yellow-green ( 4 G5/2 → 6 H5/2,), orange ( 4 G5/2 → 6 H7/2), and red ( 4 G5/2 → 6 H9/2 and 4 G5/2 → 6 H11/2) bands in the visible spectrum. In contrast, seven emission peaks were observed at the NIR region corresponding to the radiative emission from the 4 G5/2 state to 6 FJ (J = 3/2, 5/2, 7/2, 9/2) transitions and the 6 F11/2 state to 6 HJ (J = 7/2, 9/2, 11/2) transitions. It is observed that the photoluminescence emission intensities measured from visible to NIR decrease with increasing Yb 3+ concentrations. This quenching process is attributed to resonant energy transfer processes from Sm 3+ to Yb 3+ due to a large absorption cross-section of Yb 3+ . Two emission peaks were observed from the Sm 3+ :Yb 3+ -codoped glasses at 1040 and 1125 nm, with a 976 nm excitation correlating to the Yb 3+ ( 2 F5/2 → 2 F7/2) and Sm 3+ ( 4 G5/2 → 6 F9/2) transitions, whereas the photoluminescence lifetime decreases with increasing Yb 3+ concentrations. These initial photoluminescence property results of the Sm 3+ :Yb 3+ -codoped glasses suggest new NIR windows for developing lasers suitable for medical diagnostics and imaging in anatomical environments.

Conclusions
The Sm 3+ -and Sm 3+ :Yb 3+ -doped barium fluoride-substituted zinc tellurite glasses were fabricated and characterised for their numerous properties such as density, refractive index, structural, -OH content, and photoluminescence. Adding the appropriate amount of Yb 3+ ions into the glass network composition leads to a structural reformation, and the -OH content increases significantly. The photoluminescence emission and decay dynamics of the RE 3+ ion single-doped and codoped barium fluorotellurite glasses were investigated under 450 and 976 nm band excitations. The photoluminescence emission spectra were recorded using a 450 nm laser diode excitation to monitor the glasses' visible and NIR emission spectra. This measurement revealed four intense emission peaks at the yellow-green ( 4 G 5/2 → 6 H 5/2 ,), orange ( 4 G 5/2 → 6 H 7/2 ), and red ( 4 G 5/2 → 6 H 9/2 and 4 G 5/2 → 6 H 11/2 ) bands in the visible spectrum. In contrast, seven emission peaks were observed at the NIR region corresponding to the radiative emission from the 4 G 5/2 state to 6 F J (J = 3/2, 5/2, 7/2, 9/2) transitions and the 6 F 11/2 state to 6 H J (J = 7/2, 9/2, 11/2) transitions. It is observed that the photoluminescence emission intensities measured from visible to NIR decrease with increasing Yb 3+ concentrations. This quenching process is attributed to resonant energy transfer processes from Sm 3+ to Yb 3+ due to a large absorption cross-section of Yb 3+ . Two emission peaks were observed from the Sm 3+ :Yb 3+ -codoped glasses at 1040 and 1125 nm, with a 976 nm excitation correlating to the Yb 3+ ( 2 F 5/2 → 2 F 7/2 ) and Sm 3+ ( 4 G 5/2 → 6 F 9/2 ) transitions, whereas the photoluminescence lifetime decreases with increasing Yb 3+ concentrations. These initial photoluminescence property results of the Sm 3+ :Yb 3+ -codoped glasses suggest new NIR windows for developing lasers suitable for medical diagnostics and imaging in anatomical environments.