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Abstract
TeO2–ZnO–Na2CO3–La2O3 (TZNL) tellurite glasses were prepared using a conventional melt-quenching method. The effects of the Mn2+ ions and (Yb3+–Mn2+–Mn2+) trimer on the enhancement upconversion (UC) and near-infrared (NIR) emissions intensity of the co-doped Ho3+/Yb3+ bands in TZNL tellurite glasses were investigated. With the formation of the (Yb3+–Mn2+–Mn2+) trimer and the energy transfer (ET) from Mn2+ and (Yb3+–Mn2+–Mn2+) trimer into Ho3+, the UC/NIR emissions intensity of the co-doped Ho3+/Yb3+ bands was significantly increased. In addition, the ET processes between the Mn2+ and (Yb3+–Mn2+–Mn2+) trimer with Ho3+ were shown.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
In recent years, co-doped rare-earth (RE) vitreous materials have attracted significant attention owing to their extensive applications in fiber lasers, remote sensing, solar cells, fiber amplifiers, and temperature sensors [1–7]. In fact, co-doped RE materials have physical and spectroscopic properties depending on the host glass. Thus, with the desire to optimize the physical and optical properties of RE ions in the host materials, many scientific studies have investigated the optical properties of glasses and developed applications for RE ions in various glass systems, such as silicate [8,9], borophosphate [10,11], oxyfluoride [12], and tellurite glasses [13,14]. Tellurite glasses have certain advantages over silicate and borophosphate glasses for optical applications owing to their low phonon energy, and high refractive index [15]. Tellurite glasses were discovered by J.E. Stanworth [16] in 1952, and were revisited in 1994 by J.S. Wang et al. [17]; interest in such materials has since increased.
Among the existing trivalent RE ions, Ho3+ are extremely popular active ions owing to their abundant energy levels within both the visible and infrared wavelength ranges. Within the visible wavelength region, the UC emissions of Ho3+ exhibit blue, green, and red corresponding to the 5F3 → 5I8, 5S2 → 5I8, and 5F5 → 5I8 transitions for white light luminescence [18,19]. For the NIR emissions, the Ho3+ band centered at 1.2 µm corresponds to a 5I6 → 5I8 transition, which is useful for fiber-optical amplifiers within the telecommunication band [20]. A few studies have investigated broadband NIR emissions at 1.2 µm for use in the O–band (1,260–1, a 360 nm) applied in fiber-optical amplifiers [21] and photodynamic therapy [22]. In addition, laser emissions within the 1.2 µm region can be used for sensors and solar cell applications. For instance, a 1,178 nm laser frequency-doubled to generate a wavelength of 589 nm for use in guide star applications in the field of astronomy was reported by S.B. Wang et al. [7]. However, the 5I5 → 5I8, 5I6 → 5I8, and 5I7 → 5I8 transitions of Ho3+ cannot be directly pumped using an available commercial 808- or 980-nm LD owing to a lack of absorption bands. Therefore, Ho3+ needs to use a combination with other RE ions or metal elements as a sensitizer, and ET to achieve efficient infrared emissions. Recently, there many papers have been reported on the enhancement NIR emission of the Ho3+ band centered at ∼1.2 µm [7,21,23], and enhancement UC emission of the Ho3+ through the ET process from RE ions and alkali metals to Ho3+, such as energy transferred from Ce3+, Yb3+, Tm3+, Nd3+ and Li+ to Ho3+ ions [24–30]. Most recently, Du et al. [31] reported the local symmetry distortion-induced enhancement of UC luminescence in Gd2O3: Ho3+/Yb3+/Zn2+ nanoparticles for solid-state lighting and bioimaging. The results of this study indicate that with the introduction of Zn2+, the local symmetry around the dopants is decreased, leading to an enhancement in the UC emission intensity of the co-doped Ho3+/Yb3+. In the present study, we investigated the enhancement of the UC and NIR emissions intensity of co-doped Ho3+/Yb3+ in TZNL tellurite glasses. At the same time, the mechanism of the ET processes between Mn2+ and a (Yb3+–Mn2+–Mn2+) trimer with Ho3+ are also proposed and discussed.
2. Experiment details
High-purity TeO2, ZnO, Na2CO3, La2O3, MnO, Ho2O3, and Yb2O3 (99.99%) were used as the starting materials. The compositions chosen in the present study are shown in Table 1. Mixtures with a sufficient weight of approximately 10 g, compacted into a platinum crucible, were set in an electric furnace. After holding at 920 °C for 30 min in an electric furnace, the melts were quenched by placing them onto a polished plate of stainless steel. According to the glass transition temperature Tg in a differential thermal analysis, all glasses were annealed at 310 °C for 5 h to remove the thermal strains. The samples were cut into a size of 10 mm × 10 mm × 2 mm and polished for optical measurements. Optical images of the original glass and a TZNL-0.5Ho0Mn0Yb glass sample after being cut and polished are shown in the inset of curve (b) in Fig. 2. The optical absorption spectra within the range of 400–2000nm were measured on a Hitachi U–4100 spectrophotometer. The measurement resolution of the absorption spectra is 1.0 nm. UC/NIR fluorescence spectra and the lifetime curves were measured on an Edinburgh Instruments FLS980 fluorescence spectrometer using a µF920 microsecond flash lamp as the excitation source and detected using a liquid nitrogen cooled PbS detector upon excitation at 980 nm. All spectral measurements were conducted at ambient temperatures.
Table 1. Chemical composition of TeO2–ZnO–Na2CO3–La2O3–MnO–Ho2O3–Yb2O3 tellurite glasses (in mol. %)
3. Results and discussion
A DTA curve of the TZNL-0.5Ho2Mn2Yb glass sample is shown in Fig. 1. As this figure indicates, there are three temperature parameters: the glass transition temperature (Tg) located at 310 °C, the crystallization onset temperature (Tx) located at 533 °C, and two crystallization peaks temperatures (Tp1, Tp2) located at 546 °C and 571 °C, respectively. At the same time, the difference ΔT (ΔT = Tx − Tg) between Tx and Tg is used as a rough indicator of the thermal stability of the glass [32]. Compared with silicate [33,34] and fluoride [35] glasses, ΔT of TZNL tellurite glasses is calculated to be ΔT = (533−310) °C = 223 °C > 100 °C, indicating that the prepared glasses are stable and suitable for application, such as in fiber-optical amplifiers, solar cells, and lasers [32,36].
The absorption spectra of TZNL-0.5Ho2Mn2Yb, TZNL-0.5Ho0Mn0Yb, and TZNL-0Ho2Mn2Yb glass samples within the range of 400–2000nm are shown in Fig. 2. Curve (a) of Fig. 2 shows an absorption spectrum of a TZNL-0Ho2Mn2Yb glass sample, in which the absorption spectrum of the Mn2+ ions is assigned to the transition 6A1g → 4T1g [29,34]. Curves (b) and (c) of Fig. 2 show the absorption spectra of TZNL-0.5Ho0Mn0Yb and TZNL-0.5Ho2Mn2Yb glass samples, respectively. From the results of curve (b) in Fig. 2, the absorption spectrum of Mn2+ was not observed owing to the TZNL-0.5Ho0Mn0Yb glass sample not containing a Mn2+ composition. The absorption spectrum of doped Ho3+ consists of six absorption bands centered at 446, 493, 542, 645, 1,167, and 1,952 nm, each peak of which corresponds to the transitions from the ground state 5I8 to the excited states 5G6, 5F3, (5F4, 5S2), 5F5, 5I6, and 5I7, respectively. In contrast, the absorption spectra of Mn2+ in Fig. 2(a, c) are extremely different owing to an overlap of the 5I8 → 5F3 transition of the Ho3+ and 6A1(6S) → 4T1(4G) transition of Mn2+. In addition, the absorption spectrum of the Yb3+ band centered at ∼977 nm was identified based on a transition originating from Yb3+ ground multiple 2F7/2 to the excited multiple 2F5/2 [39].
Fig. 2. Absorption spectra of (a) TZNL-0Ho2Mn2Yb, (b) TZNL-0.5Ho0Mn0Yb, and (c) TZNL-0.5H02Mn2Yb glass samples.
Usually, the absorption spectrum of the Yb3+ peak near 980 nm in tellurite glass is stronger and narrower [26,37]. However, in this study, the absorption spectrum of the Yb3+ peak at ∼977 nm is weaker and wider. This is possible owing to (i) the overlap in adjacent transitions, namely, the 5F4, 5S2 → 5I8 and 5F5 → 5I8 transitions of Ho3+ with the 4T2(4G) → 6A1(6S) transition of Mn2+; and (ii) the formation of the (Yb3+–Mn2+–Mn2+) trimer, through which the absorption spectrum of Yb3+ combined with |2F5/2, 6A1g(S) 6A1g(S)〉 and |2F7/2, 6A1g(S) 6A1g(S)〉 has levels creating an overlap of the adjacent transitions, namely, the |2F5/2, 6A1g(S) 6A1g(S)〉 → |2F7/2, 6A1g(S) 6A1g(S)〉 transition of the (Yb3+–Mn2+–Mn2+) trimer and the 5F5/2 → 5F7/2 transition of the Yb3+. In addition, the absorption spectrum of Yb3+ in this study is also similar to the results by Azam [38]. Curve (c) of Fig. 2 shows the absorption spectrum of co-doped Ho3+/Yb3+/Mn2+ in the TZNL-0.5Ho2Mn2Yb glass sample. From this figure, all absorption spectra of Ho3+, Yb3+, and Mn2+ ions are clearly shown. In addition, the strong absorption spectra of the co-doped Ho3+/Yb3+/Mn2+ band are centered at 493 nm owing to the overlap of the 5I8 → 5F3 transition of the Ho3+ and 6A1(6S) → 4T1(4G) transition of the Mn2+ ions [40–42].
As is well known, a UC emission is a non-linear process. The UC emission properties are strongly excitation power-density dependent. Therefore, the UC emission spectra of the TZNL-0.5Ho2Yb7Mn glass sample under different pumping power excitations were measured, and the results are shown in Fig. 3. Under a continuous adjustment of the pumping power, the UC emission intensity of the co-doped Ho3+/Yb3+ in the TZNL-0.5Ho2Yb7Mn glass sample bands centered at 545 and 659 nm was increased with an increase in the pumping power excitation.
The UC emission spectra of the TZNL-1 glass samples within the range of 500–800 nm under a 980-nm LD excitation and pumping power of 2.0 W are shown in Figs. 4(a, b). Clearly, from results of Figs. 4(a, b), with an increase in the Mn2+ concentration, the UC emission intensity of the co-doped Ho3+/Yb3+ bands centered at 545, 659, and 755 nm, corresponding to the 5F4, 5S2 → 5I8, 5F5 → 5I8, and 5F4, 5S2 → 5I7 transitions of Ho3+, were significantly increased [30,41,42]. Interestingly, in this study when the Mn2+ concentration increases from 0 up to 7.0 mol.%, the UC emission intensity of the co-doped Ho3+/Yb3+ bands centered at ∼545, 659, and 755 nm are increased by approximately 11-, 23-, and 9-fold, respectively. In previous studies [18,27,43–47], the UC emission intensity of the co-doped Ho3+/Yb3+ band centered at ∼755 nm was weakly observed, and the ratio of the relative peak intensities of red UC emissions at 659 nm to green UC emissions at 545 nm is (I659nm/I545nm) > 1, whereas in some previous studies, it is I659nm/I545nm < 1 [46,48–50]. The value of the UC intensity of the co-doped Ho3+/Yb3+ band centered at 659 nm is maximized when the concentration of Mn2+ reaches 7.0 mol.%.
The relationship between the Mn2+ concentration with the UC emission intensity of the co-doped Ho3+/Yb3+ band centered at 659 nm is shown in the inset of Fig. 4(a). This result can be explained in two ways: (i) the ET process from the |2F7/2, 6A1g(S) 4T1g(G)〉 →|2F7/2, 6A1g(S) 6A1g(S)〉 transition of the (Yb3+–Mn2+–Mn2+) trimer [40,41] to the 5F4, 5S2 → 5I8 transition of the Ho3+ [42–44], and (ii) the ET process from the 4T2(4G) → 6A1(6S) transition of Mn2+ to the 5F5 → 5I8 transition of Ho3+. The mechanism of the ET processes from the (Yb3+–Mn2+–Mn2+) trimer and Mn2+ to Ho3+ is proposed as follows:
Fig. 4. (a) UC emission spectra of TZNL-1 glass samples under 980 nm LD excitation and a pumping power of 2.0 W within the range of 500–700 nm; (b) UC emission spectra of TZNL-1 glass samples under 980 nm LD excitation and the pumping power of 2.0 W within the range of 700–800 nm.
|2F7/2, 6A1g(S) 4T1g(G)〉 (Yb3+–Mn2+–Mn2+) trimer + (5F4, 5S2)(Ho3+) → 5I8(Ho3+) + |2F7/2, 6A1g(S) 6A1g(S)〉 (Yb3+–Mn2+–Mn2+) trimer (ET1).
4T2(4G)(Mn2+) + 5F5(Ho3+) → 6A1(6S)(Mn2+) + 5I8 (Ho3+) (ET2).
The NIR emission spectra of co-doped Ho3+/Yb3+ in TZNL-1 glass samples within the range of 1,100–1,650 nm under 980-nm LD excitation and a pumping power of 2.0 W are shown in Fig. 5. As the figure indicates, the NIR emission intensity of the co-doped Ho3+/Yb3+ band centered at 1.55 µm appears to be weak; in addition, with an increase in the Mn2+ concentration, the NIR emission intensity of the co-doped Ho3+/Yb3+ band centered at 1.55 µm remains almost unchanged, whereas with an increase in the Mn2+ concentration, the NIR emission intensity of the co-doped Ho3+/Yb3+ band centered at 1,190 nm is significantly increased (see the inset of Fig. 5). These results indicate that the ET process from the |2F5/2, 6A1g(S) 6A1g(S)〉 →|2F7/2, 6A1g(S) 6A1g(S)〉 transition of the (Yb3+–Mn2+–Mn2+) trimer to the 5I6 → 5I8 transition of Ho3+ occurs. The following mechanism of the ET process from the (Yb3+–Mn2+–Mn2+) trimer to Ho3+ is proposed: |2F5/2, 6A1g(S) 6A1g(S)〉 (Yb3+–Mn2+–Mn2+) trimer + 5I8 (Ho3+) → 5I6 (Ho3+) +|2F7/2, 6A1g(S) 6A1g(S)〉 (Yb3+–Mn2+–Mn2+) trimer (ET3). In addition, as indicated in Figs. 4 and 5, with an increase in the Mn2+ concentration, the UC emission intensity of the co-doped Ho3+/Yb3+ bands centered at 545, 659, and 755 nm is increased and reaches the maximum value when the Mn2+ concentration is at 7.0 mol.% [51], whereas the NIR emission intensity of the co-doped Ho3+/Yb3+ band centered at 1,190 nm continuously increases [52].
Fig. 5. NIR emission spectra of the co-doped Ho3+/Yb3+ TZNL-1 glass samples under 980 nm LD excitation and a pumping power of 2.0 W within the range of 1100–1650 nm.
The mechanism of the ET processes of the (Yb3+–Mn2+–Mn2+) trimer and Ho3+ in TZNL glasses under 980 nm of LD excitation is shown in in Fig. 6. First, for the Yb3+ emission at ∼991 nm, the 2F5/2 level of the Yb3+ is directly excited by the 980 nm LD. Next, the Yb3+ promotes a cooperative energy, which is transferred to the 5F3 level of the Ho3+. At the same time, the pump energy absorbed by the 2F5/2 → 2F7/2 transition of the Yb3+ is transferred to the 5I6 level of the Ho3+ leading to an increase in the NIR emission intensity of the co-doped Ho3+/Yb3+ band centered at 1,190 nm. The UC/NIR emissions of the co-doped Mn2+/Yb3+ bands centered at 588, 612, and 1,190 nm cannot be observed [33,34]. However, the Mn2+ is combined with Yb3+ to form a (Yb3+–Mn2+–Mn2+) trimer [30,41], and the energy of the Mn2+ is transferred to Ho3+, contributing to an increase in the NIR emissions intensity of the co-doped Ho3+/Yb3+ [43].
The variations in the molar Yb3+ concentration, while maintaining the Ho3+ and Mn2+ concentrations in the glass composition, are also given for comparison in the second component of the TZNL-2 glass. Figure 7 shows the NIR emission spectra of TZNL-2 glass samples within the range of 900–1,250 nm under 980 nm of LD excitation, at a pumping power of 2.0 W. As shown in Fig. 7, with the increase in the Yb3+ concentration, the NIR emissions intensity of the Yb3+ band centered at ∼991 nm is significantly increased. At the same time, the NIR emissions intensity of the Ho3+ band centered at 1,190 nm also increases. These results confirm that the ET process from Yb3+ to Ho3+ occurs. The following mechanism of the ET process from Yb3+ to Ho3+ is proposed [18,41,53]: 2F5/2(Yb3+) + 5I8(Ho3+) → 2F7/2(Yb3+) + 5I6(Ho3+) (ET4).
Fig. 7. NIR emission spectra of the Ho3+/Yb3+ co-doped TZNL-2 glass samples in the range of 900–1250 nm.
To further validate the ET processes between the (Yb3+–Mn2+–Mn2+) trimer and Ho3+, the fluorescence lifetime τYb-Mn-Mn of the (Yb3+–Mn2+–Mn2+) trimer in the TZNL-1 glass samples at 612 nm, corresponding to the |2F7/2, 6A1g(S) 4T1g(G)〉 →|2F7/2, 6A1g(S) 6A1g(S)〉 transition under 980 nm of LD excitation at a pumping power of 2.0 W, is measured and the results are shown in Fig. 8(a). The average fluorescence lifetime τYb-Mn-Mn for the TZNL-1 glass samples can be calculated using the following formula [5,54]:
Fig. 8. (a) Fluorescence lifetime of the (Yb3+–Mn2+–Mn2+) trimer at 612 nm in TZNL-1 glass samples; (b) Fluorescence lifetime of Yb3+ ions at 991 nm in TZNL-2 tellurite glass samples; (c) Fluorescence lifetime of Ho3+ ions at 659 nm in TZNL-1 glass samples; (d) Fluorescence lifetime of Ho3+ ions at 1,190 nm in TZNL-1 glass samples.
Here, parameters A1 and A2 are constants, and τ1 and τ2 are the rapid and slow lifetime of the exponential components, respectively. The average fluorescence lifetimes of TZNL-0.5Ho2Yb0Mn, TZNL-0.5Ho2Yb1Mn, TZNL-0.5Ho2Yb2Mn, TZNL-0.5Ho2Yb3Mn, TZNL-0.5Ho2Yb5Mn TZNL-0.5Ho2Yb7Mn and TZNL-0.5Ho2Yb9Mn glass samples were calculated to be approximately 216.8, 199.5, 162.6, 147.8, 128.5, 125.2, and 120.1 µs, respectively.
From the result of Fig. 8(a), the lifetime has only a relative dependence on the Mn2+ composition ratio. Therefore, this is evidence of the non-radiative (NR) processes contributing to the enhancement of the NIR/UC emissions of the co-doped Ho3+/Yb3+. The fluorescence lifetime of the (Yb3+–Mn2+–Mn2+) trimer at 612 nm were found to decrease with the increase in the Mn2+ concentration, which is strong evidence for the existence of ET from the |2F7/2, 6A1g(S) 4T1g(G)〉 → |2F7/2, 6A1g(S) 6A1g(S)〉 transition of the (Yb3+–Mn2+–Mn2+) trimer to the 5F4, 5S2 → 5I8 transition of the Ho3+. The ET efficiency (ETE) ηETE(Yb-Mn-Mn/Ho) from the (Yb3+–Mn2+–Mn2+) trimer to Ho3+ ions can be calculated using the following formula [31]:
Similarly, to further validate the ET process between Yb3+ and Ho3+ ions, the fluorescence lifetime τYb of the Yb3+ ions at ∼991 nm in TZNL-2 glass samples under a 980-nm LD excitation and a pumping power of 2.0 W were measured, the results of which are shown in Fig. 8(b). Using formula (1), the average fluorescence lifetimes τYb of TZNL-0.5Ho2Mn1Yb, TZNL-0.5Ho2Mn1.5Yb, TZNL-0.5Ho2Mn2Yb, TZNL-0.5Ho2Mn2.5Yb, and TZNL-0.5Ho2Mn3Yb glass samples were calculated as approximately 241.8, 229.1, 222.6, 205.2, and 190.5 µs, respectively. The fluorescence lifetimes of Yb3+ at ∼991 nm corresponding to a 2F5/2 → 2F7/2 transition were found to decrease with an increase in the Yb3+ concentration, which is strong evidence of ET from a 2F5/2 → 2F7/2 transition of the Yb3+ to 5I6 → 5I8 transition of the Ho3+ ions.
As is well known, the lifetime is a very important parameter for the ET process in luminescent materials, and thus to obtain more evidence for the ET process, decay curves of the Ho3+ at 659 and 1,190 nm in TZNL-1 glass samples were measured, the results of which are shown in Figs. 8(c, d). As the results indicate, the fluorescence lifetimes of Ho3+ at 659 and 1,190 nm, corresponding to 2F5 → 5I8 and 5I6 → 5I8 transitions were found to decrease with an increase in the Mn2+ concentration, which is strong evidence for the occurrence of ET from a |2F7/2, 6A1g(S) 4T1g(G)〉;|2F5/2, 6A1g(S) 6A1g(S)〉 → |2F7/2, 6A1g(S) 6A1g(S)〉 transition of the (Yb3+–Mn2+–Mn2+) trimer to a 5F6; 5I6 → 5I8 transition of the Ho3+ ions.
4. Conclusions
To summarize, TeO2–ZnO–Na2CO3–La2O3 tellurite glasses were successfully prepared using a conventional melt-quenching method. The effects of the (Yb3+–Mn2+–Mn2+) trimer on the enhancement of the UC/NIR emissions intensity of the co-doped Ho3+/Yb3+ bands in TeO2–ZnO–Na2CO3–La2O3 tellurite glasses centered at 545, 659, 755, and 1,190 nm was successfully investigated. Owing to the ET processes from the (Yb3+–Mn2+–Mn2+) trimer to Ho3+, the UC/NIR emissions intensity of the co-doped Ho3+/Yb3+ bands centered at 545, 659, 755, and 1,190 nm was significantly increased. When the Mn2+ concentration increased from 0 to 7.0 mol.%, the UC emissions intensity of the co-doped Ho3+/Yb3+ bands centered at 545, 659, and 755 nm were increased approximately 11-, 23-, and 9-fold, respectively. This result indicates that the Mn2+ and (Yb3+–Mn2+–Mn2+) trimer might provide a new option for studying the enhancement of the UC/NIR emissions intensity of co-doped Ho3+/Yb3+ in tellurite glasses. In addition, the tellurite glasses used in this study are characterized by their thermal stability, which is required for laser material applications.
Funding
Vietnam National Foundation for Science and Technology Development (NAFOSTED) (103.03-2019.56)
Disclosures
The authors declare are no conflicts of interest related to this article.
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