Tm3+ co-doped tungsten-tellurite glasses

Accepted Manuscript Enhanced mid-infrared emission in Er3+/Tm3+ co-doped Tungsten-Tellurite glasses Dan Wang, Tao Zheng, Jingwen Lv PII: DOI: Article Number: Reference:

S1350-4495(19)30278-6 https://doi.org/10.1016/j.infrared.2019.102986 102986 INFPHY 102986

To appear in:

Infrared Physics & Technology

Received Date: Revised Date: Accepted Date:

14 April 2019 10 June 2019 16 July 2019

Please cite this article as: D. Wang, T. Zheng, J. Lv, Enhanced mid-infrared emission in Er3+/Tm3+ co-doped Tungsten-Tellurite glasses, Infrared Physics & Technology (2019), doi: https://doi.org/10.1016/j.infrared. 2019.102986

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Enhanced mid-infrared emission in Er3+/Tm3+ codoped Tungsten-Tellurite glasses Dan Wang1, Tao Zheng1, *, Jingwen Lv1, * 1 School of Materials Science and Engineering, Changchun University of Science and Technology, Changchun 130022, PR China *Corresponding author1:[email protected] *Corresponding author2: [email protected]

Transparent Er3+/Tm3+ co-doped tungsten-tellurite (TWL) glasses with variable Er3+ concentration were prepared by the conventional melt-quenching method. The samples prepared were investigated by differential scanning calorimetry (DSC), absorption spectra, up-conversion, near and mid-infrared emission and lifetime decays. DSC analyses indicates a high transition temperature (Tg) and a large value of △T. Enhanced 2.7 μm emission in Er3+/Tm3+ co-doped TWL glasses presents its superiority in mid-infrared application. Based on above emission spectra and lifetime decays, the energy transfer mechanism between Er3+ and Tm3+ ions was discussed. Tm3+ ions are an effective sensitization for 2.7 μm emission of Er3+ ions in this glass for its high energy transfer efficiency (83.6%). A comparative study on emission performance suggests that this Er3+/Tm3+ codoped TWL glass is a competent candidates for mid-infrard laser materials.

Keywords: tungsten-tellurite glasses; Er3+/Tm3+ codoped; 2.7 μm emission 1. Introduction Mid-infrared lasers operating at around 3 μm have attracted extensive attentions owing to the possible applications in medicine, sensing and LIDAR recently [1, 2]. Based on the energy levels diagrams of Er3+ and Tm3+ ions, Er3+ ions is an important source for operating at 2.7 μm region and Tm3+ ions at 1.8 μm region [3, 4]. From the energy levels diagram (Fig. 1), we can find out that there are several levels close to each other between Er3+ and Tm3+ ions, like Er3+: 4I13/2 and Tm3+: 3F4 levels, Er3+: 4I 3+ 3 3+ 4 3+ 3 3+ 4 4 3+ 1 9/2 and Tm : H4 levels, Er : F9/2 and Tm : F3,2 levels, and Er : F7/2, F5, 3/2 and Tm : G4 levels. It indicates that Tm3+ ions can increase the pumping efficiency of 808 nm laser diode and can transfer energy to Er3+ ions. Additionally, the introduced Tm3+ ions affect the up-conversion and 1.5 μm emissions of Er3+ ions directly. Previous researches shows that the energy transfer process between Er3+ and Tm3+ ions is efficient and easy to happen [5-7]. In their researches, we can also find out that Er3+: 4I 13/2 ions can transfer energy through transitions: ET1: Er3+: 4I13/2+Tm3+: 3H6→Er3+: 4I15/2+Tm3+: 3F4; (1) ET2: Er3+: 4I13/2+Tm3+: 3F4→Er3+: 4I15/2+Tm3+: 3H4. (2) These transitions can depopulate the Er3+: 4I13/2 level which is beneficial to 2.7 μm emission of Er3+ ions. Thus, introducing Er3+ and Tm3+ ions in host can not only improve the 1.47 μm emission of Tm3+ ions but also improve the 2.7 μm emission of Er3+ ions. Among different host materials, glass is the most attractive matrix and has been numerously studied. To date, fluoride, chalcogenide, fluorophosphates and heavy metal oxide (tellurite and germanate) based glasses as well as glass ceramics have been investigated and fluoride glasses have emerged as natural candidates to get powerful 2.7 μm mid-infrared emission from Er3+ ions [8-14].

However, fluoride and chalcogenide glasses require a more stringently controlled fabrication and have a relatively poor thermal stability which limits their applications. Loads of efforts were did in controlling the phonon energy of matrix

[15-18].

Therefore, the heavy metal oxide glasses, germinate,

tellurite and bismuthate glasses attract great interest owing to their excellent solubility for rare-earth ions, lower phonon energy, higher refractive index, higher glass transition temperature and good infrared transmission. Among these heavy metal oxide glasses, tellurite glass has several properties, such as low phonon energy (about 700 cm-1), large refractive index (around 2.0), and good chemical durability and thermal stability and so on. Besides, compared with the traditional TeO2-ZnO-Na2O glasses, TWL glasses have high Tg (glass transition temperature) and low coefficient of thermal expansion, which will be helpful for fiber drawing and fiber device applications [19, 20]. Based on these outstanding properties, SIOM researchers successfully obtained 2.0 μm laser in Tm3+ doped tungstentellurite glass fiber

[2, 21].

Additionally, the Jilin University researchers also successfully obtained

supercontinuum spectra in tellurite zinc glass

[22].

While, the researchers in South China University of

Technology (SCUT) also studied the 2.7 μm laser performance in Er3+-doped glass and obtained 2.7 μm emission in Er3+-doped glass ceramic fibers [23, 24]. Combination with the energy gap (~700 cm-1) between Er3+: 4I13/2 state and Tm3+ :3F4 state and the phonon energy (~770 cm-1 and 930 cm-1, Te—O and W—O respectively) of TWL glass, it can be concluded that Tm3+ ions will be a perfect activating ions to stimulate the Er3+ ions in TWL glass. Therefore, tungsten-tellurite glass was chosen as matrix in this paper. In present study, Er3+/Tm3+ codoped TWL glasses were investigated. Up-conversion, near-infrared and mid-infrared emissions were measured to elaborate the energy transfer processes between Er3+ and Tm3+ ions.

Fig. 1 the energy levels diagrams of Er3+ and Tm3+ ions 2. Experimental Details Glasses with molar composition of 65TeO2 - 20WO3 - (15-x-y) La2O3 - xEr2O3 - yTm2O3 (x=0, 0.5, 1, 2, 3 and 4, y= 0 and 0.5, named as Er0.5, Erx-Tm and Tm, respectively) were prepared by the conventional melting-quenching method in an alumina crucible at 900 ℃ for 40 min. After that, the melts were casted on a preheated steel mould and annealed below the glass transition temperature for several hours before they were cooled to room temperature with a rate of 20 ℃/h. The obtained glass

samples were cut and polished into a shape of 10×10×1 mm for optical measurements. The glass transition temperature (Tg) and peak crystallization temperature (Tp) were tested by differential scanning calorimetry (DSC) measurement. The absorption spectra were recorded with a Perkin-Elmer-Lambda 900UV/VIS/NIR spectrophotometer in the range of 400-1700 nm. The emission spectra were tested with a Triax 320 type spectrometer (JobinYvon Co. France) in the range of 400-850 nm, 1400-2100 nm and 2550-2850 nm by using a 980 nm laser diode (LD) as excitation source. The fluorescence lifetime was measured with instrument FLSP920 (Edinburgh instruments Ltd., UK). All measurements were performed at room temperature. 3. Results and Discussions 3.1 Thermal stability Figure 2 shows the DSC curve labeled the characteristic temperatures (Tg and Tp). Firstly, It can be found that the values of Tg and Tx does not change obviously with the doped rare-earth ions (Er3+ and Tm3+ ions) and it is comparable with previous researches

[15].

The values of ΔT (Tx―Tg) is almost

200 ℃ which is much higher than that of ZBLAN glasses and tellurite zinc glasses [25, 26]. High laser damage threshold and large working range for drawing fiber can be expected in present tungstentellurite glass. Secondly, the thermal parameters, such as Tg and Tp, does not show an obvious change. It can be concluded that the additive of rare-earth ions has little effect on the glass structure. In brief, Er3+/Tm3+ doped TWL glass is suitable for active fiber laser matrix.

Fig. 2 the DSC curve of TWL glasses without rare-earth ions and doped with Er3+, Er3+/Tm3+ ions. 3.2 Absorption analysis Figure 3 shows the absorption spectra of Er3+, Tm3+, and Er3+/Tm3+ doped tungsten-tellurite glasses in the spectra region from 400 to 2000 nm. All absorption bands are assigned to the transitions of Er3+ and Tm3+ ions from the ground level to the labeled levels. The absorption bands are similar with other glasses

[4, 5, 7].

There is little shift between singly doped rare-earth and co-doped rare-earth ions,

which indicates that Er3+ and Tm3+ ions are homogeneously incorporated into the TWL glass network without clusters in the local ligand field

[7].

From the absorption spectra, there are obvious overlaps of

absorption bands between Er3+ and Tm3+, such as Er3+: 4I13/2 and Tm3+: 3F4, Er3+: 4I9/2 and Tm3+: 3H4, Er3+: 4F9/2 and Tm3+: 3F3, 2 which match with the levels and ET process present in Fig. 1. It can be expected that the lower level of transition Er3+: 4I11/2→4I13/2 (2.7 μm emission) can be depopulated by introduced Tm3+ ions.

Fig. 3 Absorption spectra of Er3+, Tm3+, and Er3+/Tm3+ doped TWL glasses; inset, the transmittance spectrum of TWL glass.

Fig. 4 Detailed absorption spectra of Er3+ and Er3+/Tm3+ doped TWL glasses ranges from 650 to 850 nm and from 1400 to 2000 nm; inset, the tendency of 1.5 μm, 980 nm and 800 nm absorption spectra with the concentration of Er3+ ions. Figure 4 shows the detailed absorption spectra of Er3+ and Tm3+ ions with the concentration of

Er3+ ion. As expected, the absorption intensity increases with the concentration of Er3+ ions in all of these bands and it is affected by co-doped Tm3+ ions barely. But, we can also found that the intensity of these bands increase in a linear until the concentration of Er3+ ions is 4 mol. Thus, it can be deduced that the emission may be quenched when the concentration of Er3+ ions reaches 4 mol. As shown in Fig. 3 and 4, the absorption band around 800 nm increase clearly with the Tm3+ ions. Concerning the absorption of Tm3+ ions in 800nm, the emission properties of Er3+/Tm3+ co-doped TWL glasses were investigated with a 980 nm LD pumping source to illustrate the energy transfer mechanism from Er3+ to Tm3+ ions. 3.3 Emission spectra pumped by 980 nm LD Figure 5 shows the up-conversion emission spectrum of TWL glasses under 980nm LD pumping source. There are three typical peaks of Er3+ ions at 521nm, 553nm and 672nm, respectively. With the introduction of Tm3+ ions, the change of up-conversion is easy to notice which is consistent with previous reports

[27, 28].

It indicates an active energy transfer between Er3+ and Tm3+ ions. The inset of

figure 5 shows the ratio of each peak which is divided by the intensity of Er3+-doped TWL glass. We can find out that the intensity of 553 nm is sharply decreased by introducing Tm3+ ions. As a result, increasing the concentration of Er3+ ions will not result in a higher 553nm. Combining the energy levels diagram in fig. 1, Er3+ ions can effectively transfer energy to Tm3+ ions through ET3: Er3+: 2H11/2, 4S3/2+Tm3+: 3H6Er3+: 4I15/2+Tm3+: 1G4 While there are few reports on 553 nm up-conversion of may owe to its short lifetime

[29, 30].

Tm3+

(3)

ions, especially 553 nm

(1G

4

level). It

However, with the introduction of Tm3+ ions, the intensity of 672

nm holds steady. Furthermore, the intensity of 672 nm increases with the concentration of Er3+ ions before 4%. For 672 nm up-conversion, it could involve the following energy transfer process (ET4): Er3+: 4F9/2+Tm3+: 3H6Er3+: 4I15/2+Tm3+: 3F2, 3 While there are lots of reports on 650~670 nm up-conversion of

Tm3+

ions

(4) [29, 31].

From above analysis,

we can only draw a simple conclusion that the energy transfer processes between Er3+ and Tm3+ ions are active and complex.

Fig. 5 up-conversion spectra of Er3+/Tm3+ codoped TWL glasses; the inset shows the

ratio of each peak corresponding to the Er3+-doped TWL glasses.

Fig. 6 Near-infrared emission of Er3+/Tm3+ codoped TWL glasses (left) and the tendency of 1552 nm and 1800 nm (right); partiularly, the isolated black block in right figure represents the intensity of 1552 nm in Er0.5 singly doped TWL glass. Figure 6 shows the near infrared emission spectra of Er3+-doped and Er3+/Tm3+-codoped TWL glasses. The top attention caused by the spectra is that the intensity of 1.5 μm emission is cuttingly decreased by the introduction of Tm3+ ions. Correspondingly, the 1.8 μm emission of Tm3+ ions appears which is caused mainly by the ET1 [5, 6, 32]. It is another evidence of energy transfer between Er3+ and Tm3+ ions. Actually, it also can be proved by the lifetime of Er3+: 4I13/2 state which is shown in fig. 7. The energy transfer efficiency of this process can be estimated by the measured fluorescence lifetime [13]. The measured fluorescence lifetime and the calculated energy transfer efficiency (ηET) of Er3+/Tm3+ codoped TWL glasses were listed in Table 1 both.

ηET=1‒ τEr/Tm/τEr

(5)

Table 1 the measured fluorescence lifetime and the calculated energy transfer efficiency (ηET) of Er3+/Tm3+ codoped TWL glasses Sample

Er0.5

Lifetime (ms)

Er0.5-

Er1-

Er2-

Er3-

Er4-

ZBLAY

Bismuthate

[7]

glass[33]

Tm

Tm

Tm

Tm

Tm

3.61

0.812

0.590

0.660

0.649

0.719

6.1

—

—

77.5

83.6

81.7

82

80.1

58.22

57

Calculated energy transfer efficiency ηET (%) The calculated energy transfer efficiency (ηET) of ET1 is about 83.6%, which is much higher than that of ZBLAY(58%) and bismuthate glass (57%) [7, 33]. On the basis of the energy level diagrams of Er3+ and Tm3+ ions, the high value of ηET can be attributed to the prefect match between the energy gap and phonon energy of TWL glass. That is to say, the introduced Tm3+ ions in Er3+-doped TWL glass can transfer the energy from Er3+ ions competently with the assistant of phonon energy, which will benefit the transition of Er3+: 4I11/2 state. This phenomenon is coincide with the overlap in absorption spectra as expected. As we illustrated in previous research[13], a shorten lifetime of 1.5 μm foresees a strengthening 2.7 μm emission. And then we are going to notice the tendency of each peak in this

region. In Er0.5-Tm sample, it exhibits the strongest 1.5 μm emission as well as 1.8 μm emission compared with other Er3+/Tm3+-codoped TWL glasses. It is also in keeping with the tendency of lifetime in Er3+/Tm3+-codoped TWL glasses. The explanation for non-increment intensity of 1.8 μm emission with the multiplying concentration of Er3+ ions is invalid energy transfer process ET6: Er3+: 4I13/2+a photo+Tm3+: 3F4Er3+: 4I15/2+Tm3+: 3F2, 3 This energy transfer process is harm to the population of beneficial to the population of

Tm3+: 3F

2, 3

Er3+: 4I

13/2

(6) and

Tm3+: 3F

4

states. But, it is

state. Thus, the red up-conversion rises while the 1.8 μm

emission declines. That prediction can be proved by figure 5 and figure 6, especially the separation of red up-conversion. Furthermore, the 2.7 μm emission will be improved by depopulated Er3+: 4I13/2 states. In general, the introduction of Tm3+ ions in Er3+-doped TWL glass is good for 2.7 μm emission.

Fig. 7 Luminescence decay curves of Er3+/Tm3+ codoped TWL glasses Figure 8 shows the 2.7 µm emission spectra in Er3+/Tm3+ codoped TWL glasses. As expected, with the introduction of Tm3+ ions, the intensity of 2.7 µm emission shows a roughly 30% increment. In the wake of multiplying concentration of Er3+ ions, the intensity of 2.7 µm emission is also multiplying. From the right line chart in fig. 8 (right), the linear relation between the concentration of Er3+ ions and intensity of 2.7 µm emission is clear. The Er3-Tm sample shows the strongest 2.7 µm emission, which matches with the predicts made by absorption spectra as well as near-infrared emission spectra. These results indicates that Tm3+ ions is an effective sensitizing ions for Er3+ ions to reach an intense 2.7 µm emission.

Fig.8 Mid-infrared emission of Er3+/Tm3+ codoped TWL glasses (left) and the tendency of each peaks (right) 4. Conclusion In brief, transparent Er3+/Tm3+-codoped TWL glasses with proportionally varying Er3+ ions concentration were successfully prepared and studied. The DSC curves indicate that there is barely affection caused by rare-earth ions. That makes a promise for highly dopants remaining its good thermal stability. This is not only beneficial for 2.7 µm emission but also beneficial for fiber drawing, which make a promission for fiber laser applications. On the basis of synthesis analysis of absorption spectra, emission spetra and energy level diagrams, it can be concluded that Tm3+ ions is an effective sensitization for Er3+ ions in TWL glass to enhance its 2.7 µm emission. The transfer efficiency from Er3+: 4I13/2 state to Tm3+: 3H6 state is as high as 83.6%. Additionally, there are many other energy transfer process between Er3+ and Tm3+ ions. That is to say, the energy transfer processes are active and complex which can be proved by the up-conversion and near-infrared emission.

Acknowledgement This work was supported by National Natural Science Foundation of China [No. 51502022, 61605115]; and Changchun University of Science and Technology [XQNJJ-2014-12].

Conflict of interest statement

The authors declared that they have no conflicts of interest to this work.

The introduction of Tm3+ ions enhance the 2.7 μm emission of Er3+-doped tungsten-tellurite glasses, which is valued for mid-infrared laser.

1. An enhanced 2.7 μm emission is obtained in Er3+/Tm3+-codoped tungsten-tellrutie glasses. 2. Tm3+ ions are an effective sensitization for 2.7 μm emission of Er3+ ions for its high energy transfer efficiency (83.6%). 3. The 1.8 μm emission of Tm3+ ions appears while the red up-conversion raises.

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