Yb3+ co-doped oxyfluorotellurite glasses – spectroscopic and temperature sensor properties

Journal Pre-proof 3+ 3+ Thermosensitive Tm /Yb co-doped oxyfluorotellurite glasses – Spectroscopic and temperature sensor properties Barbara Klimesz, Radosław Lisiecki, Witold Ryba-Romanowski PII:

S0925-8388(20)30116-X

DOI:

https://doi.org/10.1016/j.jallcom.2020.153753

Reference:

JALCOM 153753

To appear in:

Journal of Alloys and Compounds

Received Date: 7 October 2019 Revised Date:

4 January 2020

Accepted Date: 7 January 2020

3+ Please cite this article as: B. Klimesz, Radosł. Lisiecki, W. Ryba-Romanowski, Thermosensitive Tm / 3+ Yb co-doped oxyfluorotellurite glasses – Spectroscopic and temperature sensor properties, Journal of Alloys and Compounds (2020), doi: https://doi.org/10.1016/j.jallcom.2020.153753. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier B.V.

Thermosensitive Tm3+/Yb3+ co-doped oxyfluorotellurite glasses – spectroscopic and temperature sensor properties Barbara Klimesz *,a, Radosław Lisiecki b, Witold Ryba-Romanowski b a

Department of Physics, Opole University of Technology, ul. Prószkowska 76, 45-758 Opole, Poland b Institute of Low Temperature and Structure Research, Polish Academy of Sciences, ul. Okólna 2, 50-422 Wrocław, Poland

ABSTRACT Oxyfluorotellurite (65-x)TeO2–20ZnF2–12PbO–3Nb2O5–xTm2O3 (x = 0.5, 2 and 5) and (65-x-y)TeO2–20ZnF2–12PbO–3Nb2O5–xTm2O3–yYb2O3 (x = 0.5, y = 2 and x = 0.5, y = 5) glass systems were manufactured. Glass thermal stability was examined by means of differential thermal analysis (DTA) and some thermal stability criteria ∆T, H’ and S were determined. Absorption spectrum was utilized to prepare Judd-Ofelt (J-O) calculations and consequently oscillator strengths, intensity parameters Ω2,4,6, radiative transition probabilities, branching ratios and radiative lifetimes of luminescent levels were estimated. Emission spectra and luminescence decay curves were measured as a function of temperature in the 300 - 625 K range. Based on the upconversion luminescence of Tm3+ the optical temperature sensor qualities have been evaluated for Tm3+/Yb3+ co-doped sample. The fluorescence intensity ratio (FIR) between transitions 700 nm (Tm3+:3F2,3 → 3H6)/800 nm (Tm3+:3H4 → 3 H6) for 975 nm excitation and 700 nm (Tm3+:3F2,3 → 3H6)/ 470 nm (Tm3+:1G4 → 3H6) for 803 nm excitation were determined and an adequate relative thermal sensitivity S was estimated. The significant value of relative thermal sensitivity 1.12 %K-1 was determined when NIR Yb3+ and Tm3+ emission bands were analyzed, however. Effective emission cross section was calculated for 3F4 → 3H6 potential laser transition of thulium around 1.9 µm in single-doped Tm3+ glass. Furthermore, effect of temperature on down-converted and up-converted thulium and ytterbium emission and on relaxation dynamic of involved excited states was studied in details.

PACS codes: 42.70.- a, 42.70.Ce, 64.70.Pf, 64.70.kj, 65.60.+ a, 78.55.Qr, 78.90.+ t, 81.05.Kf, 81.70.Pg Keywords: thermal properties, spectroscopy properties, rare earth-doped oxyfluorotellurite glasses, luminescent temperature sensors

*Corresponding author. Barbara Klimesz Tel.: +48–77–4498837; e-mail address: [email protected]

1. INTRODUCTION

The dynamic development of computer networks, telecommunications and photonics still supports a long lasting interest directed to glass materials, especially those characterized by low phonon energies such as fluorides, tellurites, chalcogenides and heavy metal oxide glasses [1-6]. In the search for more efficient optical materials based on glasses doped with rare earth ions a proper choice of the type of glass matrix and of the active ion concentration is of paramount importance. On one hand, the host glass matrix should have a low phonon cut-off energy to minimize multiphonon relaxation rates for excited states of incorporated rare earth ions. On the other hand, the type and concentration of rare earth ions introduced into different host materials significantly affect the range of emission spectra (UV to NIR) recorded. The properties and advantages of tellurites or fluorides glasses for photonic and optoelectronic applications are now well known [2-9]. The tellurite glasses are characterized by chemical durability, high refractive index (~1.8 - 2.3), low phonon energy (700 - 800 cm1

), good thermal stability, high rare earth ions solubility as well as a wide transmission

window (usually ~0.4 - 6 µm). The fluoride glasses have slightly poorer mechanical strength and chemical durability, but a broader optical transmittance region between 0.3 and 7 µm and lower phonon energy (~550 cm-1) [8-10]. Besides, among the different compositions studied, fluorotellurite glasses have proven to possess a relatively large transparency window, a high refractive index and as well as high thermal stability [11-13]. For these reasons, fluorotellurite glasses with the addition of Nb2O5 are gaining increasing attention of researchers [13-15] and constitute a good compromise between the properties of pure fluoride and tellurite glass. As reported in the literature [11,15,16] the addition of F- ions to tellurite glass doped with luminescent rare earth ions increases the emission cross-section, removes OH- groups and improves luminescence lifetime.

2

Among the RE3+ ions, thulium (Tm3+) ion has attracted a great interest for its the specific transition 3H4→3F4 which offer a way to extend the optical fiber transmission window to 1450 - 1500 nm (S-band amplifier region). Moreover, thulium emission around 1800 nm offer a lasing capability within the atmospheric window (1600-1900 nm) [17-19]. Co-doping with other rare earth elements such as ytterbium ions (Yb3+) is generally used to improve the pump absorption and to discriminate partially non-radiative processes (excited state absorption, cross-relaxation, upconversion). The ytterbium ion is also very good sensitizer for other 980 nm pumped RE3+ ions [5,16,20]. In sum, the Tm3+ doped and Tm3+/Yb3+ co-doped fluorotellurite glasses have very good spectroscopic properties (large absorption and emission cross-sections, wide luminescence effective line widths, high value of linear and non-linear refractive indices, low value of phonon energy) and compared to other glass matrix they are relatively attractive material for a wide photonic applications (optical storage, lasers, optical amplifiers, photosensitivity materials). The luminescent temperature sensors based on rare earth (RE3+) doped materials are becoming more and more popular in the non-contact optical thermometry. They have many different applications in various fields of science, industry and engineering. In biological systems for indicate early symptoms of diseases, testing of biological fluids and determination of cellular processes [21-23]. In power plants, oil refineries and highly corrosive media for temperature monitoring [21,24-26]. For to predict gas outbursts in coal mines, building fire detection and for monitoring volcanic activity [21,24-27]. Because some spectroscopic properties i.e. shape of the emission spectra, emission intensity, bandwidth, spectral shift and lifetimes (luminescence decays) of RE3+ ions embedded in the matrix depend on temperature changes [28-30] they can be successfully used as optical sensors. The most popular is the fluorescence intensity ratio (FIR) technique that characterizes fast measurement, high precision and good resolution [21,25]. The FIR method gives

3

information of temperature-dependent changes of intensity ratio of emission bands, corresponding to the two thermally coupled levels of some ions RE3+ (e.g. Pr3+, Nd3+, Eu3+, Ho3+, Er3+,Tm3+) with an appropriate energy separation between the thermalized states in the range of 200 ≤ ∆E ≤ 2000 cm−1 [21,28]. Up-conversion (UC) luminescence of RE3+ ions is an excellent process to obtain specific optical thermometers when sensitizer ions (e.g. Yb3+) are excited with NIR laser. The absorbed energy is then transferred to the neighboring activator ions (e.g. Ho3+, Er3+ or Tm3+) pumping their excited states via energy transfer up-conversion (ETU) processes [21,31]. On the other hand, the efficiency of up-conversion (UC) emissions is also strongly dependent on the characteristics of the host (probabilities of the emitting levels must be high enough to show relatively large emission intensities) [28]. Therefore, it is so important to look for both a proper RE3+ ion and the matrix for use in temperature sensors. In the present paper we report on the synthesis, thermal stability and fundamental spectroscopic properties of (65-x)TeO2–20ZnF2–12PbO–3Nb2O5–xTm2O3 (x = 0.5, 2 and 5) and (65-x-y)TeO2–20ZnF2–12PbO–3Nb2O5–xTm2O3–yYb2O3 (x = 0.5, y = 2 and x = 0.5, y = 5) glass systems. Formula of the 65TeO2–20ZnF2–12PbO–3Nb2O5 undoped glass is shortly labelled as TZPN. The single-doped glasses (65-x)TeO2–20ZnF2–12PbO–3Nb2O5–xTm2O3 (x = 0.5, 2 and 5) are correspondingly denoted as TZPN:0.5%Tm, TZPN:2%Tm, TZPN:5%Tm, respectively. The co-doped samples (65-x-y)TeO2–20ZnF2–12PbO–3Nb2O5–xTm2O3–yYb2O3 (x = 0.5, y = 2 and x = 0.5, y = 5) are labelled as TZPN:0.5%Tm,2%Yb and TZPN:0.5%Tm,5%Yb, respectively. The thermal stability parameters, oscillator strengths, Judd-Ofelt intensity parameters, branching ratios and radiative transition probabilities were calculated and compared to available literature data. Furthermore, optical temperature sensitiveness of Tm3+/Yb3+ co-doped TeO2–ZnF2–PbO–Nb2O5 glass was examined in wide range of temperature.

4

2. EXPERIMENTAL

The oxyfluorotellurite glass samples were manufactured from a mixture of high purity (4N or 5N, Alfa Aesar) powders of tellurium (TeO2), niobium (Nb2O5) and lead (PbO) oxides, anhydrous zinc fluoride (ZnF2), 5N thulium oxide (Tm2O3) and 5N ytterbium oxide (Yb2O3). The chemical compositions of glasses were following (in mol%) 64.5TeO2–20ZnF2–12PbO– 3Nb2O5–xTm2O3 (x = 0.5, 2 and 5 mol%) and (65-x-y)TeO2–20ZnF2–12PbO–3Nb2O5– xTm2O3–yYb2O3 (x = 0.5, y = 2 and x = 0.5, y = 5). The starting substrates were thoroughly mixed and placed in corundum crucible. The batches were melted in a resistance furnace at 830 oC for 30 min in normal atmosphere. The melt was poured onto a preheated steel plate and then was annealed for a few hours below the glass transition temperature (Tg) in order to eliminate internal stresses. The shape of the glass samples was comparable i.e. their length was around 10 mm while thickness was about 3 mm. Differential thermal analysis (DTA) measurement was performed in atmospheric air under normal pressure using a NETZSCH differential scanning calorimeter DSC 404/3/F with Pt/PtRh DSC measuring head and platinum sample pans. An empty platinum crucible was employed as the reference. The heating rates of 10 K/min in the DTA measurements were the same for all samples studied. Optical

absorption

spectra

were

recorded

with

a

Varian

5

Absorption

Spectrophotometer at spectral steps of 0.2 nm in UV and visible and of 0.5 nm in near infrared. Emission measurements were carried out in the visible and near infrared spectral range. Luminescence spectra were excited by a xenon lamp, AlGaAs 803 nm diode laser and InGaAs 975 nm diode laser and were dispersed by an Optron DongWoo monochromator with 750 mm focal length. Signal was detected by a Hamamatsu R-955 photomultiplier , an

5

InGaAs detector or a PbS detector depending on spectral region. Some emission spectra were measured utilizing FLS980 fluorescence spectrometer from Edinburgh Instrument equipped with a 450 W xenon lamp as an excitation source. The recorded spectra were corrected for the sensitivity and wavelength of the experimental setup. Luminescence decay curves were recorded upon selective excitation provided by a Continuum Surelite optical parametric oscillator (OPO) pumped with the third harmonic of Nd:YAG. The resulting transients were acquired with Hamamatsu or a cooled InSb Janson J10D detector connected to a 500 MHz MDO4054B-3 Tektronix Mixed Domain Oscilloscope. The glass sample was placed into a chamber furnace for measurements prepared at higher temperatures within 295-625 K. The temperature of TZPN:Tm,Yb glass was detected by a copper-constant an thermocouple and controlled by a proportional-integral-derivative (PID) Omron E5CK controller.

3. RESULTS AND DISCUSSION

3.1 Thermal analysis The DTA curves for TZPN glass single-doped with 0.5, 2 and 5 mole ratio of Tm2O3 and two co-doped TZPN:0.5%Tm,2%Yb and TZPN:0.5%Tm,5%Yb samples are shown on the Fig.1, respectively. The characteristic glass temperatures, glass transition temperature (Tg) and glass crystallisation temperature (Tc) were estimated in accordance with the Keavney and Eberlin method [32] and are defined by the point obtained by extrapolating an almost straight-line section of the DTA curve to the intersection with the baseline. The value of Tg indicates onset of dilatomeric glass softening and glass crystallisation temperature Tc indicates the initiation of the glass devitrification process.

6

Marked with an asterisk the Tpc values are maxima of exothermal crystallization peaks but they depend on technical parameters of experiment (mass, grain size and heating rate of sample) as well as on constructional features of apparatus. Therefore in the physicochemical sense taking Tpc as a characteristic point is a bit problematic. The asymmetry of recorded bands, suggesting simultaneous or almost simultaneous crystallization all of components of these structurally complex (multi-component) materials is the resultant of many physical and chemical processes. The results collected in Table 1 show that the Tg values (361 - 368 oC) for all doped glass samples are comparable to the Tg = 365 oC determined for the undoped glass matrix while the initiation temperatures of crystallization are significantly different. For samples doped with Tm3+ the Tc values range from 511 to 520 oC, and for samples doped with Yb3+ the Tc has values between 498 and 610 oC. For double-doped samples the Tc is about 515 516 oC. Observed fluctuations in Tg and Tc temperatures for glass samples doped with Tm3+ and Yb3+ ions may be related to a partial substitution of TeO2 by the Tm2O3 and / or Yb2O3 oxides. Lanthanide oxides have a higher number of cations per mole and therefore an increased number of bonds per unit volume which leads to an increase of all characteristic glass temperatures especially of the glass crystallization temperature (Tc) [33-36]. In addition, F- ions introduced into the glass matrix form ionic, non-bridging M–F bonds (where M is a cation) between each of the main structural units (TeO4, TeO3 etc.), breaking up the strong covalent TeO2 glass network and resulting in a decrease in Tg (liquid forming the glass flows more easily at lower temperatures) [11]. O’Donnell et al. [37] also point out that the presence of fluoride in fluorotellurite glasses leads to increased competition between different crystallization phases and thus better thermal stabilization.

7

In order to determine the thermal stability of studied glass samples we also calculated a thermal stability criteria ∆T [32], H' and S [38]. The ∆T values for our glasses range from 137 to 246 °C and hence the TZPN single doped with Tm3+ or Yb3+ and Tm3+/Yb3+ co-doped glasses are very stable against devitrification. The trend is the same for both thulium- and ytterbium-doped samples. Their thermal stability (∆T, H' and S values) initially increases with an increase of Tm3+ or Yb3+ concentration up to 2 mol% and then decreases for TZPN:5%Tm and TZPN:5%Yb. It can be seen (see Table 1) that TZPN compared with other fluorotellurite glasses single-doped with Tm3+ or Yb3+ and double-doped with Tm3+/Yb3+ ions has similar or a little better thermal stability and can be considered as attractive candidate for photonic applications.

3.2 Analysis of optical absorption spectra

Fig.2 presents a representative VIS-NIR absorption spectrum of TZPN:2%Tm glass sample. In the wavelength range from 400 to 2000 nm the spectra show five bands corresponding to transitions from the 3H6 ground state to the higher levels of Tm3+: 1G4 (469 nm), 3F3,2 (688 nm), 3H4 (792 nm), 3H5 (1204 nm), and 3F4 (1710 nm), respectively. The positions of the absorption peaks remain unchanged for the other two glass samples TZPN:0.5%Tm and TZPN:5%Tm and are similar to those for other thulium doped oxyfluorotellurite glasses, e.g. TeO2-PbF2 [39] or (TeO2)(Nb2O5)(TiO2) [40]. The absorption peak at 1710 nm corresponds to the 3H6 → 3F4 transition is a hypersensitive transition because it is very sensitive even to small changes in Tm3+ local symmetry and fulfills the selection rules: |∆S| = 0, |∆L| ≤ 2, and |∆J| ≤ 2. Energy levels higher than 1G4 are not observed because of the intrinsic bandgap absorption in the host glass.

8

The band positions along with assignment of the absorption transitions, estimated experimental oscillator strengths Pexp and corresponding theoretical oscillator strengths Pcal are collected in Table 2. According to the Judd-Ofelt (J-O) procedure [41-43] and equations described in details in [44] these data were used to calculate the J-O intensity parameters Ωt (t = 2, 4, 6). The obtained root-mean-square deviation RMS = 7.45 × 10-7 indicates the good fit between experimental and calculated oscillator strengths. The physical meaning of Ωt (t = 2, 4, 6) has been already discussed by many authors [4,5,11,39,45] and it is commonly known that the value of Ω2 reflects the symmetry around the rare earth ion, the value of Ω4 indicates the nature of the bond and Ω6 is associated with lattice rigidity. In the present oxyfluorotellurite glasses the J-O parameters values are similar to those obtained for other Tm3+ doped glasses (see Table 3) and are consistent with the trend Ω2 > Ω4 > Ω6. Parameters Ω4 and Ω6 are related to the rigidity of the host matrix in which the ions are located and to obtain large emission cross section the high values of Ω4 and Ω6 parameters are desirable. These Ω4 and Ω6 parameters are insensitive to the local environment but connected with the RE3+ site degree of covalence. Their values can increase by lowering the covalence of the chemical bonds between RE3+ ions and ligand anions and they change only slightly with concentration [46]. In turn parameter Ω2 does not influence luminescence branching ratio but the spectroscopic quality factor X = Ω4 / Ω6 defines luminescence intensity [47]. Its high values suggest that the RE3+ doped glass is fairly suitable as a laser material. For our glasses the spectroscopic quality factor is equal 1.47. The J-O parameters (Ω2,4,6) were utilized to calculate the values of the radiative transition rates Arad, luminescence branching ratios β for excited states of Tm3+ and radiative lifetime τrad of emission level in TZNP glass (see Table 4).

9

The total radiative emission probability WT involving all the intermediate terms is given by the sum of the WJ’ terms calculated over all terminal states. Radiative lifetime τrad of an excited level is given by the inverse of the total radiative emission probability τrad = 1/WT that is equal to 414 µs and 255 µs for 3H4 and 1G4 excited levels, respectively. Experimental lifetimes of luminescent levels of Tm3+ and Yb3+ ions in our TZPN glasses are gathered in Table 5. This table shows that tenfold increase of Tm3+ concentration results in effective reduction of 3F4, 3H4 and 1G4 experimental lifetimes. This quenching process in TZPN:Tm samples is efficient and can be estimated to be around 94 - 99%. The impact of ytterbium co-doping on relaxation dynamic of thulium luminescent levels was observed in TZPN:Tm,Yb glasses. Increase of Yb3+ concentration results in reduction of experimental lifetimes of Tm3+ luminescent levels around 17% - 50% and for higher Yb3+ concentration, the lifetime of 2F5/2 Yb3+ level is lowered by 32% as well. Based on these data the efficiencies ηET of energy transfer from the 1G4 level of Tm3+ to Yb3+ were assessed employing the formula [48]: = 1−

%

(1)

where τx% is the Tm3+ lifetime in a sample containing x%Yb3+ ions and τ0 denotes the lifetime of Tm3+ donor ions in the absence of acceptors. The obtained values are 0.36 and 0.56 for TZPN:0.5%Tm,2%Yb and TZPN:0.5%Tm,5%Yb samples, respectively. It can be noticed that recently, the energy transfer efficiency from Bi3+ to Yb3+ in Lu2GeO5 was found to be 65% [49]. The evaluation of the effectiveness of the Tm3+–2Yb3+ quantum cutting (QC) process was based on the dependence [48]: =

1−

+2

(2)

where ηET denotes the Tm3+ → Yb3+ transfer efficiency, ηTm is efficiency of thulium (Tm3+) emission and ηYb denotes the efficiency of ytterbium (Yb3+) emission. To neglect the non10

radiate energy loss by defects and non-intentional impurities, both ηTm and ηYb are commonly assumed to be unity [50-53]. With this assumption the upper limit of quantum efficiency (ηQE) was estimated to be 156% for TZPN:0.5%Tm,5%Yb sample. For a comparison a slightly higher value of quantum efficiency ηQE = 169.8% has been found for 0.5Tm3+/5Yb3+ co-doped phosphate glasses [50] and lower value ηQE = 143.2% was found for 0.5Tm3+/5Yb3+ co-doped oxyfluoride glass [51].

3.3. Excitation spectra

Lower part of Fig.2 shows the excitation spectrum of the Yb3+ emission monitored at 1017 nm for TZPN:0.5%Tm,5%Yb glass sample. The spectrum consists of three bands centered around 470, 690 and 790 nm that correspond to transitions from the 3H6 ground state to 1G4, 3F3,2 and 3H4 excited states of Tm3+, respectively. This reveals that energy transfer from Tm3+ to Yb3+ occurs in the glass under study and thulium ions contribute to the feeding of Yb3+ luminescence. The weak band at about 335 nm can be associated with the O2-–Te4+ charge transfer [54]. Inset depicts energy level schemes of donor (Tm3+) and acceptor (Yb3+) ions and possible mechanisms of down-conversion (DC) processes in our glass host. Various energy transfer mechanisms (ET) from Tm3+ to Yb3+ are possible. The energy of 1G4 (Tm3+) level is approximately twice as much as energy of 2F5/2 (Yb3+) level pointing at the possibility of a resonant (RDC) or nearly resonant down-conversion [48]: Tm3+(1G4) + 2Yb3+(2F7/2) → Tm3+(3H6) + 2Yb3+(2F5/2)

(3)

that transfers the Tm3+ excitation to two adjacent Yb3+ ions which emit two NIR photons by 2

F5/2 → 2F7/2 transition.

11

The other possibility for DC mechanism is the quantum cutting (QC) in which the Yb3+(2F5/2) excitation is transferred to two Tm3+(3F4) neighbors. Such a mechanism is able to satisfy resonance conditions and can compete with the RDC mechanism considered above. In our oxyfluotellurite glasses the energy transfer from Yb3+ to Tm3+ occurs with relatively high efficiency (ηET ≈ 0.93) and therefore it is also necessary to analyze the upconversion mechanism (UC) in which Yb3+ sensitizes the Tm3+ emission. As suggested by Y. Huang et al. [55] this mechanism can be described in several stages. Firstly, the electrons in the ground state 2F7/2 of Yb3+ absorb photons and are promoted to the excited state 2F5/2: Yb3+(2F7/2) + hν0 → Yb3+(2F5/2)

(4)

Next, the energy transfer from excited Yb3+ ions to unexcited Tm3+ ions feeds the 3H5 level which relax to the 3F4 level rapidly via a non-radiative (NR) decay: Yb3+(2F5/2) + Tm3+(3H6) → Yb3+(2F7/2) + Tm3+(3H5) Tm3+(3H5) → Tm3+(3F4)

[ET1]

(5)

[NR]

(6)

A second energy transfer step involving the interaction between excited Yb3+ ions and Tm3+ ions with populated 3F4 level results in the excitation of the 3F2,3 level: Yb3+(2F5/2) + Tm3+(3F4) → Yb3+(2F7/2) + Tm3+(3F2,3)

[ET2]

(7)

followed by a nonradiative relaxation to the lower metastable state 3H4 and a third energy transfer process that feeds the 1G4 level: Tm3+(3H4) → Tm3+(3F4) Yb3+(2F5/2) + Tm3+(3H4) → Yb3+(2F7/2) + Tm3+(1G4)

[NR]

(8)

[ET3]

(9)

The Tm3+(1G4) excitation relaxes to the ground state 3H6 and the 3F4 level by radiative transitions emitting the blue and the red light, respectively: Tm3+(1G4) → Tm3+(3H6) + hν

(10)

Tm3+(1G4) → Tm3+(3F4) + hν

(11)

12

In equations given above the expressions hν0 and hν denote the excitation photon and the phonon generated in the non-radiative relaxation (NR) processes, respectively.

3.4. VIS-NIR emission spectra

Fig.3 shows the luminescence spectra excited at 457 nm for the single-doped TZPN:Tm and co-doped TZPN:Tm,Yb glasses with different concentrations. It can be seen that the luminescence band related to the 1G4 → 3H6 transition of Tm3+ is the strongest for TZPN:0.5%Tm sample and diminishes with increasing Tm3+ concentration. For TZPN:0.5%Tm,2%Yb sample the band intensity 1G4 → 3H6 is comparable to that for TZPN:0.5%Tm sample and slightly decreases with increasing content of Yb3 +. A similar tendency can be observed in the case of bands related to transitions 1G4 → 3F4, 3H4 → 3H6, 3

H5 → 3H6. In addition, for both co-doped TZPN:0.5%Tm,2%Yb and TZPN:0.5%Tm,5%Yb

samples a band associated with the 2F5/2 → 2F7/2 ytterbium transitions appears around 980 nm. It is worth noticing that emission of single doped TZPN:Yb glasses was studied as well [15]. It can be concluded considering excitation spectrum of Yb3+ NIR emission and luminescence spectra of co-coped glass samples that effective Tm3+ ↔ Yb3+ energy transfer processes occur in materials under study.

3.5. Temperature dependence of UC emission spectra

Fig.4 presents the upconverted (UC) emission spectra of TZPN:0.5%Tm,5%Yb glass measured in the temperature range of 295 - 625 K under excitation at 803 nm (part A) and 975 nm (part B). The spectra show three emission bands that correspond to the 1G4 → 3H6 (~475 nm), 3F3,2 → 3H6 (~700 nm) and 3H4 → 3H6 (~800 nm) transitions, respectively.

13

In part A (under excitation 803 nm) it is clearly observed that the intensity of the 3F3,2 → 3H6 transition increases and the intensity of the 1G4 → 3H6 transition decreases gradually with temperature increasing. In part B (under excitation 975 nm) the intensity of prominent emission at about 800 nm (3H4 → 3H6) gradually drops with growing temperature and the 3F3,2 → 3H6 emission increases with temperature elevating. The peak wavelengths of emission bands assigned to 1

G4 → 3H6 and 3F3,2 → 3H6 transitions did not change in considered spectra. So, due to the temperature dependent emission, oxyfluorotellurite glasses co-doped

with Tm3+/Yb3+ ions may be promising for application as optical temperature sensors.

3.6. Sensitivity of the thermometry

Fig.5 shows the temperature dependence of fluorescence intensity ratios (FIR) associated with 3F3,2 → 3H6 / 1G4 → 3H6 (A) and 3F3,2 → 3H6 / 3H4 → 3H6 transitions (D) in TZPN:0.5%Tm,5%Yb glass and relative thermal sensitivities (S) estimated for 803 nm (C) and 975 nm (F) excitations. The experimental data can be well fitted with the following equations: R700/470 = -19 + (4.3 × exp(0,05T) )

(12)

R700/800 = 11 exp (-2528/T) + 0.02

(13)

and

where T is the temperature in K. For thermometric applications, it is very important to examine the absolute temperature sensitivity defined as: =

∆

=

14

(14)

To reasonable compare the luminescence thermometers features, relative sensitivity is usually applied because this relevant parameter determines normalized change of FIR with temperature variation and is defined as [56,57]: =

" 100% =

∆

" 100%

(15)

The 3F2,3 and 3H4 levels are in thermal equilibrium in agreement with the Boltzmann statistics. The value of ∆E determined during fitting procedure was estimated to be 2528 cm-1. For our sample the Sr value exponentially decreases with growing temperature when excited at 803 nm (C). For excitation at 975 nm (F) the Sr value initially increases until reaching a maximum at ~450 K and then slightly decreases. Our result of thermal sensitivity is better than that for Tm3+/Yb3+ co-doped oxyfluoride glass ceramic [58] which is considered a very promising candidate for the production of optical temperature sensors. This creates the possibility of the potential use of Tm3+/Yb3+ co-doped oxyfluorotellurite glass as luminescent temperature sensors based on the upconversion process. Moreover, our results presented in Fig.5 reveal that the relative sensitivity of the TZPN:0.5%Tm,5%Yb glass is about 0.54 %K-1 at 295 K for the FIR (I700 /I470) and 0.80 %K-1 at 450 K for the FIR (I700 / I800). It is worth noticing that relative sensitivity 1.3 x 10-2 %K-1 at T=293 K was found for Er-Yb-Tm -doped LaF3 nanoparticles [59] and value of 2.3 %K-1 at 300 K was estimated for LaPO4:Tm,Yb [60].

3.7. Absorption and emission cross sections

The absorption and emission cross-section spectra of Tm3+ in TZPN glass are shown in Fig.6 (upper part). The determined previously βexp and τr values were used to estimate the stimulated emission cross-section (σem) according to the Füchtbauer-Ladenburg formula:

%$& = *+,

'() ( - ./( (

15

(

(16)

where n is the refractive index of refraction, β denotes luminescence branching ratio, τr is 3F4 excited state radiative lifetime and I(λ) is the emission intensity versus wavelength. The investigated spectrum shows that the Tm3+ ions in our glass have relatively high values of emission cross-section in the wavelength range 1750 - 1950 nm. It is found that the maximum values of absorption cross-section (σabs) and emission cross-section (σem) of Tm3+ reach 0.71 × 10−20 cm2 at 1698 nm and 0.74 × 10−20 cm2 at 1854 nm, respectively. For a comparison, the following peak emission cross-sections σem = 1.12 × 10−20 cm2 , σem = 0.49 × 10−20 cm2 and σem = 0.59 × 10−20 cm2 were found for tellurite glasses [61], lead silicate glasses [62] and ZBYA fluoride glasses [63], respectively. The lower part of Fig.6 presents spectra of effective stimulated emission cross-section as a function of wavelength. The effective stimulated emission cross-section for different excited state population (P = 0.5, 0.4, 0.3, 0.2 and 0.1) was calculated using the relation (17): $%00 = 1$% − 1 − 1 $

2

(17)

where P is the population inversion parameter defined as the density ratio of Tm3+ ions in the excited state to the total Tm3+ ions in glass system The cross-section spectra obtained in this way for different P values imply that the optical gain associated with the 3F4 → 3H6 transition of Tm3+ in oxyfluotellurite glass will be positive within 1850 - 2040 nm wavelengths range.

3.8. Temperature dependence of NIR emission spectra

Fig.7 and 8 present the emission spectra of TZPN:0.5%Tm,5%Yb glass measured in the NIR spectral range under excitation 803 nm at different temperatures (295 − 625 K). The respective bands of emission spectra show a variation of intensity with temperature.

16

The intensity of spectral band centered at 1050 nm related to the 2F5/2 → 2F7/2 transition of Yb3+ ions gradually decreases with increasing temperature. Also, a gradual decrease the emission intensity with increasing temperature was observed in the case of two bands centered at 1460 nm and 1700 nm assigned to Tm3+ ions i.e. (3H4 → 3F4) and (3F4 → 3H6). It can been seen that the intensity of the emission band picked around 1200 nm rises with temperature increasing. Because at higher temperature the population of 3F3,2 Tm3+ levels is more effective and accordingly, this band may be associated with 2F3,2 → 3H5 transition. The ratiometric approach for temperature sensing was applied evaluating NIR Yb3+ band attributed to 2F5/2 → 2F7/2 transition at around 1050 nm and NIR Tm3+ emission band located at 1200 nm. The right part of Fig.8 shows the fluorescence intensity ratio (FIR) attributed to 3

F3,2-3H5 (1200 nm)/ 2F5/2-2F7/2 (1050 nm) and the determined relative thermal sensitivity. It

follows from these findings that the maximal value of relative thermal sensitivity 1.12 %K-1 was found at T=295 K for TZPN:0.5%Tm,5%Yb glass.

3.9. Energy transfers and luminescence kinetics analysis

It can be concluded from previous chapters that various Tm3+–Tm3+ and Yb3+–Tm3+ energy transfer phenomena occur in materials under study. As result of that the quite effective down-conversion, down-shifting and up-conversion processes have been observed and described. It is known that these inter-ions processes can effectively affect the relaxation dynamic of involved excited states of optically active ions. Fig.9 shows decay curves of 1G4 and 3H4 thulium luminescence and 2F5/2 ytterbium luminescence registered for single-doped (Tm3+ or Yb3+) and co-doped (Tm3+/Yb3+) TZPN glass samples. The experimental lifetimes of 1G4 and 3H4 excited states in TZPN:0.5%Tm sample are equal to 55 and 186 µs, respectively. Increase of Tm3+ concentration to 2 mol% results in over

17

tenfold reduction of luminescence lifetimes. It means that the observed self-quenching process resulting from an enhancement of Tm3+–Tm3+ interaction in a glass host is quite effective. On the other hand the measured 1G4 and 3H4 lifetimes for a diluted sample are lower than their radiative counterparts estimated on τrad (1G4) = 255 µs and τrad (3H4) = 414 µs (Table 4). Quantum efficiency (η) of the corresponding emitting levels was calculated by relation (18): =

3 4 .56

× 100%

(18)

and accordingly it amounts to 22% for 1G4 and 45% for 3H4 in TZPN:Tm glass. The decay curve of 2F5/2 level for TZPN:2%Yb sample has a single exponential nature with experimental lifetime of 763 µs. The presence of 0.5 mol% of Tm3+ effectively reduces the lifetime to 50 µs. This proves that the non-radiative energy transfer from Yb3+ to Tm3+ takes place. Its efficiency ηET = 93% was calculated according to equation (19) [64]: = 1−

8 8

(19)

where τR and τR0 are donor (sensitizer) luminescence lifetimes in the presence and absence of acceptors (activators), respectively. Fig.10 presents the lifetime values as a function of temperature for luminescent levels of Tm3+ (3F4, 3H4) and Yb3+ (2F5/2) in TZPN:0.5%Tm,5%Yb glass. As shown in Fig.10 the3F4 (Tm3+) lifetime gradually decreases in the temperature range 295 - 675 K from ~1.3 ms at 295 K to 815 µs at 675 K. A similar trend is observed for the luminescent level 3H4 (Tm3+) for which the maximum value of lifetime is 30 µs at 300 K and with increasing temperature gradually decreases to ~20 µs. In the case of the Yb3+ ion the lifetime of the excited state 2F5/2 is 34 µs at 300 K and when the temperature reaches 700 K the lifetime drops to 16 µs.

18

This shows that the temperature increase causes thermal quenching of luminescence in materials under study and TZPN:Tm,Yb glass is characterized by high temperature sensitivity in the tested temperature range (295 - 675 K).

19

4. CONCLUSIONS

The thermal and spectroscopic properties of singly-doped (Tm3+) and co-doped (Tm3+/Yb3+) oxyfluorotellurite glasses with different activator concentrations have been investigated in details. The analysis of experimental data indicates that our glasses combine good physicochemical properties (thermal, chemical and mechanical stability) with desirable spectroscopic parameters (long experimental lifetimes, high quantum efficiencies of metastable levels and large peak values of emission cross-sections for Tm3+) and can be considered attractive candidates for optical temperature sensors. The TZPN glass host allows strong interaction between incorporated luminescent ions and efficient Tm3+–Yb3+ energy transfer competes with an adverse self-quenching of the Tm3+ luminescence. Therefore, the level of doping should be selected taking into account the luminescent properties for the specific application. Luminescence spectra of TZPN:Tm and TZPN:Tm,Yb glasses reveal intense blue (1G4 → 3H6), red (1G4 → 3F4) and infrared (3H4 → 3

H6, 3H4 → 3F4 and 3F4 → 3H6) emission bands. In TZPN:Tm glass an efficient laser

generation can be obtained around 1.9 µm (3F4 → 3H6 transition). Increase of temperature brings about a significant enhancement of thermal quenching of luminescence in TZPN:Tm,Yb glass which displays thereby a high temperature sensitivity in the temperature range 295 - 675 K. The maximum relative sensitivity values 0.54 %K-1 (I700/I470) at 295 K, 0.80 %K-1 (I700/I800) at 450 K and 1.12 %K-1 (I1200/I1050) at 295 K were found for TZPN:0.5%Tm,5%Yb glass sample.

20

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https://doi.org/10.1021/acsami.8b02853. [61] S. Balaji, K. Biswas, A.D. Sontakke, G. Gupta, K. Annapurna, Enhanced 1.8µm emission in Yb3+/Tm3+ co-doped tellurite glass: Effects of Yb3+↔Tm3+ energy transfer and back transfer, J. Quant. Spectrosc. Radiat. Transf. 147 (2014) 112-120. https://doi.org/10.1016/j.jqsrt.2014.05.025. [62] T. Zhu, G. Tang, X. Chen, M. Sun, Q. Qian, Z. Yang, Enhanced 1.8 µm emission in Er3+/Tm3+ co-doped lead silicate glasses under different excitations for near infrared laser, J. Rare Earths 34 (2016) 978-985. https://doi.org/10.1016/S1002-0721(16)601242. [63] H.Y. Zhao, A.Z. Li, Y.T. Yi, M. Tokurakawa, G. Brambilla, S.J. Jia, S.B. Wang, P.F. Wang, A Tm3+-doped ZrF4-BaF2-YF3-AlF3 glass microsphere laser in the 2.0 µm wavelength

region,

J.

Lumin.

212

(2019)

207-211.

https://doi.org/10.1016/j.jlumin.2019.04.046. [64] K.S. Kumar, C. Lou, Y. Xie, L. Hu, A.G. Manohari, D. Xiao, H. Ye, L. Tang, D. Pribat, Energy transfer in co- and tri-doped Y3Al5O12 phosphors, J. Rare Earths 35 (2017) 775782. https://doi.org/10.1016/S1002-0721(17)60975-X.

27

FIGURE CAPTIONS Fig.1. Differential thermal analysis (DTA) curves recorded for Tm3+- doped glass samples (65-x)TeO2-20ZnF2-12Pb2O5-3Nb2O5-xTm2O3 with x = 0.5, 2 and 5 (upper) as well as for co-doped TZPN:0.5%Tm,2%Yb and TZPN:0.5%Tm,5%Yb glass samples (lower).

Fig.2. Absorption spectrum of TZPN:2%Tm glass (upper) and excitation spectrum of TZPN:0.5%Tm,5%Yb luminescence monitored at 1017 nm. (lower).

Fig.3. Emission spectra of TZPN:Tm and TZPN:Tm,Yb glasses recorded under excitation at 457 nm.

Fig.4. Upconverted emission spectra of oxyfluorotellurite glass (TZPN:0.5%Tm,5%Yb) measured as a function of temperature (295 - 625 K) under excitation at 803 nm and 975 nm. (Below) The Tm3+ and Yb3+ energy level schemes related to observed upconversion phenomena. Fig.5. Fluorescence intensity ratios related to 3F3,2 → 3H6 / 1G4 → 3H6 and 3F3,2 → 3H6 / 3H4 → 3H6 transitions in TZPN:0.5%Tm,5%Yb glass as well as corresponding absolute and relative thermal sensitivities estimated for 803 nm and 975 nm excitations. Fig.6. Absorption cross-section and emission cross-section versus wavelength for 3F4 ↔ 3H6 Tm3+ transitions in TZPN glass (upper) and effective stimulated emission cross-section vs. wavelength determined for several values of population inversion parameter P (lower). Fig.7. Emission spectra assigned to the 3F4 → 3H6 transition of Tm3+ in TZPN:0.5%Tm,5%Yb glass measured as a function of temperature from 295 K to 625 K under excitation at 803 nm.

Fig.8. NIR emission spectra of TZPN:0.5%Tm,5%Yb glass measured as a function of temperature from 295 K to 625 K under excitation at 803 nm (right). Fluorescence intensity ratios related to 3F3,2 → 3H5 (Tm3+)/ 2F5/2 → 2F7/2 (Yb3+) transitions in TZPN:0.5%Tm,5%Yb glass as well as corresponding relative thermal sensitivity (right). 28

Fig.9. Decay curves of 1G4 and 3H4 thulium luminescence and 2F5/2 ytterbium luminescence measured for single doped and co-doped TZPN glasses. Fig.10. Lifetime values versus temperature for luminescent levels of Tm3+ and Yb3+ in TZPN:0.5%Tm,5%Yb glass.

29

Table 1. Characteristic temperatures (Tg, Tc, Tpc) and the thermal stability criteria (∆T, H’, S) in various glasses doped with Tm3+ and Yb3+ and co-doped Tm3+/Yb3+ Tg

Tc

Tpc

∆T

[oC]

[oC]

[oC]

[oC]

TZPN

365

552

588

187

0.51

18.44

[33]

TZPN:0.5%Tm

363

520

599

157

0.43

34.17

this work

TZPN:2%Tm

366

532

577

166

0.45

20.41

this work

TZPN:5%Tm

367

511

676

144

0.39

64.74

this work

TZPN:0.5%Tm_2%Yb

368

515

610

147

0.40

37.95

this work

TZPN:0.5%Tm_5%Yb

364

516

650

152

0.42

55.96

this work

TZPN:0.5%Yb

361

498

633

137

0.40

51.23

[34]

TZPN:2%Yb

364

610

703

246

0.68

62.85

[34]

TZPN:5%Yb

364

580

667

216

0.59

51.63

[34]

F(0GeO2)T5

321

−

−

46

−

−

[4]

TZF2

263

380

−

117

−

−

[6]

NTY1

353

>540

−

>187

−

−

[35]

NTY2

354

>540

−

>186

−

−

[35]

Glass code

H’

S [oC]

Ref.

Table 2. Electric-dipole oscillator strengths of Tm3+ transitions in TZPN glass 6

H6 →

2S+1

LJ

-6 Centre of the band gravity Oscillator strength Pp [×10 ]

[cm-1]

Pexp

Pcal

∆P

3

F4

5849

3.11

3.47

0.36

3

H5

8308

2.54

1.92

0.62

3

H4

12629

3.58

3.63

0.05

3

F3,2

14541

3.45

3.60

0.15

1

G4

21332

1.89

1.44

0.45

Ω2 = 4.35×10-20 cm2

,

Ω4 = 1.43×10-20 cm2 RMS = 7.45×10-7

,

Ω6 = 0.97×10-20 cm2

Table 3. Comparison of the Judd-Ofelt parameters of the Tm3+ doped oxyfluorotellurite glass with the other reported Tm3+ doped glasses JO parameters [×10-20 cm2]

Trends of Ω2,4,6

References

0.97

Ω2 > Ω 4 > Ω6

this work

1.81

1.37

Ω2 > Ω 4 > Ω6

[4]

4.09

1.36

1.19

Ω2 > Ω 4 > Ω6

[17]

4BaF2–4AlF3–6La2O3– 16MgO–70TeO2– 0.5wt%Tm2O3–0.5wt%Yb2O3

4.46

1.47

1.08

Ω2 > Ω 4 > Ω6

[45]

41SiO2–10 Al2O3–25LiF– 23SrF2–1Tm2O3

5.57

2.94

2.21

Ω2 > Ω 4 > Ω6

[46]

Glass host

Ω2

Ω4

Ω6

63TeO2–20ZnF2–12PbO– 3Nb2O5–2Tm2O3

4.35

1.43

75TeO2–0GeO2–10ZnF2– 12PbO–3Nb2O5–5mol%Tm3+

3.96

80TeO2–5TiO2–15Nb2O5– 0.5wt%Tm2O3

Table 4. Radiative transition rates, branching ratios of luminescence and radiative lifetimes estimated for TZPN:Tm glass

Transition

Radiative probability [s-1]

Branching ratio

Lifetime [µs]

Arad

βcalc

τrad

5849

448

1

2234

Average frequency [cm-1]

3

F4 →

3

H6

3

H5 →

3

F4

2459

35

0.08

3

H6

8308

409

0.92

3

H4 →

3

H5

4321

97

0.04

3

F4

6780

144

0.06

3

H6

12629

2177

0.90

3

F3,2 →

3

H4

1912

10

0.01

3

H5

6233

510

0.15

3

F4

8692

536

0.15

3

H6

14541

2341

0.69

1

G4 →

3

F3,2

6791

130

0.05

3

H4

8703

456

0.11

3

H5

13024

1213

0.30

3

F4

15483

232

0.06

3

H6

21332

1885

0.48

2251

414

294

255

Table 5. Experimental lifetimes of Tm3+ and Yb3+ luminescent levels in TZPN:Tm and TZPN:Tm,Yb glasses 3

3

H4 [µs] (Tm3+)

1

G4 [µs] (Tm3+)

2

Glass TZPN

F4 [µs] (Tm3+)

F5/2 [µs] (Yb3+)

0.5 at.% Tm

1560

186

55

-

2 at.% Tm

479

11

4

-

5 at.% Tm

86

1

1

-

0.5 at.% Tm, 2 at.% Yb

1492

72

35

50

0.5 at.% Tm, 5 at.% Yb

1242

36

24

34

(65-x)TeO2-20ZnF2-12PbO-3Nb2O5-xTm2O3 (as-melted)

DTA (a.u.)

Tg= 367,1 oC

Tpc= 675,9 oC *

Tg= 365,5 oC

x=5

Tc= 510,9 oC Tpc= 577,1 oC

*

Tg= 363,4 oC Tc= 531,6 oC

Tpc= 598,9 oC *

x=2 Tc= 519,8 oC

x = 0.5 200

300

400

500

600

700

800

Temperature (oC) (65-x-y)TeO2-20ZnF2-12PbO-3Nb2O5-0.5Tm2O3-yYb2O3 (as-melted) Tg= 363,6 oC

DTA (a.u.)

y=5 * Tpc= 650,0 oC Tg= 368,5 oC Tc= 515,8 oC

y=2

* Tpc= 610,4 oC Tc= 515,1 oC 200

300

400

500

600

Temperature (oC)

700

800

Time [ms]

1400

TZPN:0.5%Tm,5%Yb

1200 1000 800

3F (Tm3+) 4

600 300

400

500

600

700

T [K]

Time [ms]

35

TZPN:0.5%Tm,5%Yb

30 25

3H (Tm3+) 4

20 300

400

500

600

700

T [K]

Time [ms]

35

TZPN:0.5%Tm,5%Yb

30 25 20 15

3+ 2F (Yb ) 5/2

300

400

500

600

700

T [K]

Absorption coefficient [cm-1]

10

TZPN:2%Tm

8

3 H4

6 3 F3,2

3 H6

3 H5 3 F4

4

2

1 G4

0 400

600

800

1000

1200

1400

1600

1800

2000

Luminescence Intensity [a.u.]

Wavelength [nm]

TZPN:0.5%Tm,5%Yb ldet.=1017 nm

3

F3,2

O2--Te4+

3

1

300

400

H4

G4

500

600

700

800

Wavelength [nm]

TZPN:Tm,Yb 1

3

G 4 - H6

3

1

G4-3F4

3

3

3

H 4 - H6

Intensity [a.u.]

Exc. 457 nm

H4-3F4 3

H5-3H6

F4-3H6

0.5%Tm 2

2

2%Tm

3+

F5/2- F7/2 (Yb )

5%Tm 0.5%Tm,5%Yb 0.5%Tm,2%Yb 400

600

800

1000

1200

Wavelength [nm]

1400

1600

1800

1

28 26

22

1

20

10 8 6 4 2 0

3

F

3

G 4

18 16

ET

3, 2

14

H4

12

2

3

H 5 3 F 4

F

10

5 /2

8 6 4 2

3

1

20

G 4

18

12

2

24

22

14

D

26

24

16

1

28

D 2

2

H 6

Tm

3+

3+ Yb

F 7 /2

0

3

F

3

3 ,2 ET

H4

2

3

H 5 3 F 4 3

F

2

H 6

Tm

3+

3+ Yb

5 /2

F

7 /2

1 2 0

0 ,2 5

T Z P N :0 .5 % T m ,5 % Y b

1 0 0

F IR (7 0 0 /8 0 0 )

E x c . 8 0 3 n m

8 0

F IR (7 0 0 /4 5 0 )

T Z P N :0 .5 % T m ,5 % Y b 0 ,2 0

6 0 4 0

A

2 0

E x c . 9 7 5 n m 0 ,1 5 0 ,1 0

D

0 ,0 5 0 0 ,0 0

2 5 0

3 0 0

3 5 0

4 0 0

4 5 0

5 0 0

5 5 0

6 0 0

6 5 0

2 5 0

3 0 0

3 5 0

4 0 0

T [K ]

4 5 0

5 0 0

5 5 0

6 0 0

6 5 0

5 0 0

5 5 0

6 0 0

6 5 0

T [K ] 0 ,0 0 1 5

0 ,0 0 7 0 ,0 0 6

0 ,0 0 1 0

A

[K

0 ,0 0 4

0 ,0 0 0 5

S

S

A

[K

-1

]

-1

]

0 ,0 0 5

E

0 ,0 0 3

B

0 ,0 0 2

0 ,0 0 0 0

0 ,0 0 1

2 5 0

3 0 0

3 5 0

4 0 0

4 5 0

5 0 0

5 5 0

6 0 0

2 5 0

6 5 0

3 0 0

3 5 0

4 0 0

4 5 0

0 ,6 0 ,8

]

]

0 ,4

F

S

R

R

[%

[%

K

K

-1

-1

0 ,6

S

0 ,2

0 ,4

C 0 ,2

0 ,0

2 5 0

3 0 0

3 5 0

4 0 0

4 5 0

5 0 0

5 5 0

6 0 0

6 5 0

2 5 0

3 0 0

3 5 0

4 0 0

4 5 0

5 0 0

5 5 0

6 0 0

6 5 0

1.0

Cross section 10-20 [cm2]

TZPN:Tm 0.8

0.6

3F -3H 4 6

3 H -3 F 6 4

0.4

0.2

0.0 1400

1500

1600

1700

1800

1900

2000

2100

2200

Wavelength [nm]

0.5

Eff. cross section 10-20 [cm2]

TZPN:Tm 0.4

0.3

P = 0.5; 0.4; 0.3; 0.2; 0.1

0.2

0.1

0.0 1400

1500

1600

1700

1800

1900

2000

2100

Wavelength [nm]

2200

Exc. 803 nm

3F -3H 4 6

Intensity [a.u.]

TZPN:0.5%Tm,5%Yb

295 K 375 K 425 K 475 K T [K] 525 K 575 K 625 K 1500

1600

1700

1800

1900

Wavelength [nm]

2000

2100

2200

0 ,4 3

H 4

-3F 4

(T m

3 +

)

2

F

5 /2

-2F

7 /2

3 +

(Y b

In te n s ity [a .u .]

E x c . 8 0 3 n m )

F IR (1 2 0 0 /1 0 5 0 )

T Z P N :0 .5 % T m ,5 % Y b

0 ,3

0 ,2

0 ,1 R = -0 .0 4 + (0 .1 6 e x p (0 .0 0 5 T ))

0 ,0 3

F

3

3 ,2

- H 5

(T m

3 +

3 0 0

)

4 0 0

5 0 0

6 0 0

7 0 0

6 0 0

7 0 0

T [K ]

2 9 5 K

1 ,2

3 7 5 K 4 2 5 K K S [%

5 2 5 K

T [ K ]

4 7 5 K

-1

]

1 ,0

0 ,8

5 7 5 K 6 2 5 K 9 0 0

1 0 0 0

1 1 0 0

1 2 0 0

1 3 0 0

W a v e le n g th [n m ]

1 4 0 0

1 5 0 0

0 ,6 3 0 0

4 0 0

5 0 0 T [K ]

Intensity [a.u.]

100

TZPN:0.5%Tm texp= 55 ms

10-1 10-2 -3

10

1G 4

TZPN:2%Tm texp= 4 ms

10-4 0

50

(Tm3+)

100

150

200

Time [ms] Intensity [a.u.]

100

TZPN:0.5%Tm texp = 186 ms

10-1 10-2

3H

4 3+

TZPN:2%Tm texp= 11 ms

10-3 0

200

(Tm ) 400

600

Time [ms] Intensity [a.u.]

100

TZPN:2%Yb texp= 763 ms

10-1 10-2 10-3

2F 5/2

TZPN:0.5%Tm,2%Yb texp= 50 ms

-4

10

(Yb3+)

10-5 0

500

1000

1500

2000

2500

3000

Time [ms]

3500

Highlights • Oyfluorotellurite glasses singly-doped and co-doped Tm3+ and Yb3+ were prepared. • Thermal and optical properties of tested materials were determined. • Thermal properties of glasses depend on the Tm3+/Yb3+ ions concentrations. • Investigated materials have advantageous properties for NIR optical gain. • TZPN glasses activated with RE3+ ions can be used in luminescence thermometry.

Author Contribution Statement: Barbara Klimesz – Resources, Investigation, Methodology, Formal analysis, Writing Original Draft, Writing - Review & Editing, Visualization Radosław Lisiecki – Investigation, Methodology, Formal analysis; Writing - Review & Editing, Visualization Witold Ryba-Romanowski – Formal analysis, Conceptualization, Supervision

Declaration of interests ⌧ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: