TeO2-Ga2O3-ZnO ternary tellurite glass doped with Tm3+ and Ho3+ for 2 µm fiber lasers

Journal of Non-Crystalline Solids 531 (2020) 119855

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TeO2-Ga2O3-ZnO ternary tellurite glass doped with Tm3+ and Ho3+ for 2 µm fiber lasers L.Y. Mao, J.L. Liu, L.X. Li, W.C. Wang

T

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State Key Laboratory of Luminescent Materials and Devices, Guangdong Engineering Technology Research and Development Center of Special Optical Fiber Materials and Devices, Guangdong Provincial Key Laboratory of Fiber Laser Materials and Applied Techniques, South China University of Technology, Guangzhou 510640, PR China

A R T I C LE I N FO

A B S T R A C T

Keywords: Tellurite glass Glass-forming region Physical property 2 µm fluorescence

The glass-forming region of TeO2-Ga2O3-ZnO (TGZ) ternary tellurite glass was predicted via a thermodynamic method and then confirmed by a few experiments. Upon cooling the melts at a rate of ~20 K/s, the glass formation was observed within the range of 60–97 TeO2, 0–20 Ga2O3, and 0–40 ZnO (in mol%). For a better understanding of their fundamental physical, thermal and optical properties, the density, characteristic temperatures, Abbe number, linear and nonlinear refractive indices were comprehensively investigated. The possibility of TGZ glass host for 2 µm fiber lasers was studied through Tm3+ and Ho3+ doping and rare-earth sensitization. The results show such newly developed TGZ glass owns high thermal stability, low phonon energy and efficient 2 µm fluorescence, which enriches the tellurite glass system and may contribute to the research of high gain fiber lasers.

1. Introduction

include TeO2-ZnO-Na2O (TZN) [4] and TeO2-WO3-La2O3 (TWL) glasses [6]. The former has a problem that the glass transition temperature is low (~300 °C), resulting in low laser induced damage threshold (LIDT) and difficult to withstand higher laser pumping and output power. The latter faces a limitation of higher phonon energy (~930 cm−1) and larger nonradiative relaxation rate of the RE ions as well as low luminescence efficiency. Therefore, it is urgent to explore a new type of tellurite glass system with suitable physicochemical and optical properties. In our most recent work [7,8], several tellurite glasses such as TeO2-Ga2O3-BaO (TGB) and TeO2-Mo2O3-ZnO (TMZ) glasses with excellent glass-forming ability and spectroscopic properties have been developed. In particular, the previous study on Ga2O3-TeO2 binary glasses has proved the introduction of Ga2O3 can remarkably influence the refractive index, thermal expansion coefficient, chemical resistance and glass transition temperature of tellurite glass [9]. Here Ga2O3 plays a role of network intermediate component and TeeOeTe and GaeOeTe bridging bonds connect the tellurite building units in a continuous network. Suitable glass hosts and efficient luminescence properties are the key to promote the applications of fiber lasers, yet have not been fully investigated. On the other hand, emission around 2 µm is generally originated from the radiative transitions of Tm3+: 3 F4 → 3H6 and Ho3+: 5I7 → 5I8, respectively. Both ions exhibit wide gain bandwidths ranging from 1.7 to 2.1 µm, which provide a wide selection range of laser operation wavelengths when operating in

Due to its unique physical and optical properties, tellurite glass has received extensive attention from researchers and has gained important applications in practice [1–3]. For example, the typical tellurite glass system exhibits lower phonon energy and wider infrared transmission range than the silica, phosphate and germanate glasses. In such a glass system, TeO2 generally cannot form glass by itself, but when combining with one or more network modifiers or intermediates, it will form a very stable glass with good physical, thermal and optical properties [4]. The presence of [TeO4] bipyramid, [TeO3] pyramid, and [TeO3+1] polyhedron provides a range of sites for rare-earth (RE) ions such as Tm3+ and Ho3+, thus significantly reduces the concentration quenching effect and enables the efficient luminescence of RE ions. Besides, tellurite glass has relatively low phonon energy (~650–800 cm−1) in all oxide glasses, which is beneficial to achieving efficient near- and mid-infrared fluorescence or laser. Indeed, recent reports [5] have shown laser wavelengths of up to 2.3 µm could be achieved in tellurite glass, which is still not realized in any other oxide glasses. Moreover, tellurite glasses also show the highest nonlinear indices ever found for oxide glasses, this fascinating feature make them potential applications in the current hot fields such as supercontinuum spectroscopy and all-optical Switch. At present, the most mature and widely used tellurite glass systems

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Corresponding author. E-mail address: [email protected] (W.C. Wang).

https://doi.org/10.1016/j.jnoncrysol.2019.119855 Received 10 October 2019; Received in revised form 5 December 2019; Accepted 6 December 2019 0022-3093/ © 2019 Elsevier B.V. All rights reserved.

Journal of Non-Crystalline Solids 531 (2020) 119855

L.Y. Mao, et al.

continuous-wave (CW) or Q-switched mode [10–12]. As a continuation of our previous work, this paper conducts a study on the glass-forming region of TeO2-Ga2O3-ZnO (TGZ) ternary glass system by the thermodynamic calculation and conventional meltquenching methods, together with the physical, thermal and spectroscopic parameters. Moreover, the 2 µm emission properties and the related energy transfer mechanisms of Tm3+ single-doped, Tm3+/Ho3+ and Nd3+/Ho3+ co-doped TGZ glasses were compared and analyzed to study the application possibility of TGZ glass in mid-infrared laser glass. The results indicate the TGZ glass is a promising gain medium for 2 µm fiber lasers. 2. Experiments Two series of tellurite glasses with nominal compositions of 80TeO2(20-x)Ga2O3-xZnO (x = 5, 10, 15 mol%, denoted as TGZ5, TGZ10 and TGZ15, respectively) and (95-y)TeO2-5Ga2O3-yZnO (y = 15, 20, 25 mol %, denoted as TGZ15, TGZ20 and TGZ25, respectively) were prepared by melt-quenching method in air atmosphere at a cooling rate of ~20 K/s. In addition, three other glasses with an additional 1.0 mol% Tm2O3 doping, 0.5 mol% Tm2O3/0.5 mol% Ho2O3 co-doping, and 0.5 mol% Nd2O3/0.5 mol% Ho2O3 co-doping were prepared based on the TGZ25 glass. The raw materials are TeO2 (Aladdin, 99.99%), Ga2O3 (Aladdin, 99.99%), ZnO (Macklin, 99.99%), Tm2O3 (Aladdin, 99.99%), Ho2O3 (Aladdin, 99.99%), and Nd2O3 (Aladdin, 99.99%). A batch of 15 g was melted in corundum crucibles in an electric furnace at 850 °C for 30 min. Then the melt was cast into preheated graphite molds (~220 °C) for quenching and soon transferred to a muffle furnace and kept at 320 °C for 2 h, followed by cooling to room temperature at a rate of around 3–5 K/h. The annealed samples were cut and polished or grind to powders for further measurements. The densities were determined through Archimedes principle at 25 °C, using deionized water as immersion liquid. The refractive indices at 488, 589, 633, 655, 1309 and 1533 nm were measured by prism coupler instrument (Metricon Model 2010M, American) with an accuracy within ± 0.0005. The characteristic temperatures were obtained by differential scanning calorimetry (DSC; STA449C Jupiter, Netzsch, German) with a heating rate of 10 K/min in the N2 protected atmosphere. The infrared (IR) cut-off wavelength was acquired on a Vector33 Fourier transform infrared spectrophotometer (FTIR; Bruker, Switzerland). The thermal conductivity was measured by a Physical Property Measurement System (PPMS-9, Quantum Design, US) at room temperature. The thermal expansion coefficient was tested by a thermomechanical analyzer (DIL 402C, Netzsch, German) with a heating rate of 5 °C/min from 25 to 360 °C. The hardness and elastic modulus were measured through nanoindentation tests performed on a nanoindentation tester (TTX-NHT3, Anton Paar, Austria) with a diamond Berkovich (three-sided pyramid) indenter tip (loading rate: 10.0 mN/ min, maximum load: 5.00 mN, pause: 5.0 s, unloading rate: 10.0 mN/ min). The Raman spectra were tested by Raman spectrometer (Renishaw, UK) using a 532.8 nm laser as excitation source. The absorption spectra were recorded by Perkin-Elmer Lambda 900 UV/VIS/ NIR double beam spectrophotometer (Waltham, MA) with a step length of 1 nm. The visible, near-infrared and mid-infrared photoluminescence spectra were measured on a Triax 320 type spectrofluorometer (JobinYvon Corp) under the excitation of an 808 nm LD by using photomultiplier, InGaAs and PbSe detectors, respectively. The luminescence decay curves were captured by a function generator (TFG3051C, Tektronix Inc., US) and a digital oscilloscope (TDS3012C, Tektronix Inc., US).

Fig. 1. The glass-forming region of TGZ ternary glass system (in mol%, the area enclosed by red triangle is the theoretical GFR, and the area enclosed by blue dotted lines is the experimental GFR). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

principle that glass is usually formed easily near the eutectic points in the phase diagram. Its theoretical significance lies in combining the glass formation ability and the phase diagram organically. The practical significance is to determine the glass-forming region more quickly and efficiently and guide the discovery of new glass. In this paper, the theoretical GFR of ternary TGZ glass system is predicted utilizing the thermodynamic calculation method [13], which is enclosed by the red triangle, as shown in Fig. 1. Based on this, the experimental GFR (enclosed by the blue dotted lines in Fig. 1) is further determined by some experiments, which is within the range of 60–97 TeO2, 0–20 Ga2O3, and 0–40 ZnO (in mol%). The representative glass components in the experimental GFR were selected to study their physical and thermal parameters, as shown in Table 1. TGZ glass exhibits a large density (5.238–5.371 g/cm3), large linear refractive index (n@633 nm = 1.9905–2.0189) and long IR cutoff wavelength (5.53–5.87 µm). The DSC curves of TGZ glasses are displayed in Fig. 2. The Tg of TGZ glass is 366–377 °C, between the traditional TZN glass (303 °C) and TWL glass (476 °C). Besides, the ΔT of TGZ glass ranges from 81 to 111 °C. TGZ25 has the largest ΔT of 111 °C, which is comparable to that of TZN glass (114 °C) but less than that of TWL glass (134 °C) [14, 15]. Generally, a glass with a ΔT larger than 100 °C is considered thermal stable enough for fiber drawing. Therefore, it can be considered that TGZ25 has the best thermal stability among these different glass compositions. The refractive indices of TGZ glasses were further measured and then fitted by a three-term form of Cauchy's equation, as shown in Fig. 3. Then the refractive indices at 486.1, 587.6, and 656.3 nm (denoted as nF, nd, and nC, respectively) were determined from the fitted curves. Using the formula νd = (nd − 1)/(nF − nC) [16], the Abbe number νd is calculated as 20.92–22.14, comparable to that of TeO2TiO2-WO3 glass (22.78) [17] and TeO2-BaO-La2O3 glass (20.53) [18], as shown in Table 1. The Abbe number characterizes the dispersion of materials, the high values of which indicate low dispersion. Moreover, the nonlinear refractive index n2 is estimated by the following empirical relationship [16]:

3. Results and discussion 3.1. Glass-forming region and essential physical properties The prediction of the glass-forming region (GFR) is based on the 2

Journal of Non-Crystalline Solids 531 (2020) 119855

L.Y. Mao, et al.

Table 1 The physical and thermal parameters of TGZ glasses. Glass

ρ (g cm−3)

TGZ5 TGZ10 TGZ15 TGZ20 TGZ25

5.338 5.355 5.238 5.353 5.371

Error

± 0.001

n2 (10−13 esu) =

n2 (10−13 esu)

νd

Refractive index 488 nm

589 nm

633 nm

655 nm

1309 nm

1533 nm

2.0466 2.0618 2.0307 2.0591 2.0533

2.0145 2.0271 1.9995 2.0256 2.0199

2.0048 2.0189 1.9905 2.0165 2.0118

2.0008 2.0131 1.9863 2.0124 2.0064

1.9614 1.9736 1.9491 1.9735 1.9688

1.9585 1.9710 1.9460 1.9702 1.9660

21.71 20.92 22.14 21.49 21.55

19.91 21.61 18.74 20.69 20.38

Thermal parameter (°C) Tg

Tx

Tp

△T

377 366 366 375 369

458 455 466 470 480

471 468 482 488 494

81 89 100 95 111

± 0.0005

νd ⎡1.517 + ⎣

6n d

Raman spectra of TGZ glasses, as shown in Fig. 4. Table 2 summarizes the Raman vibration peaks and corresponding structural units. The peaks around 440 cm−1 are assigned to the bending vibrations of mixed GaeOeTe bridging bonds and symmetric stretching and bending vibrations of TeeOeTe linkages formed by vertex-sharing of [TeO4] trigonal bipyramids (tbps), [TeO3+l] polyhedra and [TeO3] triangular pyramids (tps). The peaks near 680 cm−1 are ascribed to the antisymmetric vibrations of TeeOeTe linkages constructed by two inequivalent TeeO bonds. The peaks around 760 cm−1 are attributed to both a vibration of continuous network composed of [TeO4] tbps and a stretching vibration of tellurium and non-bridging oxygen atoms in [TeO3+l] polyhedra and [TeO3] tps. All of these Raman bands have been verified by Ilieva and Sekiya et al. [9,21,22]. The maximum phonon energy of TGZ is around 760 cm−1, which is less than that of silicate glass (~1000 cm−1), germanate glass (~900 cm−1), phosphate glass (~1200 cm−1) and fluorophosphate glass (~1128 cm−1) [23]. A glass host with relatively low maximum phonon energy contributes to a higher radiative transition rate of RE ions.

1/2

νd ⎤ ⎦

5.81 5.83 5.53 5.64 5.87

±1

68(nd − 1)(nd2 + 2)2 (nd2 + 2)(n d + 1)

IR cut-off wavelength (µm)

(1)

The estimated n2 ranges from 18.74 × 10−13 to 21.61 × 10−13 esu (i.e., 3.93–4.46 × 10−15 cm2/W), which is 10 times larger than that of silicate and phosphate glasses (1.351 × 10−13 and −13 1.01–1.18 × 10 esu, respectively) [19, 20], and also much larger than that of TZN glass (1.90 × 10−15 cm2/W) [14]. Besides, the thermal conductivity, thermal expansion coefficient, hardness and elastic modulus of TGZ25 glass were investigated. Thermal conductivity characterizes the ability of heat transfer and thermal expansion coefficient indicates the change in material geometrical size with temperature. In general, higher thermal conductivity and lower thermal expansion coefficient are favorable for the mitigation of thermal damage of fiber lasers [15]. The thermal conductivity of TGZ25 is 0.79 W m−1 K−1, which is much larger than that of phosphate glasses (0.56–0.58 W m−1 K−1) used for high power lasers [20]. The thermal expansion coefficient of TGZ25 is 14.7 × 10−6 K−1 (40–300 °C), ~23% lower than that of TZN glass (19.08 × 10−6 K−1) [14]. Therefore, TGZ25 is considered to be more resistant to thermal damage than TZN during fiber drawing or laser operation. The hardness and elastic modulus of TGZ25 are tested to be 421.8 Vickers and 62.2 GPa, respectively. The hardness is ~16% larger than that of TZN (364 Vickers) [14], indicating TGZ25 has better mechanical property. The Raman spectra of glasses reflect their internal structure, from which the information of phonon energy can also be obtained. Three distinct peaks around 440, 680 and 760 cm−1 are observed from the

3.2. Absorption spectra and Judd-Ofelt analysis Based on the analysis of thermal parameters and essential physical properties of TGZ glasses, TGZ25 with best thermal stability and longest IR cut-off wavelength among all samples was selected for RE doping. Fig. 5 displays the absorption spectra of Tm3+ single-doped, Tm3+/ Ho3+ and Nd3+/Ho3+ co-doped TGZ25 glasses ranging from 300 to 2150 nm. For Tm3+ single-doped sample, five absorption bands centered at 465, 687, 794, 1213 and 1700 nm were detected,

Fig. 2. Differential scanning calorimetry patterns of TGZ glasses. 3

Journal of Non-Crystalline Solids 531 (2020) 119855

L.Y. Mao, et al.

Fig. 3. Dispersion curves of TGZ glasses (scatter points represent the measured refractive indices and the curves were fitted by Cauchy's dispersion formula).

Fig. 5. Absorption spectra of Tm3+-doped, Tm3+/Ho3+ and Nd3+/Ho3+ codoped TGZ25 glasses.

Fig. 4. Raman spectra of TGZ glasses.

ground state 4I9/2 to excited levels (4G7/2 + 4G9/2 + 2K13/2), (4G5/ 2 4 4 4 4 2 4 2 + G7/2), F9/2, ( F7/2 + S3/2), ( F5/2 + H9/2) and F3/2, respectively. The shape and positions of these absorption peaks are highly similar to those reported in other Refs. [24,25]. Besides, these glasses all have absorption bands around 808 nm, indicating they can be efficiently excited by commercial 808 nm LDs. The Judd-Ofelt theory is extensively accepted and employed to evaluate the spectroscopic properties of RE-doped glasses [26, 27]. Based on the absorption spectra of Tm3+ single-doped and Nd3+/Ho3+ co-doped samples, the J-O parameters of Tm3+ and Ho3+ are

corresponding to the transitions from ground state 3H6 to excited levels 1 G4, 3F2,3, 3H4, 3H5, and 3F4, respectively. In Tm3+/Ho3+ and Nd3+/ Ho3+ co-doped samples, other seven absorption bands centered at 418, 454, 486, 539, 644, 1155 and 1951 nm were observed, corresponding to the transitions of Ho3+ from ground state 5I8 to excited levels 5G5, (5G6 + 5F1), (5F2,3 + 3K8), (5S2 + 5F4), 5F5, 5I6 and 5I7, respectively. In Nd3+/Ho3+ co-doped TGZ25 glass, besides the absorption bands of Ho3+, other six absorption bands of Nd3+ were obtained at 526, 585, 683, 748, 805 and 875 nm, which originate from the transitions from

Table 2 Raman vibration peaks and corresponding structural units of TGZ glasses. Raman shift ( ± 1 cm−1)

Structural units

Ref.

440 680 760

TeeOeTe linkages at vertex-sharing sites and GaeOeTe bridging bonds TeeOeTe linkages constructed by two inequivalent TeeO bonds Continuous network composed of [TeO4]; tellurium and non-bridging oxygen atoms in [TeO3+1] and [TeO3]

[9,21] [21,22] [21,22]

4

Journal of Non-Crystalline Solids 531 (2020) 119855

L.Y. Mao, et al.

Table 3 The J-O intensity parameters of Tm3+ and Ho3+ in TGZ25 and other glass matrixes. Glass matrix

Tm3+

Ho3+

Ωt (10−20 cm2) Ω2 Silicate Germanate Phosphate Fluorophosphate ZBLAN Sulfide TGZ25

3.08 5.55 5.63 3.19 2.31 5.80 4.78

Ω4 [39] [40] [42] [44] [46] [48]

Ωt (10−20 cm2)

Ω4/Ω6

0.99 2.03 1.75 1.75 1.28 1.60 1.71

Ω6 [39] [40] [42] [44] [46] [48]

0.40 1.26 1.11 1.66 1.17 1.30 1.20

Ω2 [39] [40] [42] [44] [46] [48]

2.48 1.61 1.58 1.05 1.09 1.23 1.43

[39] [40] [42] [44] [46] [48]

3.14 3.35 8.58 2.50 2.30 0.10 5.19

Ω4/Ω6 Ω4

[38] [41] [43] [45] [47] [49]

3.04 2.16 4.31 3.09 2.30 4.97 3.05

Ω6 [38] [41] [43] [45] [47] [49]

0.94 0.94 2.88 1.31 1.71 0.98 1.53

[38] [41] [43] [45] [47] [49]

3.23 2.30 1.50 2.36 1.35 5.07 1.99

[38] [41] [43] [45] [47] [49]

large spontaneous radiative transition probability is beneficial to achieve an efficient emission.

calculated and compared with those in other glass matrixes, as shown in Table 3. The fitting root-mean-square deviation for Tm3+ and Ho3+ are 0.39 × 10−6 and 0.32 × 10−6, respectively, confirming the reliability of the calculated results. The J-O intensity parameters are affected by the glass composition and local environment of doped RE ions. More specifically, parameter Ω2 is related to the degree of covalency between RE ions and ligand anions, and parameter Ω6 relates to the overlap integrals of 4f and 5d orbits [25]. Besides, the values of Ω4 and Ω6 reflect the rigidity and viscosity of glass matrixes [24]. As displayed in Table 3, the Ω2 of Tm3+ and Ho3+ in TGZ25 glass are larger than those in silicate, fluorophosphate and ZBLAN glasses but less than those in phosphate glasses, indicating the moderate covalence between doped ions and surrounding ligands. Utilizing the obtained J-O parameters, several key spectroscopic parameters like spontaneous radiative transition probability (Arad), fluorescence branching ratio (β) and radiative lifetime (τrad) of Tm3+ and Ho3+ are calculated, as listed in Table 4 and Table 5, respectively. The spontaneous radiative transition probabilities of Tm3+: 3F4 → 3H6 and Ho3+: 5I7 → 5I8 transitions are 592.91 s−1 and 171.15 s−1, respectively, which are larger than those of TZN (475.81 and 136.07 s−1) [28] and germanate-tellurite glass (333.13 and 102.05 s−1) [29]. The

3.3. Emission spectra, fluorescence lifetime and energy transfer mechanism The near- and mid-infrared emission spectra of Tm3+-doped, Tm /Ho3+ and Nd3+/Ho3+ co-doped TGZ25 glass under the excitation of 808 nm LD are shown in Fig. 6(a). There are two typical emission bands located around 1.48 and 1.81 µm in the spectrum of Tm3+ single-doped sample, originating from 3H4 → 3F4 and 3F4 → 3H6 radiative transitions, respectively [30]. Among them, the fluorescence intensity of 1.48 µm emission band is much weaker than that of 1.81 µm emission band, which ascribes to the small fluorescence branching ratio of 3H4 → 3F4 transition and strong cross-relaxation process of Tm3+: 3H6 + 3H4 → 3F4 + 3F4. In the Tm3+/Ho3+ co-doped sample, besides these two emission bands, another intense emission located around 2.04 µm is observed, which originates from the radiative transition of Ho3+: 5I7 → 5I8 [24]. As Ho3+ cannot directly absorb the energy of 808 nm LD, this emission benefits from the energy transfer process from Tm3+:3F4 level to Ho3+:5I7 level. The accompanying visible luminescence decreases the efficiency of 3+

Table 4 The spectral line strength (Sed), electric- and magnetic-dipole transition probability (Aed, Amd), fluorescence branching ratio (β) and radiative lifetime (τrad) of Tm3+ in TGZ25 glass sample. Initial state

Final state

3

3

F4 H5

3

H6 H6 F4 3 H6 3 F4 3 H5 3 H6 3 F4 3 H5 3 H4 3 H6 3 F4 3 H5 3 H4 3 F3 3 H6 3 F4 3 H5 3 H4 3 F3 3 F2 3 H6 3 F4 3 H5 3 H4 3 F3 3 F2 1 G4 3

3 3

H4

3

F3

3

F2

1

G4

1

D2

Sed (10−20 cm2)

Aed (s−1)

4.26 1.67 0.72 1.90 1.08 0.89 1.55 0.22 3.60 1.35 0.31 1.51 1.20 1.89 0.15 0.35 0.13 0.97 1.18 0.52 0.19 0.65 2.96 0.02 0.85 0.90 0.83 1.21

592.91 510.88 4.83 2521.77 217.73 48.61 4126.42 119.78 791.41 9.41 1291.62 1407.36 472.28 37.26 0.03 2258.46 313.31 1444.44 525.12 105.07 30.51 17354.14 38971.16 214.32 3848.81 2710.92 2216.79 462.60

5

Amd (s−1)

121.44 0.21 29.40 12.39 85.17 0.51

0.03 12.58 212.78 51.72 4.98

80.51 49.15

β (%)

τrad (ms)

100.00 99.21 0.79 89.11 8.73 2.16 80.39 3.99 16.03 0.19 40.26 43.86 14.72 1.16 0.00 45.54 6.57 33.42 11.63 2.22 0.62 86.53 66.39 0.37 6.36 4.42 3.46 0.95

1.69 1.57

0.35

0.19

0.31

0.20

0.02

Journal of Non-Crystalline Solids 531 (2020) 119855

L.Y. Mao, et al.

Table 5 The spectral line strength (Sed), electric- and magnetic-dipole transition probability (Aed, Amd), fluorescence branching ratio (β) and radiative lifetime (τrad) of Ho3+ in TGZ25 glass sample. Initial state

Final state

5

5

I7 I6

5

5

I5

5

F5

5

F4

5

F3

5

G5

I8 I8 5 I7 5 I8 5 I7 5 I6 5 I8 5 I7 5 I6 5 I5 5 I8 5 I7 5 I6 5 I5 5 F5 5 I8 5 I7 5 I6 5 I5 5 F5 5 F4 5 I8 5 I7 5 I6 5 I5 5 F5 5 F4 5 F3 5 F2 5 K8 5 G6 5

Sed (10−20 cm2)

Aed (s−1)

Amd (s−1)

β (%)

τrad (ms)

2.20 0.90 1.61 0.13 1.01 1.45 2.12 1.75 1.02 0.31 1.60 0.74 1.11 0.97 1.45 0.36 1.13 0.55 0.81 0.60 0.75 1.90 3.48 1.46 0.32 2.20 1.82 1.44 0.55 0.03 1.51

119.28 260.54 29.94 102.03 124.53 15.06 4258.08 1063.34 189.96 14.20 6637.53 1152.54 708.51 244.37 133.69 2639.28 3525.40 809.55 575.94 64.69 5.10 14521.29 12986.31 2935.64 370.35 746.57 153.57 31.47 6.82 0.32 4.16

51.87

100.00 82.27 17.73 39.82 48.60 11.57 77.06 19.24 3.44 0.26 74.39 12.92 7.94 2.74 2.01 34.61 46.24 10.62 7.55 0.85 0.13 45.73 40.89 69.08 8.71 17.57 3.61 0.74 0.16 0.00 0.02

5.84

the near- and mid-infrared fiber lasers, therefore, the visible spectra of Tm3+-doped, Tm3+/Ho3+ and Nd3+/Ho3+ co-doped TGZ25 glass are further measured to clarify the energy transfer mechanism and the competitive relationship between infrared down-conversion (DC) and visible up-conversion (UC) processes. In the Tm3+ single-doped sample, one distinct emission peak centered at 706 nm is observed (in Fig. 6(b)), which attributes to the 3F2,3 → 3H6 radiative transition of Tm3+ [30]. Differently, another green emission centered at 547 nm from the Ho3+:5F4 + 5S2 → 5I8 transition is detected in the Tm3+/Ho3+ codoped sample [30]. For Nd3+/Ho3+ co-doped sample, three bands centered at 545 nm, 659 nm and 688 nm are observed, the latter two emissions ascribe to the transitions of Ho3+: 5F5 → 5I8 and Nd3+: 4F9/ 4 3+ /Ho3+ co-doped sample also ex2 → I9/2 [25,31], respectively. Nd hibits strong near-infrared emissions at 1338, 1060 and 901 nm, while the Tm3+ single-doped and Tm3+/Ho3+ co-doped samples only show weak emission at 1.48 µm, as shown in Fig. 6(c). Moreover, the fluorescence decay curves of these glasses excited by 808 nm pulsed LD are shown in Fig. 6(d). All three curves satisfy the single-exponential decay characteristics. The lifetime of Tm3+: 3F4 level in Tm3+ single-doped sample is 299.42 µs, longer than that reported in Tm3+-doped TeO2ZnO-Na2O-La2O3 glass (193 µs) with the same doping concentration [24]. The lifetime of Ho3+: 5I7 level in Nd3+/Ho3+ and Tm3+/Ho3+ co-doped samples is 529.95 and 1260 µs, respectively. The energy level diagram helps to understand the energy transfer process and luminescence mechanism. From the partial energy level diagram of Tm3+, Ho3+ and Nd3+ displayed in Fig. 7, the near-/midinfrared and visible spectra of these glasses can be explained. For the Tm3+ single-doped sample, the Tm3+ ions are excited from the ground state 3H6 level to the higher 3H4 level via a ground state absorption (GSA) process under the 808 nm excitation. Among the excited Tm3+ ions at 3H4 level, a small part of them decay to 3H5 level via a nonradiative transition process and then the Tm3+ ions at 3H5 level are excited to 1G4 level through an excited state absorption (ESA:

26.19

14.59

45.56

4.78

0.89

3.16

3.90

0.18

0.12

0.13

0.03

3 H5 + one photon → 1G4) process, after which the Tm3+ at 1G4 level decay to 3F2,3 level non-radiatively, followed by a radiative transition from 3F2,3 level to ground state 3H6 and then the red emission at 710 nm is produced. Another small part of them radiate to the lower 3F4 level and generates the 1.48 µm emission, while most of them quickly decay to the 3F4 level through cross-relaxation (CR1: Tm3+:3H6 + 3H4 → 3 F4 + 3F4) and nonradiative transition. When the Tm3+ at 3F4 level returns to the 3H6 ground state, the 1.81 µm emission band is generated. For Tm3+/Ho3+ co-doped sample, there exists a quasi-resonant energy transfer from Tm3+:3F4 to Ho3+:5I7 upon the excitation of 808 nm LD due to the small energy gap between these two levels. Through the energy transfer process (ET1: Tm3+:3F4 + Ho3+:5I8 → Tm3+:3H6 + Ho3+:5I7), Ho3+ ions are excited from 5I8 ground state to 5 I7 level. When the Ho3+ at 5I7 level returns to 5I8 level, the emission at 2.04 µm generates. Meanwhile, some Ho3+ ions are populated to 5I5 level through an energy transfer upconversion process (ETU1: Tm3+:3F4 + Ho3+:5I7 → Tm3+:3H6 + Ho3+:5I5) [32] and then decay to 5I6 level non-radiatively, after which the Ho3+ at 5I6 level are excited to 5F3 level via the ESA process (5I6 + one photon → 5F3) and soon decay to 5S2,5F4 levels non-radiatively, followed by a radiative transition of Ho3+: 5S2 + 5F4 → 5I8 and generates the green emission at 547 nm. This is the reason why there is an additional emission at 547 nm in Tm3+/Ho3+ co-doped sample compared with the Tm3+ single-doped one. For Nd3+/Ho3+ co-doped sample, the Nd3+ ions are firstly excited from the 4I9/2 ground state to the 4F5/2 level through GSA and then decay to 4F3/2 level non-radiatively. When the Nd3+ at 4F3/2 level radiates to 4I13/2, 4I11/2 and 4I9/2 levels, emissions at 1338, 1060 and 901 nm are generated, respectively. Meanwhile, some Nd3+ ions at 4F3/ 2 4 2 2 level are excited to D5/2 level via an ESA ( F3/2 + one photon → D5/ 4 2) process and then decay to F9/2 level non-radiatively, followed by a radiative transition from 4F9/2 level to 4I9/2 ground state and generates

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Journal of Non-Crystalline Solids 531 (2020) 119855

L.Y. Mao, et al.

Fig. 6. The mid-infrared (a), visible (b) and near-infrared (c) luminescence spectra as well as decay curves (d) monitored at ~2 µm of Tm3+-doped, Tm3+/Ho3+ and Nd3+/Ho3+ co-doped TGZ25 glasses under the excitation of 808 nm LD.

Fig. 7. Partial energy level diagram of Tm3+, Ho3+ and Nd3+ as well as relevant transitions under the excitation of 808 nm LD. 7

Journal of Non-Crystalline Solids 531 (2020) 119855

L.Y. Mao, et al.

the red emission at 688 nm. Besides, the energy transfer from Nd3+:4F3/ to Ho3+:5I5 (ET2: Nd3+:4F3/2 + Ho3+:5I8 → Nd3+:4I9/ 2 3+ 5 : I5) takes place, followed by the population of Ho3+:5F3 2 + +Ho level via the energy transfer upconversion process (ETU2: Nd3+:4F3/ 3+ 5 : I5 → Nd3+:4I11/2 + +Ho3+:5F3) [33] and then decay to 2 + +Ho 5 5 S2, F4 level non-radiatively, after that the radiative transition of Ho3+: 5S2 + +5F4 → 5I8 occurs and the green emission at 545 nm is produced. Meanwhile, a part of Ho3+ ions are populated to the 5I7 level through the CR3 process (Nd3+:4I9/2 + +Ho3+:5I6 → Nd3+:4I13/ 3+ 5 : I7) [25] and nonradiative transition from 5I5 level, after 2 + +Ho which one part of them return to 5I8 ground state and generate the emission at 2.04 µm, while another part of them are excited to 5S2,5F4 level via an ESA process (5I7 + +one photon → 5S2 + +5F4) and then decay to 5F5 level non-radiatively, followed by the Ho3+ at 5F5 level radiate to the 5I8 ground state and generates the red emission at 659 nm. A conclusion can be drawn from the above discussion that the co-doping sensitization method is found to face a more furious competition process than the single-doped case in the aspect of achieving efficient 2 µm emission. Therefore, the upconversion luminescence in the glass system should be suppressed as much as possible, especially for the co-doped glass towards 2 µm fiber lasers.

Fig. 8. Calculated absorption and emission cross-sections corresponding to the Tm3+: 3H6 → 3F4 and Ho3+: 5I8 → 5I7 transitions, as well as the one- and twophonon emission sidebands of Tm3+: 3F4 → 3H6 transition in TGZ25 glass.

3.4. Absorption, emission cross-sections, gain coefficient and energy transfer coefficient

In order to obtain a further comprehension of the energy transfer process, a quantitative analysis is carried out in Tm3+/Ho3+ co-doped sample as an example. According to Dexter's theory, the energy transfer probability relates to the overlap integral between donor emission and acceptor absorption cross-section spectra, which can be written in the form [36]:

The absorption and emission cross-sections are important to evaluate whether RE-doped glass can achieve high-efficient emissions. The large values of which contribute to an efficient emission. Based on the absorption spectra of Tm3+-doped and Nd3+/Ho3+ co-doped samples, the absorption cross-section can be calculated according to the BeerLambert Eq. [24]:

σa =

2.303logI0 (λ )/ I (λ ) Nl

2π M WDA = ⎛ ⎞ HDA 2 SDA ⎝ℏ ⎠

(2)

where |HDA| is the matrix element of the perturbation Hamilton beM is the overlap integral between the mtween initial and final states. SDA phonon emission line shape of donor (D) ion and the k-phonon absorption line shape of acceptor (A) ion. On the basis of Dexter's theory, Tarelho et al. developed a method to estimate the microscopic probability rate of phonon-assisted energy transfer between RE ions in solids, where the energy transfer probability rate from donor to acceptor ions can be expressed by the following Eq. [37]:

where I0(λ) is the incident optical intensity, I(λ) is the optical intensity throughout the sample with thickness l, N is the doping concentration of RE ion. Utilizing the obtained absorption cross-section, the emission cross-section can be further determined according to the McCumber Eq. [34]:

σe (λ ) = σa (λ )

ZL hc ⎛ 1 1 exp ⎡ − ⎞⎤ ⎢ kB T ⎝ λZL ZU λ ⎠⎥ ⎦ ⎣ ⎜

⎟

(3)

WDA =

where ZL and ZU are the partition functions of the lower and upper states, respectively, h is Plank constant, c is the velocity of light, kB is the Boltzmann constant, T is the temperature, λZL is the zero-phonon line. For Tm3+ and Ho3+, the values of ZL/ZU are respectively taken of 1.51 and 0.81, and the λZL are taken of 1784 and 1941 nm [35], respectively. According to Eqs. (2) and (3), the absorption and emission cross-sections of Tm3+ and Ho3+ are obtained, as depicted in Fig. 8. The peak absorption and emission cross-sections of Tm3+ are 5.7 × 10−21 and 5.9 × 10−21 cm2, respectively, and those of Ho3+ are 5.8 × 10−21 and 7.2 × 10−21 cm2, respectively. The gain coefficient can be calculated based on absorption and emission cross-sections as a function of population inversion for the upper laser level, which can be denoted as [24]:

G(λ ) = N [Pσe (λ ) − (1 − P ) σa (λ )]

(5)

R6 CDA = 6C 6 R R τD

(6)

where CDA is the energy transfer coefficient, R is the distance between the donor and acceptor ions, τD is the lifetime of donor ion. RC is the critical radius of interaction, which can be expressed by [37]:

RC6 =

D 6cτD glow D 4 2 (2π ) n gup

∞

M

∑M=0 ∑m =0 ∫ σe(Dm −phonon) (λ) σa(Ak −phonon) (λ) dλ (7)

where n is the refractive index of the sample, M (M = m + +k) is the D total number of phonons involved in the energy transfer process, glow D and gup is the degeneracy of donor ion at lower and upper levels,

σe(Dm − phonon) and σa(Ak − phonon) represent the emission sideband of donor ion with m-phonon emission and absorption sideband of acceptor ion with k-phonon absorption, respectively, which can be determined by [37]:

(4)

where N is the doping concentration of RE ion, P is the population inversion. The acquired gain coefficients of Tm3+: 3F4 → 3H6 and Ho3+: 5 I7 → 5I8 transitions by setting the population inversion values ranging from 0 to 1 in an interval of 0.2 are shown in Fig. 9. The maximum gain coefficients of Tm3+ and Ho3+ are 2.70 and 1.61 cm−1, respectively, which are much larger than those in silicate and fluorophosphate glasses (as shown in Table 6), indicating the good gain properties of REdoped TGZ25 glasses.

σe(Dm − phonon) (λ ) = σeD (λm+) ≈ e−S0

S0m D (n¯ + 1)mσe(expt) (E − mℏω0) m!

(8)

σa(Ak − phonon) (λ ) = σaA (λk−) ≈ e−S0

S0k A (n¯)k σa(expt) (E + k ℏω0) k!

(9)

λm+

1 1/( λ

λk−

1 1/( λ

= − mℏω0 ) and = + k ℏω0 ) represent the correwhere sponding wavelengths of m-phonon emission and k-phonon absorption, 8

Journal of Non-Crystalline Solids 531 (2020) 119855

L.Y. Mao, et al.

Fig. 9. Gain coefficients with various population inversion values P for (a) Tm3+: 3F4 → 3H6 and (b) Ho3+: 5I7 → 5I8 transitions in TGZ25 glasses.

assistance is only with a contribution ratio of 10.82%. Besides, the energy transfer among Tm3+ ions is a resonant process, in which almost no phonon is needed and the energy transfer coefficient reaches 92.9 × 10−40 cm6/s.

Table 6 Comparison of doping concentrations and gain coefficients for several Tm3+and Ho3+-doped glasses. Tm3+

Glass matrix

Silicate Fluorophosphate TGZ25

Ho3+

Concentration (mol%)

Gain coefficient (cm−1)

Concentration (mol%)

Gain coefficient (cm−1)

1.5 [39] 6.0 [44] 2.0

1.50 [39] 0.97 [44] 2.70

0.8 [38] 1.0 [45] 1.0

0.84 [38] 0.80 [45] 1.61

4. Conclusions In summary, a TeO2-Ga2O3-ZnO (TGZ) ternary tellurite glass system was explored via a thermodynamic calculation method and then confirmed by a few experiments. This newly developed TGZ glass exhibits high glass transition temperature (366–377 °C), large density (5.238–5.371 g/cm3), high refractive index (n@ 633 nm = 1.9905–2.0189), wide infrared transmission range (5.53–5.87 µm) and low maximum phonon energy (~760 cm−1). Furthermore, an intense 2 µm emission was realized in the Tm3+ and Ho3+ doped TGZ glasses upon the excitation of 808 nm LD. The codoping sensitization method was found to face a more furious competition process than the single-doped case in the aspect of achieving efficient 2 µm emission. The results enrich the tellurite glass system and may help the research for high-gain fiber lasers.

Table 7 Energy transfer coefficients and critical radii of Tm3+ and Ho3+ in Tm3+/Ho3+ co-doped TGZ25 glass. Energy transfer

Tm

3+

→ Ho

3+

Tm3+ → Tm3+

M (number of phonons) % (phonon assisted)

0 89.18 0 99.86

1 10.82 1 0.14

Transfer coefficient (10−40 cm6/s)

RC (nm)

CDA

CDD

38.6

-

1.412

-

92.9

1.634

CRediT authorship contribution statement L.Y. Mao: Data curation, Writing - original draft, Writing - review & editing. J.L. Liu: Writing - review & editing. L.X. Li: Writing - review & editing. W.C. Wang: Conceptualization, Methodology, Writing - review & editing.

respectively. n¯ is the average occupancy of phonon mode, S0 is the Huang-Rhys factor, ℏω0 is the maximum phonon energy of the glass matrix. Based on the emission cross-section of Tm3+: 3F4 → 3H6 and Eq. (8), the one- and two-phonon emission sidebands of Tm3+ are determined, as shown in Fig. 8. To simplify, just considering the m-phonon emission of donor ion in energy transfer process (i.e., M = m, k = 0), and then the energy transfer coefficient and critical radius are determined by [37,38]:

CDA =

D 6cglow

∞

∑

D (2π ) 4n2gup m=0

e−(2n + 1) S0

S0m (n + 1)m × m!

Declaration of Competing Interest The authors declare no competing financial interests. Acknowledgments This work is financially jointly supported by the NSFC (Grant No. U1601205), NSAF (Grant No. U1830203), Fundamental Research Funds for the Central Universities, SCUT (No. 2018MS74), and Local Innovative and Research Teams Project of Guangdong Pearl River Talents Program (2017BT01X137).

∫ σeD (λm+) σa (λ) d (λ) (10)

R C6 = CDA τD

(11)

Based on Eqs. (6)–(11) and data in Fig. 8, the energy transfer coefficient and critical radius are determined and displayed in Table 7. The involved phonon number and their contribution ratio (%) to the total probability rate are also listed. It is found the energy transfer coefficient between Tm3+:3F4 level and Ho3+:5I7 level is 38.6 × 10−40 cm6/s. This process is nearly resonant and one-phonon

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