Access the full text.
Sign up today, get DeepDyve free for 14 days.
Loading next page...
/lp/multidisciplinary-digital-publishing-institute/thermal-optical-and-ir-emission-properties-of-extremely-low-hydroxyl-iaAKhA1Gzx
- Publisher
- Multidisciplinary Digital Publishing Institute
- Copyright
- © 1996-2021 MDPI (Basel, Switzerland) unless otherwise stated Disclaimer The statements, opinions and data contained in the journals are solely those of the individual authors and contributors and not of the publisher and the editor(s). MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. Terms and Conditions Privacy Policy
- ISSN
- 2304-6732
- DOI
- 10.3390/photonics8080320
- Publisher site
- See Article on Publisher Site
Abstract
hv photonics Article Thermal, Optical, and IR-Emission Properties of Extremely Low Hydroxyl TeO -WO -Bi O -La O -xEr O Glasses for 2 3 2 3 2 3 2 3 Mid-Infrared Photonics 1 , 2 3 1 , 2 1 Vitaly V. Dorofeev , Vasily V. Koltashev , Sergei E. Motorin , Alexander D. Plekhovich 2 , and Arkady V. Kim * G.G. Devyatykh Institute of Chemistry of High-Purity Substances of the Russian Academy of Sciences, 49 Tropinin Str., 603951 Nizhny Novgorod, Russia; dorofeev@ihps-nnov.ru (V.V.D.); motorin@ihps-nnov.ru (S.E.M.); plekhovich@ihps-nnov.ru (A.D.P.) Institute of Applied Physics of the Russian Academy of Sciences, 46 Ulyanov Str., 603950 Nizhny Novgorod, Russia Prokhorov General Physics Institute of the Russian Academy of Sciences, Dianov Fiber Optics Research Center, 38 Vavilov Str., 119333 Moscow, Russia; kvv@fo.gpi.ru * Correspondence: arkady.kim@gmail.com Abstract: A series of glass samples of the tungsten–tellurite system TeO -WO -Bi O -(4-x) La O - 2 3 2 3 2 3 20 3 xEr O , x = 0; 0.4; 0.5; 0.7; 1.2; 2; 4 mol%, C = 0 - 15 10 cm were synthesized from high-purity 2 3 Er oxides in an oxygen flow inside a specialized sealed reactor. In all samples of the series, an extremely 16 3 low content of hydroxyl groups was achieved (~n 10 cm , more than 4 orders of magnitude lower than the concentration of erbium ions), which guarantees minimal effects on the luminescence 3+ properties of Er . The glasses are resistant to crystallization up to 4 mol% Er O , and the glass 2 3 transition temperatures do not depend on the concentration of erbium oxide when introduced by replacing lanthanum oxide. Thin 0.2 mm plates have high transmittance at a level of 20% in the Citation: Dorofeev, V.V.; Koltashev, 4.7–5.3 m range, and the absorption bands of hydroxyl groups at about 2.3, 3, and 4.4 m, which V.V.; Motorin, S.E.; Plekhovich, A.D.; are typical for ordinary tellurite glass samples, are indistinguishable. The introduction of erbium Kim, A.V. Thermal, Optical, and oxide led to an insignificant change in the refractive index. Er O -concentration dependences of the IR-Emission Properties of 2 3 Extremely Low Hydroxyl luminescence intensities and lifetimes near the wavelengths of 1.53 and 2.75 m were found for the 4 4 4 4 3+ TeO -WO -Bi O -La O -xEr O 2 3 2 3 2 3 2 3 I – I and I – I transitions of the Er ion. The data obtained are necessary for the 13/ 2 15/ 2 11 13/ /2 2 / 3+ Glasses for Mid-Infrared Photonics. development of mid-infrared photonics; in particular, for the design of Er -doped fiber lasers. Photonics 2021, 8, 320. https:// doi.org/10.3390/photonics8080320 Keywords: high-purity tellurite glass; Er O content; hydroxyl groups; crystallization; glass 2 3 transition temperature; luminescence; lifetime Received: 12 July 2021 Accepted: 3 August 2021 Published: 9 August 2021 1. Introduction Publisher’s Note: MDPI stays neutral Modern-day applications of mid-infrared photonics, which encompasses the gener- with regard to jurisdictional claims in ation, manipulation, transmission, and detection of mid-IR radiation, have undoubtedly published maps and institutional affil- become possible with the technological advancements in the material growth and forma- iations. tion of new composites, particularly of specialty glasses, such as chalcogenide, fluoride, and telluride glasses, with their unique properties, relevant for use in photonic devices. Here, we will pay special attention to the tellurite glasses, due to their distinctive properties: they are transparent in a wide spectral range of 0.4–5.5 m; have good chemical stability Copyright: © 2021 by the authors. and solubility of rare-earth oxides; the best compositions are sufficiently stable against Licensee MDPI, Basel, Switzerland. crystallization; and low phonon energy makes it possible to achieve stimulated emission at This article is an open access article electronic transitions of rare-earth ions, which are nonradiative for most oxide glasses [1–6]. distributed under the terms and This allows the use of glasses based on tellurium dioxide as active media for fiber-optic conditions of the Creative Commons devices in the IR range of 1–3 m, in which the most important area of application lies Attribution (CC BY) license (https:// beyond 2.2 m, where step-index silicate fibers are inoperative. Tellurite glass fibers are creativecommons.org/licenses/by/ efficient up to 3–3.5 m [7], are transparent in the pumping range up to 1 m, and have 4.0/). Photonics 2021, 8, 320. https://doi.org/10.3390/photonics8080320 https://www.mdpi.com/journal/photonics Photonics 2021, 8, 320 2 of 13 already demonstrated the ability for lasing in active fibers, including generation near 2.3 m [8,9]. At present, fiber laser sources in the 1–3 m range are in great demand for solving many fundamental and applied problems. Due to the presence of absorption bands of many inorganic and organic molecules, primarily absorption bands of hydroxyl groups in solids (including biological tissues) [9–13], such sources are in demand in laser surgery, cosmetic medicine, atmospheric monitoring systems, remote sensing, and diagnostics, as well as for the well-known needs of telecommunications and radiophotonics. In this work, tungsten-tellurite glass containing lanthanum and bismuth oxides was chosen as a matrix composition for the introduction of erbium. Among most tellurite glass compositions, systems based on TeO -WO have the advantages of higher glass 2 3 transition temperatures, nonlinear optical properties, and crystallization resistance, and have a relatively low thermal expansion. Resistance to crystallization can be significantly improved by using high-purity starting materials [14] and modifying the glass composition with lanthanum oxide La O . Some TeO -WO -La O glasses are extremely resistant to 2 3 2 3 2 3 crystallization in a wide range of La O concentrations [2,15,16]. In addition, the presence 2 3 of lanthanum oxide in the glass composition allows the introduction of active additives of other rare earth oxides instead of La O , without significant changes in the physicochemical 2 3 properties. The Bi O additive is used to create fiber structures by modifying the refractive 2 3 index of the core. Bismuth oxide is excellently soluble in tungsten–tellurite matrices, has a positive effect on the stability of some compositions to crystallization [6,16,17], and significantly increases the linear and nonlinear refractive indices [18,19]. Thus, TeO -WO -La O -Bi O tellurite glass is a good candidate for the manufacture 2 3 2 3 2 3 of step-index fibers [15,16]. Studies of the thermal, optical, and emission properties of erbium-doped tellurite glasses of various compositions have been of considerable interest to researchers for several decades. In [20], the effect of the addition of Er O on the thermodynamic functions 2 3 of the tungsten–tellurite system TeO -WO -La O -Bi O was studied from the point of 2 3 2 3 2 3 view of developing a method for predicting the thermodynamic properties of unexplored glass compositions. G.N. Boetti et al. [21] fabricated samples as follows: TeO -WO -Na O-Nb O -xEr O , 2 3 2 2 5 2 3 where x = 0.01, 0.1, 0.5, 1, 3 wt%. The glass transition temperature and glass density were found to increase monotonically with Er O content. The refractive index of the 2 3 3+ glasses decreased with increasing Er ion content. While pumped with a commercial telecom 980 nm laser diode, the 1.5 m emission band became broader with the increasing concentration of Er ions. The maximum doping concentration allowed was found to 20 3 4 be around 1.77 10 ions/cm , for which a lifetime of 3.4 ms for the I level was 13/2 measured. The lifetime of the I state decreased with increasing Er ion concentration, 13/2 due to the energy transfer process. The longest lifetime of 3.7 ms was measured for the 19 3 4 sample with a doping level of 8.9 10 ions/cm . The lifetime of the I state of 11/2 3+ Er remained unchanged with the increase of its concentration and was at a level of 140 30 s. Quaternary tellurite glass systems TeO -WO -TiO -xEr O with x = 0.01, 0.1, 2 3 2 2 3 1, 3, 5, and 7 mol% of Er O were prepared and investigated in [22]. All the samples 2 3 possessed a thermal stability higher than 100 K, except the glass sample with 7 mol% of Er O characterized by a low thermal stability of 74 K. The glass transition temperature 2 3 significantly increased with the Er O content, increasing from 0.01 to 7 mol% [22]. The 2 3 authors of [23] studied the influence of Er O addition on the thermal behavior of tungsten– 2 3 tellurite glasses TeO -WO doped with 0.5 and 1.0 mol% of Er O by running detailed 2 3 2 3 differential thermal analyses. Introducing rare-earth elements into tungsten–tellurite glasses and increasing their content resulted in an increase in glass transition temperatures. The erbium ion in the tellurite matrix in the IR region is characterized by three luminescence bands, with maxima of ~1, ~1.55, and ~2.75 m, corresponding to the 4 4 4 4 4 4 electronic transitions I – I , I – I , and I – I [24]. However, for a long 11/2 15/2 13/2 15/2 11/2 13/2 time, studies of the luminescence properties of the erbium ion focused on emission from Photonics 2021, 8, 320 3 of 13 4 4 the long-lived I level. This is due to the fast nonradiative relaxation of the I level 13/2 11/2 by hydroxyl groups in tellurite glasses obtained by a trivial method in air. Only with the advent of progressive methods for drying the melt, was the study of the emission of about 2.75 m intensified. 3+ The most convenient absorption band for the activation of the Er ion in tellurite glasses is the absorption band at about 980 nm, which corresponds to the wavelengths of inexpensive standard commercial laser diodes. In this case, the level of I is excited, 11/2 4 4 and then the transition from the I level to I occurs. For silica glass, due to the high 11/2 13/2 phonon energy, the level of I is depopulated without radiation, but for tellurite glasses, 11/2 a radiative transition in the 2.7–2.8 m range is possible. The I level is filled, and 13/2 4 4 generation in the range of 1.53–1.6 m can be achieved at the I – I transition [25,26]. 13/2 15/2 3+ The influence of the concentration of Er ions on the luminescent properties of TeO -WO -ZnO glasses was studied in [27]. It was noted that with an increase in the 2 3 3+ 20 20 3 concentration of Er ions from 1.66 10 to 4.11 10 cm , the intensity and width of the luminescence band of about 1.5 m increase, and the decay time of luminescence decreases from 3.6 ms to 3.3 ms, which indicates the appearance of concentration quenching. 4 4 Luminescence in the 2.7–2.8 m region at the I – I transition in tellurite glasses 11/2 13/2 was studied in [24,25,28]. For various compositions of tungsten–tellurite glasses, the 4 4 lifetime of the I level (~100 s) is much shorter than the lifetime of the I level 11/2 13/2 (several ms), due to the moderate phonon energy of ~900 cm [2,5,25]. With continuous 4 4 wave (CW) pumping at the I – I transition, this leads to a high population at the 15/2 11/2 4 4 I level and a small population at the I level. Under laser pumping at 978 nm 13/2 11/2 3+ in Er doped tungsten–tellurite glasses, the luminescence intensity in the 2.7 m region increased with increasing concentration of Er O [24]. 2 3 3+ The main channel of nonradiative relaxation in tellurite glasses activated with Er is quenching on vibrations of OH groups. Due to the presence of an OH absorption band of 3+ 4 about 3 m, the internal energy of Er at the I level is converted into the vibration 13/2 energy of two hydroxyl groups. As a result, OH ions cause both nonradiative relaxation of 3+ the excited energy level and absorption of Er luminescence radiation at a wavelength 4 4 of about 1.5 m. These effects are even more pronounced at the I – I transition, 11/2 13/2 since only one hydroxyl group is involved [29]. Thus, to obtain an active medium for lasing, it is especially important to synthesize and study only glass with a minimum hydroxyl concentration. There has been a significant number of publications studying the properties of various erbium-activated tellurite glass compositions. However, information about the properties of erbium-activated glasses of the lanthanum–tungsten–tellurite system is insufficient, although they have already proven their promise for fiber optics applications [5,7,30]. In connection with the above, in this work we studied the properties of TeO -WO - 2 3 Bi O -La O -xEr O glasses, which are important for use in fiber optics, depending on 2 3 2 3 2 3 the erbium concentration. Crystallization stability, glass transition temperatures, the 3+ transmission spectra and absorption bands of Er and OH groups, refractive indices, and luminescence characteristics were studied. To obtain the most accurate data, special attention was paid to reducing the concentration of impurities, primarily hydroxyl groups, through the use of pure starting materials and original synthesis technology. 2. Materials and Methods 2.1. Glass Samples Preparation Erbium-doped tungsten–tellurite glasses were produced by melting the oxides in crucibles of platinum inside a sealed silica chamber in an atmosphere of purified oxygen. A “TWBL-xEr” series of glass compositions with a common formula TeO -WO -(4-x)La O - 2 3 2 3 Bi O -xEr O , where x = 0; 0.4; 0.5; 0.7; 1.2; 2; 4 mol%, was produced. The compositions of 2 3 2 3 the glasses of the studied series, designations, the content of the erbium oxide dopant, and hydroxyl groups absorption at ~3 m are listed in Table 1. Photonics 2021, 8, 320 4 of 13 Table 1. The compositions of glasses of the studied TWBL-xEr series, designations, the content of the erbium oxide dopant, and hydroxyl group absorption at ~3 m. 3+ Designation of the Er O Content, Er Ions Content, OH Groups Volume 2 3 Composition, mol% 3 1 Glasses mol% cm Absorption at ~3 m, cm 71.2TeO -23.7WO -1.1Bi O - 2 3 2 3 TWBL-0Er 0 0 - 4La O -0Er O 2 3 2 3 71.2TeO -23.7WO -1.1Bi O - 2 3 2 3 TWBL-0.4Er 0.4 1.54 10 0.003 3.6La O -0.4Er O 2 3 2 3 71.2TeO -23.7WO -1.1Bi O - 2 3 2 3 TWBL-0.5Er 0.5 1.93 10 0.005 3.5La O -0.5Er O 2 3 2 3 71.2TeO -23.7WO -1.1Bi O - 2 3 2 3 TWBL-0.7Er 0.7 2.7 10 0.005 3.3La O -0.7Er O 2 3 2 3 71.2TeO -23.7WO -1.1Bi O - 2 3 2 3 20 TWBL-1.2Er 1.2 0.006 4.61 10 2.8La O -1.2Er O 2 3 2 3 71.2TeO -23.7WO -1.1Bi O - 2 3 2 3 TWBL-2Er 2 7.64 10 0.011 3.5La O -2Er O 2 3 2 3 71.2TeO -23.7WO -1.1Bi O - 2 3 2 3 TWBL-4Er 4 15.2 10 0.009 0La O -4Er O 2 3 2 3 The gradation of the erbium oxide concentration was achieved by lanthanum oxide re- placement. All the samples were synthesized with a low and approximately equal hydroxyl con- 3 1 tent, corresponding to OH-group volume absorption at the level of n 10 cm at a wave- 3+ 20 20 3 length of ~3 m; the concentrations of Er were in the range of 1.54 10 –15 10 cm . The glass-forming matrix system for activation by erbium ions was chosen on the basis of our previous studies, which showed that glasses of similar compositions have high transparency in the IR range, resistance to crystallization, and that high-quality optical elements and fibers can be successfully made from them [5–8]. The binary glass TeO -WO 2 3 was the basis, the matrix for the compositions used, and compares favorably with other widely studied compositions based on TeO -ZnO, with higher stability and solubility of REI, better mechanical properties, and significantly lower values of thermal expansion coefficient [20,31,32]. Lanthanum oxide La O was included in the composition of tungsten–tellurite glass 2 3 to increase the resistance to crystallization and for convenience of introducing erbium oxide. Bismuth oxide Bi O was added to modify the refractive index of the core when 2 3 creating light-guide structures. The glasses were prepared from high-purity tellurium dioxide (TeO ) obtained by vacuum distillation and from commercially available high-purity tungsten (WO ), bis- muth (Bi O ), lanthanum (La O ), and erbium (Er O ) oxides. The total content of the 2 3 2 3 2 3 3d-transition metal impurities, most actively absorbed in the IR region, in the initial oxide mixture did not exceed 2 ppm wt [6]. The sample preparation technique included a number of successive stages: reduced pressure drying of the initial oxides batch, melting at 800 C for several hours, lowering the temperature of the glass-forming melt and casting samples into a cylindrical mold of silica glass, annealing at the glass transition temperature, and slow cooling to room temperature. After cooling to room temperature, the castings were mechanically cut, ground, and polished for further study. Tablets 0.2 cm thick were used for the majority of optical measurements, while longer samples (0.6–1.5 cm long) were applied for OH volume absorption evaluation of erbium containing compositions (Figure 1). The prepared samples were optically homogeneous and did not contain large scattering defects in the volume; the samples with the highest concentration of Er O (2 and 4 mol%) were characterized by 2 3 a darker color. Photonics 2021, 8, x FOR PEER REVIEW 5 of 14 the temperature of the glass-forming melt and casting samples into a cylindrical mold of silica glass, annealing at the glass transition temperature, and slow cooling to room tem- perature. After cooling to room temperature, the castings were mechanically cut, ground, and polished for further study. Tablets 0.2 cm thick were used for the majority of optical measurements, while longer samples (0.6–1.5 cm long) were applied for OH volume ab- sorption evaluation of erbium containing compositions (Figure 1). The prepared samples were optically homogeneous and did not contain large scattering defects in the volume; Photonics 2021, 8, 320 5 of 13 the samples with the highest concentration of Er2O3 (2 and 4 mol%) were characterized by a darker color. Figure 1. Photographs of the polished discs (tablets) and cylinders made of TeO -WO -Bi O -(4-x)La O -xEr O glasses. Figure 1. Photographs of the polished discs (tablets) and cylinders made of TeO 2 2-WO 33-Bi2 2O3 3-(4-x)La2 2O33 -xEr2O 2 3 glasses. 3 2.2. Methods 2.2. Methods Differential scanning calorimetry (DSC) data were obtained on a Netzsch DSC 404 Differential scanning calorimetry (DSC) data were obtained on a Netzsch DSC 404 F1 F1 Pegasus device. Samples of glass in the form of disks with a diameter of ~5 mm and a Pegasus device. Samples of glass in the form of disks with a diameter of ~5 mm and a thickness of ~1 mm were used. Measurements were carried out in platinum crucibles in thickness of ~1 mm were used. Measurements were carried out in platinum crucibles in the temperature range of 300–950 K, at a thermal scanning rate of 10 K/min in a stream of the temperature range of 300–950 K, at a thermal scanning rate of 10 K/min in a stream of pure argon, with a flow rate of 80 mL/min. pure argon, with a flow rate of 80 mL/min. The transmittance spectra of the series TWBL-xEr were recorded with Lambda 900 spec- The transmittance spectra of the series TWBL-xEr were recorded with Lambda 900 trometer in the visible and near-IR regions and by a IR Nicolet 6700 Fourier spectrometer spectrometer in the visible and near-IR regions and by a IR Nicolet 6700 Fourier spectrom- in the IR range. Absorption spectra inside the hydroxyl groups absorption band were eter in the IR range. Absorption spectra inside the hydroxyl groups absorption band were calculated from the IR transmittance spectra by the expression ln(100/T%), taking into calculated from the IR transmittance spectra by the expression ln(100/T%), taking into ac- account Fresnel reflection by subtracting the straight line. The volume absorption co- count Fresnel reflection by subtracting the straight line. The volume absorption coefficient efficient of hydroxyl groups in the band maximum was calculated using the formula of hydroxyl groups in the band maximum was calculated using the formula α = = (ln(100/T%) 2 )/L (cm ); where L (cm) is the sample length, and 2 is the absorp- −1 (ln(100/T%) − 2β)/L (cm ); where L (cm) is the sample length, and 2β is the absorption by tion by surface hydroxyl groups at the ends. surface hydroxyl groups at the ends. The refractive indices were measured using a prism-coupler Metricon-2010 at the The refractive indices were measured using a prism-coupler Metricon-2010 at the wavelengths 633, 969, and 1539 nm. Three scans were made during each measurement; the wavelengths 633, 969, and 1539 nm. Three scans were made during each measurement; error was estimated to be 0.0005. the error was estimated to be ±0.0005. The luminescence spectra were recorded with a photovoltaic InSb detector P5968, The luminescence spectra were recorded with a photovoltaic InSb detector P5968, using the excitation under pumping laser diode 975 nm with a power of 0.5 W. The radiation using the excitation under pumping laser diode 975 nm with a power of 0.5 W. The radi- was focused on the sample using a lens; the scattered emission radiation was collected ation was focused on the sample using a lens; the scattered emission radiation was col- using another lens at the input slit of the monochromator MDR-2. A filter was used to lected using another lens at the input slit of the monochromator MDR-2. A filter was used cut off the pump spectrum. The kinetics of the luminescence were registered according to cut off the pump spectrum. The kinetics of the luminescence were registered according to the same scheme, using a LeCroy oscilloscope and 976 nm optical parametric oscillator to the same scheme, using a LeCroy oscilloscope and 976 nm optical parametric oscillator 4 4 excitation with a pulse duration of ~5 ns. The decay curves for the I – I transition 13/2 15/2 4 4 excitation with a pulse duration of ~5 ns. The decay curves for the I13/2– I15/2 transition were were obtained directly from 1.53 emission data, and the lifetimes of the I level were 11/2 obtained directly from 1.53 emission data, and the lifetimes of the I11/2 level were deter- determined by recording the 0.98 m emission using an infrared PMT with a photocathode mined by recording the 0.98 µm emission using an infrared PMT with a photocathode having a time response of ~20 ns. having a time response of ~20 ns. 3. Results 3.1. Thermal Properties The thermograms of the differential scanning calorimetry of the TWBL-xEr series are shown in Figure 2. The insert shows the DSC curves imposed in the temperature area of the glass transition, to illustrate the absence of the dependence of the glass transition temperature on the concentration of erbium oxide. Photonics 2021, 8, x FOR PEER REVIEW 6 of 14 3. Results 3.1. Thermal Properties The thermograms of the differential scanning calorimetry of the TWBL-xEr series are shown in Figure 2. The insert shows the DSC curves imposed in the temperature area of the glass transition, to illustrate the absence of the dependence of the glass transition tem- perature on the concentration of erbium oxide. There are no clear thermal effects of the crystallization and melting of crystals in the thermograms, which indicates the crystallization stability of the glasses of the series. In- creasing the concentration of erbium oxide to 4 mol% practically does not change the glass transition temperature, which is equal to ~390 °C for all samples (insert in Figure 2). Thus, replacing lanthanum oxide with an equimolar amount of erbium oxide allows activating Photonics 2021, 8, 320 6 of 13 the core with Er ions, without changing the viscosity properties. This is very important in the process of making fibers. exo TWBL-4Er 350 360 370 380 390 400 410 420 430 440 450 TWBL-2Er TWBL-1.2Er TWBL-0.7Er TWBL-0.5Er TWBL-0.4Er TWBL-0Er 350 400 450 500 550 600 650 700 Temperature, C Figure 2. DSC-thermograms of TeO -WO -Bi O -(4-x)La O -xEr O glasses, x = 0; 0.4; 0.5; 0.7; 1.2; 2; 2 3 2 3 2 3 2 3 Figure 2. DSC-thermograms of TeO2-WO3-Bi2O3-(4-x)La2O3-xEr2O3 glasses, x = 0; 0.4; 0.5; 0.7; 1.2; 2; 4 mol% (heating rate 10 K/min). 4 mol% (heating rate 10 K/min). There are no clear thermal effects of the crystallization and melting of crystals in 3.2. Transmission Spectra and Hydroxyl Groups Absorption the thermograms, which indicates the crystallization stability of the glasses of the series. The TWBL-xEr glasses have high transparency in the visible and IR regions, from Increasing the concentration of erbium oxide to 4 mol% practically does not change the glass 0.47 to 5.3 µm. The spectra of the samples in the visible and IR regions are shown in Fig- transition temperature, which is equal to ~390 C for all samples (insert in Figure 2). Thus, ures 3 and 4. The absence of characteristic absorption bands of 3d-transition metals and replacing lanthanum oxide with an equimolar amount of erbium oxide allows activating undesirable impurity RE elements throughout the transparency area confirms the low im- the core with Er ions, without changing the viscosity properties. This is very important in purity content. the process of making fibers. 3+ The transmission spectra in the short-wave region contain typical Er absorption bands, with peaks at 1530, 798, 653, 543, 521, and 489 nm corresponding to the absorption 3.2. Transmission Spectra and Hydroxyl Groups Absorption 4 4 4 4 4 2 4 from the ground state of I15/2 to the excited levels of I13/2, I9/2, F9/2, S3/2, H11/2, and F7/2. The The TWBL-xEr glasses have high transparency in the visible and IR regions, from intensity of the absorption peaks of erbium increases with its concentration. For pumping, 0.47 to 5.3 m. The spectra of the samples in the visible and IR regions are shown in while studying the luminescent characteristics of glasses and fibers, an absorption band Figures 3 and 4. The absence of characteristic absorption bands of 3d-transition metals 4 4 Photonics 2021, 8, x FOR PEER REVIEW 7 of 14 corresponding to the I15/2– I11/2 transition, with a maximum at about 0.98 µm, was chosen. and undesirable impurity RE elements throughout the transparency area confirms the low The absorption coefficient in this band is directly proportional to the concentration of impurity content. Er2O3 (Figure 3). TWBL-0Er TWBL-0.4Er TWBL-0.5Er TWBL-0.7Er TWBL-1.2Er TWBL-2Er TWBL-4Er 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 Wavelength, μm Figure 3. Visible and near-IR transmission spectra of TeO -WO -Bi O -(4-x)La O -xEr O glass Figure 3. Visible and near-IR transmission spectra of TeO2-WO3-Bi2O3-(4-x)La2O3-xEr2O3 glass sam- 2 3 2 3 2 3 2 3 samples ples 0.2 c 0.2 m t cm hicthick, k, x = 0; x 0. = 0; 4; 0.5 0.4;; 0.7 0.5;; 1 0.7; .2; 2; 4 m 1.2; 2; o 4l%. mol%. The thin discs of TWBL-xEr glass, with a thickness of 2 mm (tablets, Figure 1), have a high transmittance at a level of at least 20% in the near and mid-IR ranges, up to a wave- length of ~5.3 µm (Figure 4). It is possible to note absorption bands at 5.4 and 5.7 µm in the spectra of the tablets, characteristic of the overtones of O=W bond vibrations in single and paired O=WO5 centers in the first case, and of a combination of O=W and W–O–W vibrations in pairs of single O=WO5 centers in the second [6]. The absorption bands of hydroxyl groups, with peaks of about 2.3, 3, and 4.4 microns characteristic of tellurite glasses obtained by the traditional method in open systems, are indistinguishable in the spectra of TWBL-xEr tablets. To calculate the volume absorption coefficient of the hy- droxyl groups, samples of glasses in the form of longer cylinders (0.6–1.5 cm long) were used (Figure 1). In the transmission spectra of such samples, an absorption band with a maximum near 3 microns appears and can be mathematically processed (Figure 4). 0.2 cm_TWBL-0Er 0.2 cm_TWBL-0.4Er 0.2 cm_TWBL-0.5Er 1.5 cm_TWBL-0.4Er 50 0.2 cm_TWBL-0.7Er 1.5 cm_TWBL-0.5Er 0.2 cm_TWBL-1.2Er 1.5 cm_TWBL-0.7Er 0.2 cm_TWBL-2Er 1.5 cm_TWBL-1.2Er 0.2 cm_TWBL-4Er 0.6 cm_TWBL-2Er 0.66 cm_TWBL-4Er 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5 Wavelength, μm Figure 4. IR transmission spectra of TeO2-WO3-Bi2O3-(4-x)La2O3-xEr2O3 glass samples with different thicknesses. The absorption spectra inside the hydroxyl groups absorption band, calculated from IR transmittance spectra by the expression ln(100/T%), are plotted in Figure 5. Transmittance, % DSC, signal Transmittance, % Photonics 2021, 8, x FOR PEER REVIEW 7 of 14 TWBL-0Er TWBL-0.4Er TWBL-0.5Er TWBL-0.7Er TWBL-1.2Er TWBL-2Er TWBL-4Er 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 Wavelength, μm Figure 3. Visible and near-IR transmission spectra of TeO2-WO3-Bi2O3-(4-x)La2O3-xEr2O3 glass sam- ples 0.2 cm thick, x = 0; 0.4; 0.5; 0.7; 1.2; 2; 4 mol%. The thin discs of TWBL-xEr glass, with a thickness of 2 mm (tablets, Figure 1), have a high transmittance at a level of at least 20% in the near and mid-IR ranges, up to a wave- length of ~5.3 µm (Figure 4). It is possible to note absorption bands at 5.4 and 5.7 µm in the spectra of the tablets, characteristic of the overtones of O=W bond vibrations in single and paired O=WO5 centers in the first case, and of a combination of O=W and W–O–W vibrations in pairs of single O=WO5 centers in the second [6]. The absorption bands of hydroxyl groups, with peaks of about 2.3, 3, and 4.4 microns characteristic of tellurite glasses obtained by the traditional method in open systems, are indistinguishable in the spectra of TWBL-xEr tablets. To calculate the volume absorption coefficient of the hy- droxyl groups, samples of glasses in the form of longer cylinders (0.6–1.5 cm long) were Photonics 2021, 8, 320 7 of 13 used (Figure 1). In the transmission spectra of such samples, an absorption band with a maximum near 3 microns appears and can be mathematically processed (Figure 4). 0.2 cm_TWBL-0Er 0.2 cm_TWBL-0.4Er 0.2 cm_TWBL-0.5Er 1.5 cm_TWBL-0.4Er 50 0.2 cm_TWBL-0.7Er 1.5 cm_TWBL-0.5Er 0.2 cm_TWBL-1.2Er 1.5 cm_TWBL-0.7Er 0.2 cm_TWBL-2Er 1.5 cm_TWBL-1.2Er 0.2 cm_TWBL-4Er 0.6 cm_TWBL-2Er 0.66 cm_TWBL-4Er 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5 Wavelength, μm Figure 4. IR transmission spectra of TeO -WO -Bi O -(4-x)La O -xEr O glass samples with differ- Figure 4. IR transmission spectra of TeO2-W 2 O3-Bi 3 2O2 3-(4-x)La 3 2O32 -xEr 3 2O3 g 2 la 3ss samples with different ent thicknesses. thicknesses. 3+ The transmission spectra in the short-wave region contain typical Er absorption The absorption spectra inside the hydroxyl groups absorption band, calculated from bands, with peaks at 1530, 798, 653, 543, 521, and 489 nm corresponding to the absorption IR transmittance spectra by the expression ln(100/T%), are plotted in Figure 5. 4 4 4 4 4 2 from the ground state of I to the excited levels of I , I , F , S , H , 15/2 13/2 9/2 9/2 3/2 11/2 and F . The intensity of the absorption peaks of erbium increases with its concentra- 7/2 tion. For pumping, while studying the luminescent characteristics of glasses and fibers, 4 4 an absorption band corresponding to the I – I transition, with a maximum at about 15/2 11/2 0.98 m, was chosen. The absorption coefficient in this band is directly proportional to the concentration of Er O (Figure 3). 2 3 The thin discs of TWBL-xEr glass, with a thickness of 2 mm (tablets, Figure 1), have a high transmittance at a level of at least 20% in the near and mid-IR ranges, up to a wavelength of ~5.3 m (Figure 4). It is possible to note absorption bands at 5.4 and 5.7 m in the spectra of the tablets, characteristic of the overtones of O=W bond vibrations in single and paired O=WO centers in the first case, and of a combination of O=W and W–O–W vibrations in pairs of single O=WO centers in the second [6]. The absorption bands of hydroxyl groups, with peaks of about 2.3, 3, and 4.4 microns characteristic of tellurite glasses obtained by the traditional method in open systems, are indistinguishable in the spectra of TWBL-xEr tablets. To calculate the volume absorption coefficient of the hydroxyl groups, samples of glasses in the form of longer cylinders (0.6–1.5 cm long) were used (Figure 1). In the transmission spectra of such samples, an absorption band with a maximum near 3 microns appears and can be mathematically processed (Figure 4). The absorption spectra inside the hydroxyl groups absorption band, calculated from IR transmittance spectra by the expression ln(100/T%), are plotted in Figure 5. The absorption values of hydroxyl in samples of the TWBL-xEr series at the maxi- mum of the band were found to be 0.007 for TWBL-0.4Er; 0.01 for TWBL-0.5Er; 0.011 for TWBL-0.7Er; 0.012 for TWBL-1.2Er; 0.01 for TWBL-2Er; and 0.009 for TWBL-4Er. The volume absorption coefficient of hydroxyl groups in the band maximum can be calculated from: (cm ) = (ln(100/T%) 2 )/L; where L (cm) is the sample length, is the absorp- tion at the two ends by hydroxyl groups adsorbed from the air or during polishing [5,32,33]. Taking into account the surface absorption of hydroxyl groups (on average 2 0.003 for polished samples of tungsten–tellurite glasses [32,33]) and the actual length of the samples, the volume absorption coefficients at the band peak of ~3 m were calculated (Table 1). The values are in the range of 0.003–0.011 cm , which corresponds to the concentration 16 3 of hydroxyl groups at the level of n 10 cm [5,33,34]. Thus, the concentration of 3+ 4 hydroxyl groups is inferior to the concentration of Er by at least 10 times, allowing excluding the influence of this important impurity on the accuracy of determining the emission characteristics. A very important conclusion from a practical point of view is that Transmittance, % Transmittance, % Photonics 2021, 8, 320 8 of 13 Photonics 2021, 8, x FOR PEER REVIEW 8 of 14 there is no deterioration in the effectiveness of our method for removing hydroxyl groups with an increase in the concentration of erbium in the glass-forming melt. 1.5 cm_TWBL-0.4Er 1.5 cm_TWBL-0.5Er 0.012 1.5 cm_TWBL-0.7Er 1.5 cm_TWBL-1.2Er 0.010 0.6 cm_TWBL-2Er 0.66 cm_TWBL-4Er 0.008 0.006 0.004 0.002 0.000 2.8 2.9 3.0 3.1 3.2 3.3 Wavelength, μm Figure 5. Absorption spectra of TeO -WO -Bi O -(4-x)La O -xEr O glass samples with different 2 3 2 3 2 3 2 3 Figure 5. Absorption spectra of TeO2-WO3-Bi2O3-(4-x)La2O3-xEr2O3 glass samples with different thicknesses within the hydroxyl groups band. thicknesses within the hydroxyl groups band. 3.3. Refraction Index The absorption values of hydroxyl in samples of the TWBL-xEr series at the maxi- The values of the linear refractive index were determined for samples with Er O 2 3 mum of the band were found to be 0.007 for TWBL-0.4Er; 0.01 for TWBL-0.5Er; 0.011 for contents of 0; 1.2; 4 mol% (TWBL-0Er, TWBL-1.2Er, TWBL-4Er). The dependence of the TWBL-0.7Er; 0.012 for TWBL-1.2Er; 0.01 for TWBL-2Er; and 0.009 for TWBL-4Er. The vol- refractive index values on the concentration of erbium oxide at wavelengths of 633, 969, and ume absorption coefficient of hydroxyl groups in the band maximum can be calculated 1539 nm is shown in Figure 6. The introduction of erbium oxide into the tungsten–tellurite −1 from: α (cm ) = (ln(100/T%) − 2β)/L; where L (cm) is the sample length, β is the absorption matrix by replacing lanthanum oxide leads to a very slight decrease in the refractive index, at the two ends by hydroxyl groups adsorbed from the air or during polishing [5,32,33]. even at high dopant concentrations; the values of the slope of the lines are in the order of Taking into account the surface absorption of hydroxyl groups (on average 2β ≈ 0.003 for Photonics 2021, 8, x FOR PEER REVIEW 9 of 14 0.001 (Figure 6). Having only a minor change in the refractive index is highly desirable polished samples of tungsten–tellurite glasses [32,33]) and the actual length of the sam- 3+ when designing fibers with a core activated with Er . ples, the volume absorption coefficients at the band peak of ~3 µm were calculated (Table −1 1). The values are in the range of 0.003–0.011 cm , which corresponds to the concentration 16 −3 of hydroxyl groups at the level of n × 10 cm [5,33,34]. Thus, the concentration of hy- 2.18 3+ 4 droxyl groups is inferior to the concentration of Er by at least 10 times, allowing exclud- Wavelength 633 nm 2.16 Y = 2.15 - 0.0007X ing the influence of this important impurity on the accuracy of determining the emission characteristics. A very important conclusion from a practical point of view is that there is 2.14 no deterioration in the effectiveness of our method for removing hydroxyl groups with an Wavelength 969 nm increase in the concentration of erbium in the glass-forming melt. 2.12 Y = 2.11 - 0.0012X 2.10 3.3. Refraction Index Y = 2.09 - 0.001X The values of the linear refractive index were determined for samples with Er2O3 2.08 Wavelength 1539 nm contents of 0; 1.2; 4 mol% (TWBL-0Er, TWBL-1.2Er, TWBL-4Er). The dependence of the refract 2.06ive index values on the concentration of erbium oxide at wavelengths of 633, 969, and 1539 nm is shown in Figure 6. The introduction of erbium oxide into the tungsten– 01 23 4 tellurite matrix by replacing lanthanum oxide leads to a very slight decrease in the refrac- Er O content, mol% 2 3 tive index, even at high dopant concentrations; the values of the slope of the lines are in the order of −0.001 (Figure 6). Having only a minor change in the refractive index is highly Figure 6. Refractive index of TeO2-WO3-Bi2O3-(4-x)La2O3-xEr2O3 glass samples versus Er2O3 concen- Figure 6. Refractive index of TeO -WO -Bi O -(4-x)La O -xEr O glass samples versus Er O 2 3 2 3 2 3 2 3 2 3 3+ desirable when designing fibers with a core activated with Er . tration for wavelengths of 633, 969, and 1539 nm. concentration for wavelengths of 633, 969, and 1539 nm. 3.4. Luminescent Properties 3.4. Luminescent Properties At the excitation at 0.975 m, broad luminescence bands with maxima at ~1.53 and At the excitation at 0.975 µm, broad luminescence bands with maxima at ~1.53 and 4 4 4 4 3+ ~2.75 m, corresponding to electronic transitions 4 I 4 – I and 4 I 4 – I 3+ of Er for 13/2 15/2 11/2 13/2 ~2.75 µm, corresponding to electronic transitions I13/2– I15/2 and I11/2– I13/2 of Er for TWBL- TWBL-xEr series glasses, were observed. xEr series glasses, were observed. 3+ Figure 6 presents the experimental normalized luminescence spectra inside the Er band with a peak at 1.53 µm of the TeO2-WO3-Bi2O3-(4-x)La2O3-xEr2O3 glasses with differ- ent Er2O3 contents. The emission bandwidth and the luminescence intensity near 1.53 µm 3+ increase with increasing Er concentration; the samples with an erbium oxide content of 1.2 and 2% have the highest values; and an increase in concentration leads to a decrease in these characteristics of emission (Figures 7 and 8). In the region of low erbium oxide concentrations (up to 0.7 mol%), the intensity in- creases strongly; in the region of medium concentrations (1.2–2 mol%), the intensity in- creases insignificantly; and with a further increase in the concentration, the 1.53 µm emis- sion intensity drops. This may be mainly due to the complete absorption of the pump radiation at the ini- 3+ tial stage. However, when the Er concentration becomes close enough to the absorption saturation, the increase in the radiation intensity slows down. At the same time, the dis- 3+ 3+ tance between Er ions becomes closer, and the upconversion effect of Er grows with an increase in its concentration. This reduces the population of the I13/2 level and leads to a sharp decrease in the emission intensity. The behavior of the emission bandwidth as the concentration increases to 1.2 mol% can be explained by an increase in the variety of dopant sites in the glass matrix occupied 3+ by Er ions, together with an increase in the number of ions. The luminescence spectrum broadens at this stage. Further termination of the broadening of the spectrum, and its nar- rowing at the highest erbium concentration, is associated with the occupation of all pos- sible dopant places and with the increase of upconversion intensity. Similar effects were observed in [25]. Refractive index Absorption, arb.u. Photonics 2021, 8, 320 9 of 13 3+ Figure 6 presents the experimental normalized luminescence spectra inside the Er band with a peak at 1.53 m of the TeO -WO -Bi O -(4-x)La O -xEr O glasses with 2 3 2 3 2 3 2 3 different Er O contents. The emission bandwidth and the luminescence intensity near 2 3 3+ 1.53 m increase with increasing Er concentration; the samples with an erbium oxide Photonics 2021, 8, x FOR PEER REVIEW 10 of 14 Photonics 2021, 8, x FOR PEER REVIEW 10 of 14 content of 1.2 and 2% have the highest values; and an increase in concentration leads to a decrease in these characteristics of emission (Figures 7 and 8). 1.0 1.0 TWBL-0.4Er 0.9 TWBL-0.4Er 0.9 TWBL-0.5Er TWBL-0.5Er 0.8 TWBL-0.7Er 0.8 TWBL-0.7Er TWBL-1.2Er 0.7 TWBL-1.2Er 0.7 TWBL-2Er TWBL-2Er 0.6 TWBL-4Er 0.6 TWBL-4Er 0.5 0.5 0.4 0.4 0.3 0.3 0.2 0.2 0.1 0.1 0.0 0.0 1.40 1.45 1.50 1.55 1.60 1.65 1.70 1.40 1.45 1.50 1.55 1.60 1.65 1.70 Wavelength, nm Wavelength, nm 3+ 3+ Figure 7. Luminescence band of Er with peak at 1.53 µm of the TeO2-WO3-Bi2O3-(4-x)La2O3-xEr2O3 Figure 7. Luminescence band of Er with peak at 1.53 m of the TeO -WO -Bi O -(4-x)La O - 3+ 2 3 2 3 2 3 Figure 7. Luminescence band of Er with peak at 1.53 µm of the TeO2-WO3-Bi2O3-(4-x)La2O3-xEr2O3 glasses with different Er2O3 contents. xEr O glasses with different Er O contents. 2 3 2 3 glasses with different Er2O3 contents. 1.0 1.0 0.9 0.9 0.8 0.8 0.7 0.7 0.6 0.6 0.5 0.5 0.4 0.4 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 Er O content, mol% Er2O3 content, mol% 2 3 Figure 8. The 1.53 µm luminescence peak intensity as a function of Er2O3 content. Figure 8. The 1.53 µm luminescence peak intensity as a function of Er2O3 content. Figure 8. The 1.53 m luminescence peak intensity as a function of Er O content. 2 3 The experimental normalized luminescence spectra and the luminescence peak in- In The exper the region imental normalized of low erbium oxide luminconcentrations escence spectra and the (up to 0.7lum mol%), inescence the intensity peak in- 3+ tensity dependence on the Er2O3 content for the Er band near 2.75 µm for TWBL-xEr 3+ incr tensi eases ty dependence on the Er strongly; in the region 2O3 content f of medium or the Er concentrations band ne (1.2–2 ar 2.75 mol%), µm for T the W intensity BL-xEr series glasses are presented in Figures 9 and 10, respectively. The luminescence intensity increases insignificantly; and with a further increase in the concentration, the 1.53 m series glasses are presented in Figures 9 and 10, respectively. The luminescence intensity and bandwidth increase non-linearly with an increase in the doping level; there is no sat- emission intensity drops. and bandwidth increase non-linearly with an increase in the doping level; there is no sat- uration of the dependences (Figures 9 and 10). The highest emission bandwidth and lu- This may be mainly due to the complete absorption of the pump radiation at the initial uration of the dependences (Figures 9 and 10). The highest emission bandwidth and lu- 3+ minescence intensity values were found for the sample with the highest dopant content. stage. However, when the Er concentration becomes close enough to the absorption minescence intensity values were found for the sample with the highest dopant content. The observed behavior can be explained by the same effects as in the case of the 1.53 µm saturation, the increase in the radiation intensity slows down. At the same time, the The observed behavior can be explained by the same effects as in the case of the 1.53 µm 3+ 3+ band, but in the absence of an up-conversion from this level. Thus, high concentrations of distance between Er ions becomes closer, and the upconversion effect of Er grows with band, but in the absence of an up-conversion from this level. Thus, high concentrations of 4 4 3+ the dopant can be used when the transition I11/2– I13/2 of Er is exploited. 4 4 3+ an increase in its concentration. This reduces the population of the I level and leads to the dopant can be used when the transition I11/2– I13/2 of Er is explo 13/2 ited. a sharp decrease in the emission intensity. The behavior of the emission bandwidth as the concentration increases to 1.2 mol% can be explained by an increase in the variety of dopant sites in the glass matrix occupied 3+ by Er ions, together with an increase in the number of ions. The luminescence spectrum broadens at this stage. Further termination of the broadening of the spectrum, and its Peak emission intensity, r.u. In In te te nn sity sity , , r.r. uu . . Peak emission intensity, r.u. Photonics 2021, 8, 320 10 of 13 narrowing at the highest erbium concentration, is associated with the occupation of all possible dopant places and with the increase of upconversion intensity. Similar effects were observed in [25]. The experimental normalized luminescence spectra and the luminescence peak inten- 3+ sity dependence on the Er O content for the Er band near 2.75 m for TWBL-xEr series 2 3 glasses are presented in Figures 9 and 10, respectively. The luminescence intensity and bandwidth increase non-linearly with an increase in the doping level; there is no saturation of the dependences (Figures 9 and 10). The highest emission bandwidth and luminescence intensity values were found for the sample with the highest dopant content. The observed behavior can be explained by the same effects as in the case of the 1.53 m band, but in the Photonics 2021, 8, x FOR PEER REVIEW 11 of 14 Photonics 2021, 8, x FOR PEER REVIEW 11 of 14 absence of an up-conversion from this level. Thus, high concentrations of the dopant can 4 4 3+ be used when the transition I – I of Er is exploited. 11/2 13/2 1.0 TWBL-0.4Er 1.0 TWBL-0.4Er TWBL-0.5Er 0.9 TWBL-0.5Er 0.9 TWBL-0.7Er TWBL-0.7Er 0.8 TWBL-1.2Er 0.8 TWBL-1.2Er TWBL-2Er 0.7 TWBL-2Er 0.7 TWBL-4Er TWBL-4Er 0.6 0.6 0.5 0.5 0.4 0.4 0.3 0.3 0.2 0.2 0.1 0.1 0.0 0.0 2.55 2.60 2.65 2.70 2.75 2.80 2.85 2.90 2.95 2.55 2.60 2.65 2.70 2.75 2.80 2.85 2.90 2.95 Wavelength, μm Wavelength, μm 3+ 3+ Figure 9. Luminescence band of Er at 2.75 m of the TeO -WO -Bi O -(4-x)La O -xEr O glasses Figure 9. Luminescence band of Er at 2.75 µm of the TeO2-WO3-Bi2O3-(4-x)La2O3-xEr2O3 glasses 2 3 2 3 2 3 2 3 3+ Figure 9. Luminescence band of Er at 2.75 µm of the TeO2-WO3-Bi2O3-(4-x)La2O3-xEr2O3 glasses with different Er2O3 contents. Excitation 975 nm, 0.5 W. with different Er O contents. Excitation 975 nm, 0.5 W. 2 3 with different Er2O3 contents. Excitation 975 nm, 0.5 W. 1.0 1.0 0.9 0.9 0.8 0.8 0.7 0.7 0.6 0.6 0.5 0.5 0.4 0.4 0.3 0.3 0.2 0.2 0.1 0.1 0.0 0.0 0.00.5 1.01.5 2.02.5 3.03.5 4.0 0.00.5 1.01.5 2.02.5 3.03.5 4.0 Er O content, mol% Er 2 O 3 content, mol% 2 3 Figure 10. The 2.75 µm luminescence peak intensity versus Er2O3 content. Figure 10. The 2.75 µm luminescence peak intensity versus Er2O3 content. Figure 10. The 2.75 m luminescence peak intensity versus Er O content. 2 3 4 3+ The lifetimes of the I4 11/2 level of Er we 3+ re measured by registration of the 0.98 µm 4 3+ The lifetimes The lifetimesof the of the II 11/2 level of Er level of Er we wer re m e measur easured ed by by regist registration ration of t of h the e 0. 0.98 98 µm m 11/2 emission, the decay curves for glasses of the TWBL-xEr are plotted in Figure 11. The meas- emission, the decay curves for glasses of the TWBL-xEr are plotted in Figure 11. The emission, the decay curves for glasses of the TWBL-xEr are plotted in Figure 11. The meas- 4 3+ ured lifetimes of the I11 2 level 4 of Er ion were 3+ 113, 113, and 104 µs for the Er2O3 contents 4 3+ measured lifetimes of the I level of Er ion were 113, 113, and 104 s for the Er O ured lifetimes of the I11/2 level of Er ion were 113, 113, and 104 µs for the Er2O3 contents 11/ 2 2 3 of 0.4, 1.2, and 4 mol%, respectively. The I13/2 Er leve 4 l lifetimes were measured by regis- contents of 0.4, 1.2, and 4 mol%, respectively. The I Er level lifetimes were measured of 0.4, 1.2, and 4 mol%, respectively. The I13/2 Er leve 13/2 l lifetimes were measured by regis- 4 3+ 4 3+ tering the 1.53 µm emission. The measured lifetimes of the I13/2 level of Er ion were 7.0 4 3+ by registering the 1.53 m emission. The measured lifetimes of the I level of Er ion tering the 1.53 µm emission. The measured lifetimes of the I13/2 level of 13/ Er 2 ion were 7.0 and 6.5 ms for the Er2O3 contents of 0.4 and 4 mol%, respectively. An exponential attenu- were 7.0 and 6.5 ms for the Er O contents of 0.4 and 4 mol%, respectively. An exponential and 6.5 ms for the Er2O3 contents of 0.4 2 3 and 4 mol%, respectively. An exponential attenu- ation of the luminescence intensity was observed when the excitation is removed, pre- attenuation of the luminescence intensity was observed when the excitation is removed, ation of the luminescence intensity was observed when the excitation is removed, pre- sumably the excited ions interact weakly with neighboring ions in the ground state. presumably the excited ions interact weakly with neighboring ions in the ground state. sumably the excited ions interact weakly with neighboring ions in the ground state. 4 4 For both I11/2 and I13/2 energy levels, a 10-fold increase in concentration led to a de- 4 4 For both I11/2 and I13/2 energy levels, a 10-fold increase in concentration led to a de- crease in the lifetime by only 10%, due to non-radiative effects from concentration quench- crease in the lifetime by only 10%, due to non-radiative effects from concentration quench- 3+ ing. A slow reduction in the luminescence lifetimes, with a large increase in the Er con- 3+ ing. A slow reduction in the luminescence lifetimes, with a large increase in the Er con- centration, indicates weak concentration quenching. The possibility of creating high con- centration, indicates weak concentration quenching. The possibility of creating high con- centrations of dopant in the glass and weak concentration quenching confirms the absence centrations of dopant in the glass and weak concentration quenching confirms the absence 3+ of Er ion clustering in the glasses [25]. 3+ of Er ion clustering in the glasses [25]. Peak emission intensity, r.u. Peak emission intensity, r.u. Intensity, r.u. Intensity, r.u. Photonics 2021, 8, 320 11 of 13 Photonics 2021, 8, x FOR PEER REVIEW 12 of 14 TWBL-0.4Er TWBL-1.2Er -1 τ ( I ) = 113 μs 11/2 TWBL-4Er -2 τ ( I ) = 104 μs 11/2 0 50 100 150 200 250 300 Time, μs Figure 11. Decay curves of 0.98 µm luminescence from I11/2 level for TeO2-WO3-Bi2O3-(4-x) La2O3- Figure 11. Decay curves of 0.98 m luminescence from I level for TeO -WO -Bi O -(4-x) La O - 11/2 2 3 2 3 2 3 xEr2O3 glasses with Er2O3 contents of 0.4, 1.2, and 4 mol%. xEr O glasses with Er O contents of 0.4, 1.2, and 4 mol%. 2 3 2 3 4 4 4. Disc For ussion both I and I energy levels, a 10-fold increase in concentration led to 11/2 13/2 a decrease in the lifetime by only 10%, due to non-radiative effects from concentration The properties of TeO2-WO3-Bi2O3-La2O3-Er2O3 glasses were studied depending on quenching. A slow reduction in the luminescence lifetimes, with a large increase in the erbium oxide concentration. There were no clear thermal effects of the crystallization and 3+ Er concentration, indicates weak concentration quenching. The possibility of creating melting of crystals on the DSC data; the glasses were resistant to crystallization up to 4 high concentrations of dopant in the glass and weak concentration quenching confirms the mol% Er2O3. Increasing the concentration of erbium oxide to 4 mol% did not practically 3+ absence of Er ion clustering in the glasses [25]. change the glass transition temperature, which was equal to ~390 °C for all samples. Thus, replacing lanthanum oxide with an equimolar amount of erbium oxide allows activating 4. Discussion 3+ the core with Er , without changing the viscosity properties. The properties of TeO -WO -Bi O -La O -Er O glasses were studied depending on 2 3 2 3 2 3 2 3 The introduction of erbium oxide by replacing lanthanum oxide leads to an insignif- erbium oxide concentration. There were no clear thermal effects of the crystallization and icant change in the refractive index, even at high dopant concentrations. This is highly melting of crystals on the DSC data; the glasses were resistant to crystallization up to 3+ desirable when designing fibers with a core activated with Er . 4 mol% Er O . Increasing the concentration of erbium oxide to 4 mol% did not practically 2 3 The considered glasses had high transmittance in the 4.7–5.3 µm range, the absorp- change the glass transition temperature, which was equal to ~390 C for all samples. Thus, tion bands of the hydroxyl groups at about 2.3, 3, and 4.4 µm, typical for ordinary tellurite replacing lanthanum oxide with an equimolar amount of erbium oxide allows activating glass samples, were indistinguishable for the thin specimens. The concentration of hy- 3+ the core with Er , without changing the viscosity properties. 3+ 4 droxyl groups was inferior to the concentration of Er by at least 10 times, allowing ex- The introduction of erbium oxide by replacing lanthanum oxide leads to an insignif- cluding the influence of this important impurity on the accuracy of determining the emis- icant change in the refractive index, even at high dopant concentrations. This is highly sion characteristics. There is no deterioration in the effectiveness of the method for hy- 3+ desirable when designing fibers with a core activated with Er . droxyl group removal with an increase in the concentration of erbium in the glass-forming The considered glasses had high transmittance in the 4.7–5.3 m range, the absorption melt. bands of the hydroxyl groups at about 2.3, 3, and 4.4 m, typical for ordinary tellurite glass The Er2O3-concentration dependencies for the luminescence characteristics were samples, were indistinguishable for the thin specimens. The concentration of hydroxyl 4 4 4 4 3+ found for the I13 2– I15 2 and I11 2– I13 2 transitions of the Er ion under 0.975 µm pumping. / / / / 3+ 4 groups was inferior to the concentration of Er by at least 10 times, allowing excluding 4 4 The dependence of the bandwidth and the luminescence intensity at the I13/2– I15/2 transi- the influence of this important impurity on the accuracy of determining the emission tion have a maximum, after which the emission characteristics are deteriorated; for the characteristics. There is no deterioration in the effectiveness of the method for hydroxyl 4 4 I11/2– I13/2 transition the bandwidth and intensity increase without the observed saturation. group removal with an increase in the concentration of erbium in the glass-forming melt. 4 3+ 4 The measured lifetimes of the I11 2 level of the Er ion were 110 and 100 µs, and the I13 2 / / The Er O -concentration dependencies for the luminescence characteristics were 2 3 3+ levels of the Er4 ion were 4 7.0 and 4 6.5 m4 s for the Er2O3 contents of 0.4 3+ and 4 mol%, respec- found for the I – I and I – I transitions of the Er ion under 0.975 m 13/ 2 15/ 2 11/ 2 13/ 2 tively; demonstrating a decrease with an increase in the activator concentration. pumping. The dependence of the bandwidth and the luminescence intensity at the 4 4 I – I transition have a maximum, after which the emission characteristics are 13/ 2 15/ 2 5. Conclusions 4 4 deteriorated; for the I – I transition the bandwidth and intensity increase without 11 2 13 2 / / 4 3+ the observed A series of T saturation. eO2-WO The 3-Bi2measur O3-La2O ed 3-Er lifetimes 2O3 glasof sesthe was I synthes level ize of d from the Er high ion -purity were 11/ 2 4 3+ oxides in a purified oxygen flow inside a sealed silica chamber. Binary TeO2-WO3 glass 110 and 100 s, and the I levels of the Er ion were 7.0 and 6.5 ms for the Er O 13 2 2 3 wa contents s the ba ofsi 0.4 s, La and 2O4 3 was in mol%,clude respective d to inc ly;rdemonstrating ease the resistan ace to crystal decrease with liza an tion incr an ease d for con- in the 3+ venience activator concentration. of introducing an Er activator, and Bi2O3 was added to evaluate the practice of modifying the refractive index of the core of the step-index fibers. High-quality optical elements and fibers had previously been successfully manufactured from similar glasses, and detailed studies of the doping features are necessary to improve the active devices. Intensity of 0.98 μm emission, r.u. Photonics 2021, 8, 320 12 of 13 5. Conclusions A series of TeO -WO -Bi O -La O -Er O glasses was synthesized from high-purity 2 3 2 3 2 3 2 3 oxides in a purified oxygen flow inside a sealed silica chamber. Binary TeO -WO glass 2 3 was the basis, La O was included to increase the resistance to crystallization and for 2 3 3+ convenience of introducing an Er activator, and Bi O was added to evaluate the practice 2 3 of modifying the refractive index of the core of the step-index fibers. High-quality optical elements and fibers had previously been successfully manufactured from similar glasses, and detailed studies of the doping features are necessary to improve the active devices. The properties important for use in photonics and fiber optics were studied, depending on erbium concentration. To obtain the most accurate data, special attention was paid to reducing the concentration of impurities, primarily hydroxyl groups, through the use of pure starting materials and original synthesis technology. In all samples of the series, 16 3 an extremely low content of hydroxyl groups ~n 10 cm was achieved, in order to 3+ guarantee there were no effects on the luminescence properties of Er . The transparency range of the considered glasses extended from 4.7 to 5.3 m. The in- troduction of erbium oxide led to an insignificant changes in the refractive index, resistance to crystallization, and glass transition temperature. This is very important for production 3+ of optical fibers with a core activated with Er . The studies of the emission characteristics show that low concentrations of the activa- 4 4 tor are preferable for using emission at the I – I transition. To use the emission at 13 2 15 2 / / 4 4 the I – I transition, it is preferable to achieve high concentrations of the activator. 11/ 2 13/ 2 The results obtained confirm the high applicability of these glasses for creating active fiber-optic devices and are useful for calculating specific laser fibers. Author Contributions: Conceptualization, V.V.D. and V.V.K.; methodology, V.V.D. and A.D.P.; val- idation, V.V.D. and V.V.K.; formal analysis, V.V.D., V.V.K. and A.D.P.; investigation, V.V.D., S.E.M. and V.V.K.; resources, V.V.D., V.V.K. and A.V.K.; data curation, V.V.D. and S.E.M.; writing—original draft preparation, V.V.D. and V.V.K.; writing—review and editing, V.V.D. and A.V.K.; visualization, V.V.D. and V.V.K.; supervision, V.V.D. and A.V.K.; project administration, V.V.D. and A.V.K.; fund- ing acquisition, V.V.D. and A.V.K. All authors have read and agreed to the published version of the manuscript. Funding: The work was supported by the Center of Excellence «Center of Photonics» funded by the Ministry of Science and Higher Education of the Russian Federation, contract No. 075-15-2020-906. Conflicts of Interest: The authors declare no conflict of interest. References 1. Jha, A.; Richards, B.; Jose, G.; Teddy-Fernandez, T.; Joshi, P.; Jiang, X.; Lousteau, J. Rare-earth ion doped TeO and GeO glasses as 2 2 laser materials. Prog. Mater. Sci. 2012, 57, 1426–1491. [CrossRef] 2. El-Mallawany, R.A.H. Tellurite Glasses Handbook: Physical Properties and Data; CRC Press: Boca Raton, FL, USA, 2016. 3. Smayev, M.P.; Dorofeev, V.V.; Moiseev, A.N.; Okhrimchuk, A.G. Femtosecond laser writing of a depressed cladding single mode channel waveguide in high-purity tellurite glass. J. Non-Cryst. Solids 2018, 480, 100–106. [CrossRef] 4. Yakovlev, A.I.; Snetkov, I.L.; Dorofeev, V.V.; Motorin, S.E. Magneto-optical properties of high-purity zinc-tellurite glasses. J. Non-Cryst. Solids 2018, 480, 90–94. [CrossRef] 3+ 5. Anashkina, E.A.; Dorofeev, V.V.; Koltashev, V.V.; Kim, A.V. Development of Er -doped high-purity tellurite glass fibers for gain-switched laser operation at 2.7 m. Opt. Mater. Express 2017, 7, 4337–4351. [CrossRef] 6. Dorofeev, V.V.; Moiseev, A.N.; Churbanov, M.F.; Kotereva, T.V.; Chilyasov, A.V.; Kraev, I.A.; Pimenov, V.G.; Ketkova, L.A.; Dianov, E.M.; Plotnichenko, V.G.; et al. Production and properties of high purity TeO -WO -(La O , Bi O ) and TeO -ZnO-Na O-Bi O 2 3 2 3 2 3 2 2 2 3 glasses. J. Non-Cryst. Solids 2011, 357, 2366–2370. [CrossRef] 7. Anashkina, E.A.; Andrianov, A.V.; Dorofeev, V.V.; Kim, A.V. Toward a mid-infrared femtosecond laser system with suspended-core tungstate-tellurite glass fibers. Appl. Opt. 2016, 55, 4522–4530. [CrossRef] [PubMed] 3+ 8. Muravyev, S.V.; Anashkina, E.A.; Andrianov, A.V.; Dorofeev, V.V.; Motorin, S.E.; Koptev, M.Y.; Kim, A.V. Dual-band Tm -doped tellurite fiber amplifier and laser at 1.9 m and 2.3 m. Sci. Rep. 2018, 8, 16164. [CrossRef] [PubMed] 9. Denker, B.I.; Dorofeev, V.V.; Galagan, B.I.; Koltashev, V.V.; Motorin, S.E.; Plotnichenko, V.G.; Sverchkov, S.E. 2.3 m laser action in 3+ Tm -doped tellurite glass fiber. Laser Phys. Lett. 2019, 16, 015101. [CrossRef] 10. Ebrahim-Zadeh, M.; Sorokina, I.T. Mid-Infrared Coherent Sources and Applications; Springer: Dordrecht, The Netherlands, 2008. [CrossRef] Photonics 2021, 8, 320 13 of 13 11. Wartewig, S.; Neubert, R.H.H. Pharmaceutical applications of mid-IR and Raman spectroscopy. Adv. Drug Deliv. Rev. 2005, 57, 1144–1170. [CrossRef] 12. Mukherjee, A.; Von der Porten, S.; Patel, C.K.N. Standoff detection of explosive substances at distances of up to 150 m. Appl. Opt. 2010, 49, 2072–2078. [CrossRef] 13. Seddon, A.B. Mid-Infrared (IR)—A hot topic: The potential for using mid-IR light for non-invasive early detection of skin cander in vivo. Phys. Status Solidi B 2013, 250, 1020–1027. [CrossRef] 14. Moiseev, A.N.; Dorofeev, V.V.; Chilyasov, A.V.; Pimenov, V.G.; Kotereva, T.V.; Kraev, I.A.; Ketkova, L.A.; Kosolapov, A.F.; Plotnichenko, V.G.; Koltashev, V.V. Low loss, high-purity (TeO ) (WO ) glass. Inorg. Mater. 2011, 47, 665–669. [CrossRef] 2 0.75 3 0.25 3+ 15. Feng, X.; Qi, C.; Lin, F.; Hu, H. Tungsten tellurite glass a new candidate medium for Yb -doping. J. Non-Cryst. Solids 1999, 256–257, 372–377. [CrossRef] 16. Dorofeev, V.V.; Moiseev, A.N.; Churbanov, M.F.; Snopatin, G.E.; Chilyasov, A.V.; Kraev, I.A.; Lobanov, A.S.; Kotereva, T.V.; Ketkova, L.A.; Pushkin, A.A.; et al. High purity TeO -WO -(La O , Bi O ) glasses for fiber-optics. Opt. Mater. 2011, 33, 1911–1915. 2 3 2 3 2 3 [CrossRef] 17. Champarnaud-Mesjard, J.-C.; Thomas, P.; Marchet, P.; Frit, B.; Chagraoui, A.; Tairi, A. Glass Formation Study in the Bi O -TeO - 2 3 2 WO System. Ann. Chim. Sci. Mater. 1998, 23, 289–292. [CrossRef] 18. Kaky, K.M.; Lakshminarayana, G.; Baki, S.O.; Kityk, I.V.; Taufiq-Yap, Y.H.; Mahdi, M.A. Structural, thermal and optical absorption features of heavy metal oxides doped tellurite rich glasses. Results Phys. 2017, 7, 166–174. [CrossRef] 19. Yousef, E.; Hotzel, M.; Rüssel, C. Effect of ZnO and Bi O addition on linear and non-linear optical properties of tellurite glasses. 2 3 J. Non-Cryst. Solids 2007, 353, 333–338. [CrossRef] 20. Balueva, K.V.; Kut’in, A.М.; Plekhovich, A.D.; Motorin, S.E.; Dorofeev, V.V. Thermophysical characterization of TeO -WO -Bi O 2 3 2 3 glasses for optical applications. J. Non-Cryst. Solids 2021, 553, 120465. [CrossRef] 21. Boetti, G.N.; Lousteau, J.; Chiasera, A.; Ferrari, M.; Mura, E. Thermal stability and spectroscopic properties of erbium-doped niobic-tungsten–tellurite glasses for laser and amplifier devices. J. Lumin. 2012, 13, 1265–1269. [CrossRef] 22. El-Mallawany, R.; Ahmed, I.A. Thermal properties of multicomponent tellurite glass. J. Mater. Sci. 2008, 43, 5131–5138. [CrossRef] 23. Ersundu, A.E.; Karaduman, G.; Çelikbilek, M.; Solak, N.; Aydın, S. Effect of rare-earth dopants on the thermal behavior of tungsten–tellurite glasses. J. Alloys Compd. 2010, 508, 266–272. [CrossRef] 3+ 24. Ma, Y.; Guo, Y.; Huang, F.; Hu, L.; Zhang, J. Spectroscopic properties in Er doped zinc- and tungsten-modified tellurite glasses for 2.7 m laser materials. J. Lumin. 2014, 147, 372–377. [CrossRef] 25. Gomes, L.; Oermann, M.; Ebendor-Heidepriem, H.; Ottaway, D.; Monro, T.; Felipe Henriques Librantz, A.; Jackson, S.D. Energy level decay and excited state absorption processes in erbium-doped tellurite glass. J. Appl. Phys. 2011, 110, 083111. [CrossRef] 26. Anashkina, E.A. Laser Sources Based on Rare-Earth Ion Doped Tellurite Glass Fibers and Microspheres. Fibers 2020, 8, 30. [CrossRef] 3+ 3+ 27. Li, J.; Li, S.; Hu, H.; Gan, F. Emission properties of Yb /Er doped TeO –WO –ZnO glasses for Broadband Optical Amplifiers. 2 3 J. Mater. Sci. Technol. 2004, 20, 139–142. 28. Oermann, M.R.; Ebendor-Heidepriem, H.; Li, Y.; Foo, T.C.; Monro, T.M. Index matching between passive and active tellurite glasses for use in microstructured fiber lasers: Erbium doped lanthanum-tellurite glass. Opt. Express 2009, 17, 15578–15584. [CrossRef] 3+ 29. Zhan, H.; Zhou, Z.; He, J. Intense 2.7 m emission of Er -doped water-free fluorotellurite glasses. Opt. Lett. 2012, 37, 3408–3410. [CrossRef] [PubMed] 30. Anashkina, E.A.; Dorofeev, V.V.; Skobelev, S.A.; Balakin, A.A.; Motorin, S.E.; Kosolapov, A.F.; Andrianov, A.V. Microstructured fibers based on tellurite glass for nonlinear conversion of Mid-IR ultrashort optical pulses. Photonics 2020, 7, 51. [CrossRef] 31. Kut’in, A.M.; Plehovich, A.D.; Dorofeev, V.V.; Moiseev, A.N.; Churbanov, M.F.; Markin, A.V. Thermophysical Properties of TeO -WO -La O Glass. J. Non-Cryst. Solids 2013, 377, 16–20. [CrossRef] 2 3 2 3 32. Kut’in, A.M.; Plekhovich, A.D.; Balueva, К.V.; Motorin, S.E.; Dorofeev, V.V. Thermal properties of high purity zinc-tellurite glasses for fiber-optics. Thermochim. Acta 2019, 673, 192–197. [CrossRef] 33. Churbanov, M.F.; Moiseev, A.N.; Chilyasov, A.V.; Dorofeev, V.V.; Kraev, I.A.; Lipatova, M.M.; Kotereva, T.V.; Dianov, E.M.; Plotnichenko, V.G.; Kryukova, E.B. Production of high-purity TeO -ZnO and TeO -WO glasses with the reduced content of 2 2 3 OH-groups. J. Optoelectron. Adv. Mater. 2007, 9, 3229–3234. 34. Tatarintsev, B.V.; Yakhkind, A.K. The water content in tellurite glasses and its effect on infrared transmission. Opt. Promyshlennost 1975, 3, 40–43.
Journal
Photonics – Multidisciplinary Digital Publishing Institute
Published: Aug 9, 2021
Keywords: high-purity tellurite glass; Er2O3 content; hydroxyl groups; crystallization; glass transition temperature; luminescence; lifetime