Excitation powder dependent optical temperature behavior of Er3+ doped transparent Sr0.69La0.31F2.31 glass ceramics

Xiangfu Wang; Qing Liu; Peiqing Cai; Jing Wang; Lin Qin; ThiQuynh Vu; Hyo Jin Seo

doi:10.1364/OE.24.017792

1. Introduction

In recent years, Er³⁺ doped phosphors were widely explored for applications in lasers, three-dimensional displays, optical temperature sensors, biological imaging probes, and solar cells [1–6], due to their intense visible up-conversion emissions induced by infrared photon excitation. Visible up-conversion emissions of Er³⁺ ions originated from transitions from ²H_11/2, ⁴S_3/2, and ⁴F_9/2 excited states to ⁴I_15/2 ground state after sequential absorption of infrared photons or two-step energy transfers from Yb³⁺ to Er³⁺ ions [7–10]. Fischer et. al. reported that Er³⁺ doped phosphors could have intense emissions at high excitation power where they are saturated [11]. Moreover, some nonlinear properties were also observed in Er³⁺ up-conversion process at high pumping power of lasers. Chen et.al. reported the “s”-shape power dependence for the green and red emissions in Gd₂O₃:Er³⁺ nanocrystals under 970 nm excitation [12]. Instead of three-photon process, the linear dependence of intensities of green and red emissions on the excitation power density was observed by Chen et.al. in LiYF₄:Er³⁺ under 1490 nm excitation [13]. Qu et. al. reported that the green and red emissions of LaF₃:Yb³⁺, Er³⁺ nanocrystals showed a typical two-photon upconversion process under lower power density of 980 nm laser, while they exhibited as the hysteresis loops when the pump power density turns to higher region [14]. Tian et. al. observed that the green emission of Yb³⁺-Er³⁺ co-doped Gd₆WO₁₂ nanoparticles was a two-photon process when the excitation power density of 980 nm laser was low, while a three-photon process occurred when the excitation power density was high [15]. These nonlinear changes at high excitation power, owing to the competition between the linear decay and the depletion of the intermediate excited states [16].

It was found that the intensity of green emission from ²H_11/2 level increased with temperature increase at fixed excitation powder, while the intensity of green emission from ⁴S_3/2 level decreased [17,18], due to the thermal population and depopulation at high temperature [19]. The fluorescence intensity ratio (FIR) of green emissions from ²H_11/2 and ⁴S_3/2 levels of Er³⁺ ion have been used to study optical temperature sensing in glass ceramics [20–23] and oxide bulks [24–28]. In these optical thermometry processes, it was assumed that the ²H_11/2 and ⁴S_3/2 levels of Er³⁺ ion were populated by a same two-photon up-conversion, and the FIR was independent of excitation powder of infrared lasers. In fact, almost all these ²H_11/2 and ⁴S_3/2 levels have the different pump power dependencies reported so far. This deviated a fact that the photon numbers needed to populate the ²H_11/2 and ⁴S_3/2 levels might be different at low and high pump powders. The conventional optical thermometry may lead to an error of sensitivity without consideration on the dependence of FIR on the pump power. Thus, it is necessary to explore the optical temperature behavior controlled by the excitation powder. In this work, novel Er³⁺ doped transparent Sr_0.69La_0.31F_2.31 glass ceramics with high physical and chemical stability have been fabricated. The dependence of FIR on the pump power is studied through monitoring the up-conversion emissions from three couples of adjacent thermally coupled levels, ²H_11/2/⁴S_3/2, ⁴F_9/2(1)/⁴F_9/2(2), and ⁴I_9/2(1)/⁴I_9/2(2) at low (66.8 Mw/mm²) and high (374.8 mW/mm²) excitation powers. The thermal quenching ratio, population mechanism of thermally coupled levels, and temperature sensitivity are also studied by using the low and high excitation powers.

2. Experimental

Er³⁺ doped transparent Sr_0.69La_0.31F_2.31 glass ceramics were fabricated by an improved melt-quenching method and subsequent heating. The raw materials are SiO₂ (AR), Na₂CO₃ (AR), Al₂O₃ (AR), LaF₃ (99.99%) and SrF₂ (99.99%) with the composition (in mol%): 45SiO₂–20Na₂CO₃–15Al₂O₃–6.2LaF₃–13.8SrF₂. Then 0.75 wt% ErF₃ is doped into the raw materials. The materials were thoroughly mixed and ground in an mortar and heated at 1500°C for about 1 hour and then cast into a brass mold to proceed rapid quenching. The as-made glass was heat-treated at 660°C for 2 h to form transparent ceramics through a crystallization process.

The structure of the sample was investigated by X-ray diffraction (XRD) using a X'TRA (Switzerland ARL) equipment provided with Cu tube with K_α radiation at 1.54056 Å. The size and shape of the sample was observed by a JEM-2100 transmission electron microscope (JEOL Ltd., Tokyo, Japan). Luminescence spectra were obtained by the Acton SpectraPro Sp-2300 Spectrophotometer with a photomultiplier tube equipped with 980 nm laser as the excitation sources. Different temperature spectra were obtained by using an INSTEC HCS302 Hot and Cold System.

3. Results and discussions

Figure 1 illustrates the transmission electron microscope (TEM) and high-resolution transmission electron microscope (HRTEM) images, XRD, and unit cell structure of transparent Sr_0.69La_0.31F_2.31 glass ceramics. TEM image in Fig. 1(a) confirms homogeneously distributed Sr_0.69La_0.31F_2.31 spherical crystallites precipitated among the oxyfluoride glassy matrix. The size of Sr_0.69La_0.31F_2.31 crystallite in the transparent ceramic is about 10 nm. The HRTEM observation in Fig. 1(b) confirms the well-defined lattice image and good crystallinity of the resulting nano-glass ceramics. The interplanar distance d value measured through two separated crystal planes from the HRTEM image is about 0.14 nm, which can be ascribed to the (111) crystal plane of Sr_0.69La_0.31F_2.31 crystals. Structure of the resulting transparent nano-glass ceramics is studied by the XRD patterns in Fig. 1(c). One can find that the position and relative intensity of all the diffraction peaks can be readily indexed to the cubic Sr_0.6La_0.31F_2.31 according to the JCPDS file No. 78-1143. Compared to the standard XRD pattern, the intensity ratio of the (111) peak of our sample is obviously larger than that of the standard value, suggesting that the sample tends to be preferentially oriented. The spatiality of the Sr_0.69La_0.31F_2.31 structure along the [001] direction is illustrated in Fig. 1(d). The crystal structure is cubic Sr_0.69La_0.31F_2.31 (ICSD 062285) [29]. The space group is Fm-3m. Each unit cell contains 4 formula units. In the unit cell of Sr_0.69La_0.31F_2.31, there are 8c and 32f two F cation sites, and Sr and La occupy the same 4a cation site, respectively. La is substituted by the Er in Er³⁺-doped Sr_0.6La_0.31F_2.31 crystal. To our knowledge, the synthesis and optical temperature sensing of transparent Er³⁺-doped Sr_0.69La_0.31F_2.31 nano-glass ceramics has not been reported.

Fig. 1 (a) TEM and (b) HRTEM images of Er³⁺ doped Sr_0.69La_0.31F_2.31 glass ceramics. (c) XRD pattern of Er³⁺ doped Sr_0.69La_0.31F_2.31 glass ceramics. The below standard data for cubic Sr_0.69La_0.31F_2.31 (JCPDS 78-1143). (d) The schematic views of unit cell of Sr_0.69La_0.31F_2.31 structure along b-direction.

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The transmission spectrum of Er³⁺ doped Sr_0.69La_0.31F_2.31 glass ceramics is shown in Fig. 2. It shows that the resulting glass ceramics maintains a high transparency in the region from visible to infrared, which is due to the smaller size of the precipitated crystals much than the wavelength of visible and infrared light. The transmission peaks at 378 nm, 488 nm, 520 nm, 651nm, 797 nm, 980 nm and 1532 nm are observed, which are assigned to the transitions of ⁴I_15/2→⁴G_11/2, ⁴I_15/2→⁴F_7/2, ⁴I_15/2→²H_11/2, ⁴I_15/2→⁴F_9/2, ⁴I_15/2→⁴I_9/2, ⁴I_15/2→⁴I_11/2 and ⁴I_15/2→⁴I_13/2.

Fig. 2 Transmission spectra of Er³⁺ doped Sr_0.69La_0.31F_2.31 glass ceramics.

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In order to explore the property of optical temperature sensing, the temperature dependent photoluminescence spectra are studied in the temperature range from 298 K to 573 K by using a 980 nm laser with two kinds of powers of 66.8 mW/mm² and 374.8mW/mm², as shown in Fig. 3. It can be seen that the photoluminescence spectra in Fig. 3(a) and Fig. 3(b) contain green, red, and infrared emission bands, which are assigned to the ⁴I_15/2→²H_11/2 (522 nm), ⁴I_15/2→⁴S_3/2 (540 nm), ⁴I_15/2→⁴F_9/2(1) (650 nm), ⁴I_15/2→⁴F_9/2(2) (665 nm) and ⁴I_15/2→⁴I_9/2 (800 nm) transitions of Er³⁺ ion, respectively. One can find that the intensities of green and red emission bands greatly decrease as temperature increases without changing the peak positions of the emissions, while the intensity of infrared emission bands greatly increase as temperature increases with a peak shifting from 840 nm through 820 nm and to 800 nm. The emission spectra of Er³⁺ doped Sr_0.69La_0.31F_2.31 glass ceramics are converted to the Commission International de I’Eclairage (CIE) 1931 chromaticity diagram [30], as shown in Fig. 4. With increasing the temperature from 298 K to 523 K, the luminescent color changes a little in green range at low and high excitation power.

Fig. 3 Temperature dependent photoluminescence spectra of Er³⁺ doped Sr_0.69La_0.31F_2.31 glass ceramics (a) at low 66.8 mW/mm² excitation power, and (b) at high 374.8 mW/mm² excitation power.

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Fig. 4 CIE(X, Y) chromaticity coordinates diagrams (a) at low 66.8 mW/mm² excitation power, and (b) at high 374.8 mW/mm² excitation power.

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From the insets of Fig. 3(a) and Fig. 3(b), we can see only an infrared peak at 840 nm when the temperature is lower than 373 K. This peak begins to split when the temperature increases more than 373 K. To study peak splitting induced by the temperature, the temperature and excitation power dependent photoluminescence spectra from 750 nm to 900 nm are shown in Fig. 5. The infrared peak at 840 nm in Fig. 5(a) is not split at 298K with low excitation power of 66.8 mW/mm², while the asymmetric emission band at 840 nm has been resolved into two Gaussian components peaking at 800 nm and 840 nm at 298 K with high excitation power of 374.8 mW/mm², as shown in Fig. 5(b). Fixed the excitation power at 66.8 mW/mm², the infrared peak at 840 nm obtained at 298 K (Fig. 5(a)) is resolved into three Gaussian components peaking at 800 nm, 820 nm, and 840 nm at high temperature of 573 K, as shown in Fig. 5(c). Fixed the excitation power at 374.8 mW/mm², the asymmetric infrared peak is still resolved into two Gaussian components peaking at 800 nm and 840 nm at low (298 K) and high temperature (573 K), as shown in Fig. 5(b) and Fig. 5(d). Compared with the peak at 800 nm at 298 K, an obvious intensity increase of 800 nm emission is observed at 573 K. These change trends mean that the infrared emission band can be modified by changing the excitation power. At Sr_0.69La_0.31F_2.31 host, the ⁴I_9/2 level of Er³⁺ ion is split into three crystal field levels [31], such as ⁴I_9/2(1), ⁴I_9/2(2), and ⁴I_9/2(3). The ⁴I_9/2(3) is populated predominantly at low temperature, giving 840 nm emission with a ⁴I_9/2(3)→⁴I_15/2 transition. According to the Boltzmann distributing law [32], the ions on ⁴I_9/2(3) level are thermally pumped onto the ⁴I_9/2(2) and ⁴I_9/2(1) levels step by step at high temperature. This thermal population decreases the intensity of 840 nm emission, and increases the intensities of 800 nm and 820 nm emissions. Thus, the peak shifting from 840 nm to 800 nm is observed with temperature increase.

Fig. 5 Temperature and excitation power dependent photoluminescence spectra from 750 nm to 900 nm of Er³⁺ doped Sr_0.69La_0.31F_2.31 glass ceramics. The black solid line represents the experimental data, and the colored dotted lines represent the fitting data.

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To evaluate the affection of temperature on luminescence quenching, the thermal quenching ratio (R_Q) of emission bands induced by the temperature change is defined as follow:

R_{Q} = 1 - \frac{I_{T}}{I_{0}}

where I_T is luminescence intensity at different temperature T, and I₀ is luminescence intensity at room temperature. Figure 6 shows the thermal quenching ratios of five emission bands through exciting the resulting glass ceramics with low and high excitation power. For green emission bands at 522 nm and 540 nm, it shows a reverse trend with the temperature increase. The value of R_Q of 540 nm emission band increases with temperature increase from 298 K to 573 K. Oppositely, the value of R_Q of 522 nm emission band shows some negative values, and decreases firstly and then increases with temperature increase, which means that the ²H_11/2 state (522 nm) is populated thermally at high temperature [33]. Moreover, when the temperature is below 423 K, the values of R_Q of 522 nm and 540 nm emission bands induced by high excitation power are more than those induced by low excitation power. However, when the temperature is above 423 K, the values of R_Q of 522 nm and 540 nm emission bands induced by high excitation power are less than those induced by low excitation power. It means that the thermal quenching ratios of green emission bands can be inhibited by high excitation power. Differently, it shows that the thermal quenching ratios of red emissions at 650 nm and 665 nm can be increased by high excitation power. The intensity of 800 nm emission can be enhanced a lot by the increase of temperature and excitation power.

Fig. 6 Thermal quenching ratios (R_Q) of Er³⁺ doped Sr_0.69La_0.31F_2.31 glass ceramics (a) at low 66.8 mW/cm² excitation power, and (b) at high 374.8 mW/cm² excitation power.

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To explore the origin of emissions of Er³⁺ ions at high temperatures, the relation between up-conversion emission intensity I and laser light intensity P is expressed as:

I \propto p^{n}

where n is the number of absorbed photons in up-conversion process [16]. Figure 7 shows log–log plots of up-conversion intensity and pumping power for green, red, and infrared emissions at the different temperatures. The slopes of six fitted lines for 522 nm and 540 nm emissions change a little at three temperature points at 298 K, 373 K, and 573 K, and indicate that 522 nm and 540 nm emissions come from two-photon up-conversion process. The slopes of three six lines for 650 nm and 665 nm emissions are dependent on temperature, and indicate that two red emissions come from two-photon up-conversion process. The slopes of four lines for 800 nm and 820 nm emissions are dependent on temperature, and indicate that two infrared emissions come from one-photon up-conversion process. Thus, under 980 nm excitation, the excited state of ⁴I_11/2 is populated by the ground state absorption with a ⁴I_15/2→⁴I_11/2 transition, shown in Fig. 8. The slopes for the 800 nm and 820 nm emissions have significant difference at 373K and 573K. It may be attributed to the competition between the nonradiative relaxation and thermal population of the ⁴I_9/2(1) and ⁴I_9/2(2) excited states [34,35]. Associated with phonons, three adjacent energy levels split from ⁴I_9/2 are populated thermally. After an excited state absorption, the ⁴F_7/2 state is populated by a ⁴I_11/2→⁴F_7/2 transition. The ions in the ⁴F_7/2 state relax to the next lower energy levels ²H_11/2/⁴S_3/2 and ⁴F_9/2(1)/⁴F_9/2(2) through non-radiative relaxation, giving green and red emissions. According to Boltzmann distributing law, two adjacent energy levels, upper ²H_11/2 (⁴F_9/2(1)) level and lower ⁴S_3/2 (⁴F_9/2(2)) level, can be thermally populated and depopulated with temperature increase [36]. The temperature-dependent fluorescence intensity ratios, I_U1/I_L1 (522 nm/540 nm), I_U2/I_L2 (650 nm/665 nm), and I_U3/I_L3 (800 nm/820 nm), may be used to be as thermally coupled levels, such as TCL1 (²H_11/2/⁴S_3/2), TCL2 (⁴F_9/2(1)/⁴F_9/2(2)), and TCL3 (⁴I_9/2(1)/⁴I_9/2(2)), for optical temperature sensing.

Fig. 7 Log–log plots of intensity and pumping power for (a) 522 nm, (b) 540 nm, (c) 650 nm, (d) 665 nm, (e) 800 nm, and (f) 820 nm emissions at different temperatures.

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Fig. 8 Population mechanism of Er³⁺ at high temperature under 980 nm excitation.

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It was reported that the emission intensity ratio (R) from the thermally coupled energy levels of active ions is expressed as

R = A e^{- \frac{Δ E}{k T}} + B

where ΔE is the energy difference between thermally coupled levels, T the absolute temperature, and k is the Boltzmann constant. A and B are constants [3]. From the spectra in Fig. 3, the experimental values of ΔE are about 638.6 cm⁻¹, 347.02 cm⁻¹, and 304.9 cm⁻¹ for thermally coupled levels of TCL1, TCL2 and TCL3. If fitted with the Eq. (3), the values of ΔE are about 735.1 cm⁻¹ (66.8 mW/cm² excitation power) and 653.7 cm⁻¹ (374.8 mW/cm² excitation power) for thermally coupled level of TCL1, 88.4 cm⁻¹ (66.8 mW/cm² excitation power) and 93.5 cm⁻¹ (374.8 mW/cm² excitation power) for thermally coupled level of TCL2, 1097.3 cm⁻¹ (66.8 mW/cm² excitation power), 154.4 cm⁻¹ (374.8 mW/cm² excitation power) for thermally coupled level of TCL3. Compared with the experimental values of ΔE, the errors are too big to ignore. The experimental ΔE is calculated from the two emission peaks originated from ²H_11/2 and ⁴S_3/2 in the experimental spectrum. The theoretical ΔE will be obtained through fitting some data points of luminescence intensity ratios at different temperature by using the Eq. (3). The Eq. (3) was a theoretical model and established by only considering the Boltzmann distribution between two thermally coupled levels and radiative transitions from thermally coupled levels to ground state [37], without considering nonradiative relaxation and energy transfer between the host and the rare-earth ions. However, nonradiative relaxation and energy transfer are not neglected in real photoluminescence process at high temperature. Thus, the error between experimental and theoretical values of ΔE appears. A large error value means that the thermally coupled levels are populated and dispopulated at high temperature through a complex dynamic process, not only Boltzmann distribution and radiative transitions, but also nonradiative relaxation and energy transfer.

Notably, the slopes for red and infrared emissions in Fig. 7 at low and high temperatures are not identical, which suggests that the fluorescence intensity ratios of thermally coupled levels of the TCL2 and TCL3, are expected to be susceptible to the pumping powers. This unsteady phenomenon is attributed to the competition between the thermal population and the nonradiative relaxation of the intermediate excited states [34]. The nonradiative relaxation possibility is expressed as

w_{i j} = w_{0} {(1 - e^{- ℏ w / k T})}^{- P}

where w₀ is its value at zero temperature, ħw is the phonon energy, and the exponent p is the phonon numbers. Fluorescence intensity ratios of adjacent thermally coupled levels are not be fitted well with a single exponential model from Eq. (3), due to the fact that nonradiative relaxation possibility changes also with the temperature.

Considering the similar exponential model of R and w_ij, the relation between R and T is modified as

L n R = \frac{a}{T} + b

where a is constant. The b is a correction term for the comprehensive population of thermally coupled energy levels induced by the thermal population, nonradiative relaxation and so on. Figure 9 shows temperature dependent emission intensity ratios of green, red, and infrared emissions at the different excitation powers. One can find that the experimental points can be fitted well with a linear mode. The slope values of green, red, and infrared emissions are dependent on excitation powers. It means that the fluorescence intensity ratios of thermally coupled levels of TCL1, TCL2, and TCL3 are susceptible to the pumping power. It is noting that the values of fitted slopes in Fig. 9 are not equal to the theoretical values of the ΔE/K, due to a complex dynamic up-conversion process induced by the high temperature and high excitation powers.

Fig. 9 Arrhenius plots of temperature dependent emission intensity ratios of (a) 522 nm/540 nm, (b) 650 nm/665 nm, (c) 800 nm/820 nm at low 66.8 mW/cm² excitation power and at high 374.8 mW/cm² excitation power.

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The sensitivity is one of the key parameter to determine the property of optical thermometry [3]. The sensitivity is defined as

S = \frac{d R}{d T} = \frac{a}{T^{2}} e^{\frac{b T - a}{T}}

where a and b are the constants from Eq. (5). The excitation power dependent sensitivity for three thermally coupled energy levels, such as TCL1, TCL2, and TCL3, are calculated in Fig. 10. All the sensitivity lines increase and then decrease with the increase of temperature. Three thermally coupled energy levels show the high sensitivity at low excitation power. The maximum values at (452.6K, 0.0014/K), (494.8K, 0.0015/K), (62.7K, 0.0063/K), (65.8K, 0.0061/K), (584K, 0.0039/K), and (934.4K, 0.0045/K) are observed for the TCL1, TCL2, and TCL3. From Fig. 7, the slopes of fitted lines for TCL1 are very closer at three temperature points at 298 K, 373K, and 573 K, revealing that TCL1 is especially stable for monitoring the temperature in wide temperature range. It is very sensitive to measure temperature below 250 K by using the fluorescence intensity ratio of the TCL2, shown in Fig. 10. Most of sensors based on up-conversion luminescence of Er³⁺ ion showed excellent sensitivity property for temperatures ranging from room to high temperatures, with the maximum sensitivity values at high temperature more than 300 K [17–25]. Reports on optical thermometry below room temperature were very scarce. The slope change induced by the temperature in Fig. 7 shows the TCL2 is not stable when the temperature excesses 373 K. Thus, TCL2 is suitable for monitoring the temperature below 373 K. Even though the TCL3 has high sensitivity at high temperature range more than 500 K, Fig. 7 shows the TCL3 is not stable with the temperature increase. Thus, the combined use of TCL1 and TCL2 as thermally coupled energy levels will get a more precise temperature reading at low excitation power.

Fig. 10 The excitation power dependence of sensitivity.

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4. Conclusion

In summary, Er³⁺ doped transparent Sr_0.69La_0.31F_2.31 glass ceramics was fabricated by a improved melt-quenching method. XRD, TEM and HRTEM analysis show that cubic Sr_0.69La_0.31F_2.31 crystals are homogeneously precipitated among the silicate aluminum glass matrix. It was observed that the spectrum structure, thermal quenching ratio, fluorescence intensity ratio, and sensitivity from three thermally coupled levels, such as ²H_11/2/⁴S_3/2, ⁴F_9/2(1)/⁴F_9/2(2), and ⁴I_9/2(1)/⁴I_9/2(2), are strongly dependent on the change of pump powers. A new fitting method is proposed to establish the relation between fluorescence intensity ratios and temperature. The ²H_11/2/⁴S_3/2 levels has high population stability at high temperature, while ⁴F_9/2(1)/⁴F_9/2(2) levels has high population stability at low temperature. A more precise temperature measuring can be obtained through the combining use of ²H_11/2/⁴S_3/2 and ⁴F_9/2(1)/⁴F_9/2(2) as thermally coupled energy levels at low excitation power at 66.8 mW/mm². This work also overcomes difficulty in low temperature measurement for conventional optical temperature sensing technology. The resulting glass ceramic is a very promising candidate for accurate optical temperature sensors with much higher sensitivity and wide temperature range.

Funding

This work was supported by National Natural Science Foundation of China (NSFC) (11404171), Natural Science Youth Foundation of Jiangsu Province (BK20130865), the Six Categories of Summit Talents of Jiangsu Province of China (2014-XCL-021), and the Natural Science Foundation of the Jiangsu Higher Education Institutions of China (14KJB430020), and Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (NRF-2013RA1A2009154).

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Excitation powder dependent optical temperature behavior of Er³⁺ doped transparent Sr_0.69La_0.31F_2.31 glass ceramics

Abstract

1. Introduction

2. Experimental

3. Results and discussions

4. Conclusion

Funding

References and links

Cited By

Figures (10)

Equations (6)

Optics Express