1. Introduction

Lead silicate glasses belonging to wide family of heavy metal systems [1–5] are classified as a “binary network-former glass” due to presence of extraordinarily large amounts of free volume (voids), which cannot be found in conventional binary silicate glasses [6]. They exhibit high stability against crystallization, thermal conductivity, heat capacity and wide glass formation composition range up to 90 mol% PbO. The presence of heavy metal oxide and/or fluoride elements minimizes the phonon energy of the glass hosts. Based on phonon side band (PSB) associated to the ⁷F₀ → ⁵D₂ (Eu³⁺) excitation transition, the phonon energy equal to 953 cm⁻¹ was evaluated and its value is in a good agreement with the maximum intensity band of the Raman spectrum in lead silicate glass [7]. Furthermore, trivalent rare earths as an optically active ions are quite easy incorporated into PbO-SiO₂ based glass hosts. Previous studies indicate that Nd³⁺ [8–10], Yb³⁺ [11], Pr³⁺ [12], Tm³⁺ and Tm³⁺/Er³⁺ [13–15], and Ho³⁺ [16, 17] doped lead silicate glasses emitting near-infrared (NIR) radiation are promising for solid-state lasers and broadband optical telecommunication. On the other hand, Pb atoms can be also play the same role as rare earths and take part in the formation of the NIR active centers in silicate glasses and their optical fibers. Bufetov et al [18] suggest that Pb-doped silica-based optical fibers exhibiting luminescence and optical gain in the NIR wavelength range between 1100 and 1200 nm. Photodarkening losses can be also induced in silica based glasses and optical fibers. At a consequence, color centers are produced and this process is accelerated by increase in temperature or by exposing the glass/fiber to light. Numerous works suggest that the photodarkening effect is found to be reversed through temperature annealing or exposure to UV and/or visible light [19–21]. It is interesting to notice that lead silicate glasses exposed to gamma irradiation darken due to the trapping of electrons in defects [22]. Both the nature of these defects and the process of color center formation are still under the study.

In recent years a considerable interest has been directed to up-conversion luminescence and its potential application for optical temperature sensing [23]. Numerous published papers have been devoted to elaborate methods of temperature sensing. In general, the up-converted luminescence of rare earth ions plays an important role in optical temperature sensing, since two excited states are in thermal equilibrium and their relative populations, determined by Boltzmann distribution law, depend on temperature. Therefore the temperature can be well determined employing the luminescence intensity ratio (FIR) technique [24–27]. Among rare earth ions, Er³⁺ doped systems are particularly promising for optical temperature sensing. The spectrum of up-converted visible luminescence in Er³⁺ doped systems consists of a green band related to the ²H_11/2,⁴S_3/2 → ⁴I_15/2 transition and of a red band related to the ⁴F_9/2 → ⁴I_15/2 transition [28]. In particular, the green up-converted luminescence of Er³⁺ plays significant role, since two excited states ²H_11/2 and ⁴S_3/2 are thermally coupled and the luminescence intensity ratio (FIR) of band components related to the ²H_11/2 → ⁴I_15/2 and ⁴S_3/2 → ⁴I_15/2 transitions depends strongly on temperature [29]. In another case, valley-to-peak intensity ratio (VPR) thermometry based on the red up-conversion luminescence due to ⁴F_9/2 → ⁴I_15/2 transition of trivalent Er³⁺ was successfully developed suggesting that the VPR method is also a promising candidate for temperature measurement in practice [30]. Much attention has been paid to up-conversion luminescence phenomena in various amorphous [31–33] and crystalline host materials [34–36] co-doped with Er³⁺/Yb³⁺ and their optical temperature sensing ability. These aspects have not been yet presented and discussed for Er³⁺/Yb³⁺ co-doped lead silicate glasses, to the best of our knowledge.

In this work, Er³⁺/Yb³⁺ co-doped lead silicate glasses have been studied for up-conversion luminescence applications. Green and red up-conversion luminescence spectra of Er³⁺ ions were recorded under excitation of Yb³⁺. In particular, the green up-conversion luminescence spectra of trivalent Er³⁺ have been examined with temperature and the ability of these systems for optical temperature sensing has been demonstrated.

2. Experimental

Er³⁺/Yb³⁺ co-doped lead silicate glasses with the following chemical composition (in mol%): (system I referred as 0.5Er-2.5Yb) 45PbO-45SiO₂-7Ga₂O₃-0.5Er₂O₃-2.5Yb₂O₃; (system II: 0.25Er-1.25Yb) 45PbO-45SiO₂-8.5Ga₂O₃-0.25Er₂O₃-1.25Yb₂O₃; (system III: 0.1Er-0.5Yb) 45PbO-45SiO₂-9.6Ga₂O₃-0.1Er₂O₃-0.5Yb₂O₃, were prepared by mixing and melting appropriate amounts of metal oxides of high purity (99.99%, Aldrich Chemical Co.) as starting materials. A homogeneous mixture was heated in a protective atmosphere of dried argon. Mixed reagents were melted for 0.5h at 1100°C. The same procedure was applied for preparation of Er³⁺ singly doped glass sample in 45PbO-45SiO₂-9.5Ga₂O₃-0.5Er₂O₃ chemical composition. The molar ratios PbO:SiO₂ = 1:1 and Er₂O₃:Yb₂O₃ = 1:5 are the same in all three studied heavy metal glass samples. The molar ratio Er₂O₃:Yb₂O₃ was optimized based on our earlier spectroscopic studies used for heavy metal oxide glasses [37]. The third Ga₂O₃ component plays the important role as a stabilizer of glass network. Additionally, the samples were prepared under the same experimental conditions (melting temperature and time), so the luminescence changes for erbium ions are related to concentrations of Er³⁺ and Yb³⁺ in glass composition. Optical absorption spectra (Cary 5000) and results of the X-ray diffraction measurement using an INEL diffractometer revealed that all glass samples were transparent and free from crystalline inclusions. Samples in the form of polished plates 3 mm thick were prepared for spectroscopic measurement. Up-converted emission was excited with a diode laser emitting at 980 nm, dispersed by a 1-meter double grating monochromator and detected employing a photomultiplier with S-20 spectral response. Emission spectra were recorded using a Stanford SRS 250 boxcar integrator controlled by a computer. The spectral bandwidth of the fluorometer system used was set to 0.1 nm. When up-converted emission spectra were measured in temperature range 300-670 K the glass samples were placed into a chamber furnace controlled by a proportional-integral-derivative (PID) Omron E5CK controller. The temperature deviation is estimated to be less than 5% for temperatures up to 700 K.

3. Results and discussion

Up-converted luminescence spectra for Er³⁺/Yb³⁺ co-doped lead silicate glasses measured under 980 nm excitation are presented in Figs. 1-3. The spectra were recorded at several different temperatures. The observed green and red emission bands centered at about 550 nm and 670 nm correspond to ²H_11/2,⁴S_3/2 → ⁴I_15/2 and ⁴F_9/2 → ⁴I_15/2 transitions of Er³⁺. In order to calculate their integrated luminescence intensities the deconvolution procedure was applied. Inset of Fig. 1 shows an example of green emission band deconvoluted into two sub-bands, which are associated to thermally coupled ²H_11/2 → ⁴I_15/2 and ⁴S_3/2 → ⁴I_15/2 transitions of Er³⁺.

 

Fig. 1 Up-conversion emission in 0.5Er-2.5Yb lead silicate glass varying with temperature.

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Fig. 2 Up-conversion emission in 0.25Er-1.25Yb lead silicate glass varying with temperature.

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Fig. 3 Up-conversion emission in 0.1Er-0.5Yb lead silicate glass varying with temperature.

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It can be seen in Figs. 1-3 that the temperature affects the green and red up-conversion luminescence spectra of Er³⁺ ions in lead silicate glasses. First of all, the integrated intensities of luminescence bands corresponding to the ²H_11/2,⁴S_3/2 → ⁴I_15/2 (green) and ⁴F_9/2 → ⁴I_15/2 (red) transitions of Er³⁺ diminish significantly with increasing temperature. Comparison of optical absorption spectra recorded as a function of temperature revealed that the changes of absorption coefficient of inhomogenously broadened Yb³⁺ absorption bands at the pump wavelength cannot account for observed dependence. Therefore, we attribute this phenomenon to the enhancement of temperature dependent rates of nonradiative multiphonon relaxation that contributes adversely to the decay of luminescent levels involved. Secondly, it can be seen in Fig. 4 that red-to-green luminescence intensity ratios R/G (Er³⁺) decrease drastically with increasing temperature. To explain this finding a more detailed insight into processes of relaxation of luminescent levels in question is needed. In particular it should be noticed that energy gap between the ⁴S_3/2 and the next lower energy level ⁴F_9/2 is comparable to that between the ⁴F_9/2 and the ⁴I_9/2 level. Accordingly, it follows from the energy gap law that rates of multiphonon relaxation rates for the ⁴S_3/2 and ⁴F_9/2 and their enhancement with increasing temperature should be comparable, too. However, the ⁴S_3/2 level is in thermal equilibrium with the ²H_11/2 level that decays to the ground state with a very high rate of radiative transitions. With growing temperature the ²H_11/2 population increases. As a consequence the luminescence from the ²H_11/2,⁴S_3/2 group of levels is enhanced at the expense of multiphonon transitions and the R/G factor changes. This trend is similar for all studied Er³⁺/Yb³⁺ co-doped lead silicate glasses, but the changes of R/G factor with temperature are quite stronger for 0.1Er-0.5Yb system than 0.25Er-1.25Yb and 0.5Er-2.5Yb systems, respectively.

 

Fig. 4 The integrated emission intensity ratio R/G (Er³⁺) varying with temperature.

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The mechanism of up-conversion process was also examined based on log-log plots of visible luminescence intensity versus the power of infrared excitation light (not shown here). The log-log dependence of up-conversion emission intensities for both the ²H_11/2,⁴S_3/2 → ⁴I_15/2 (green) and ⁴F_9/2 → ⁴I_15/2 (red) transitions of Er³⁺ on the excitation power was determined using the relation I_UPC ~Pⁿ_pump, where I_UPC denotes the up-conversion integrated luminescence intensity, P – the laser power and n – number of photons. Independent of Er³⁺ and Yb³⁺ concentrations, the slopes for green and red electronic transitions of trivalent erbium are equal to 1.86 and 1.67 (0.5Er-2.5Yb), 1.92 and 1.78 (0.25Er-1.25Yb), 2.01 and 1.46 (0.1Er-0.5Yb), respectively. It suggests that 2-photon mechanism was involved in the up-conversion luminescence process. Similar results were obtained earlier for lead silicate glass singly doped with Er³⁺ [38]. Two-step energy transfer from Yb³⁺ to Er³⁺ (ET) and several processes like the ground state absorption GSA (I), the excited state absorption ESA (II) and cross-relaxation (III) are responsible for the population of ²H_11/2,⁴S_3/2 and ⁴F_9/2 excited states of trivalent erbium [39]. They are schematized on the energy level diagram shown in Fig. 5.

 

Fig. 5 Energy level diagram for Er³⁺/Yb³⁺ co-doped lead silicate glass.

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It is generally accepted that the absorption cross-section at around 980 nm is markedly larger for Yb³⁺ than Er³⁺ ions. Thus, the ET processes from Yb³⁺ to Er³⁺ ions in lead silicate glasses are supposed to be much efficient than ESA processes. In the first step of ET the excited Yb³⁺ ions interact with unexcited Er³⁺ ions, creating thereby a population of the ⁴I_11/2 state of Er³⁺. In the second step an interaction between excited Yb³⁺ and Er³⁺ ions raises the excitation of trivalent erbium from the ⁴I_11/2 to the ⁴F_7/2. Next, a multiphonon relaxation transfers quickly the excitation energy from the ⁴F_7/2 state to the ²H_11/2, ⁴S_3/2 states of Er³⁺. A part of the excitation energy relaxes radiatively from the ²H_11/2, ⁴S_3/2 states to the ⁴I_15/2 ground state emitting green light, whereas another part is dissipated by nonradiative relaxation and gives rise to a red luminescence related to the ⁴F_9/2 → ⁴I_15/2 transition of Er³⁺. Moreover, several possible cross-relaxation routes displayed in Fig. 5 have been proposed to account for the feeding of the ⁴F/_9/2 state of Er³⁺. When Yb³⁺ concentration is low, both the ⁴I_11/2 and ⁴F_7/2 states of Er³⁺ ions are not well populated because of small absorbed excitation energy. Thus, the feeding of the red-emitting ⁴F_9/2 state by ET, ESA and CR processes is quite poor and the green luminescence is stronger than the red luminescence. In our case the Yb³⁺ concentration is relatively higher and the energy transfer (ET) from Yb³⁺ to Er³⁺ feeds both the ⁴I_11/2 and ⁴F_7/2 excited states of Er³⁺ more efficiently thereby favouring the feeding of the lower-lying ⁴F_9/2 red-emitting state by ESA and CR processes.

Further experimental studies indicate that the green up-conversion luminescence bands corresponding to the ²H_11/2 → ⁴I_15/2 and ⁴S_3/2 → ⁴I_15/2 transitions of Er³⁺ ions in lead silicate glasses are changed significantly with temperature. To determine the optical temperature sensing ability the temperature-dependent up-conversion luminescence spectra of Er³⁺ ions are examined in the 500-580 nm spectral region, where two band components related to electronic transitions originating from thermally coupled ²H_11/2 and ⁴S_3/2 excited states to the ⁴I_15/2 ground state of Er³⁺ occur. The relative emission band intensities of the ²H_11/2 → ⁴I_15/2 and ⁴S_3/2 → ⁴I_15/2 transitions of Er³⁺ ions in Er³⁺/Yb³⁺ co-doped lead silicate glasses were determined as a function of temperature.

The Fluorescence Intensity Ratio referred usually as FIR or R for band components of luminescence related to the ²H_11/2,⁴S_3/2 → ⁴I_15/2 transition of Er³⁺ ions can be expressed by the following equation [40–45]:

 

Fig. 6 Fluorescence intensity ratio (R) as a function of inverse temperature.

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The fluorescence intensity ratios R were used to calculate the temperature sensitivity S, which is defined by:

Figure 7 shows temperature sensitivity values S for Er³⁺/Yb³⁺ co-doped lead silicate glasses. Our experimental results clearly indicate that dependences of temperature sensitivity for the studied 0.5Er-2.5Yb, 0.25Er-1.25Yb and 0.1Er-0.5Yb glass systems are quite different. Additionally, 0.5Er-2.5Yb system is compared to erbium singly doped glass sample (0.5Er). Based on up-conversion luminescence spectrum obtained for erbium singly doped glass sample (not shown here), the fluorescence intensity ratio as a function of inverse temperature was determined and then temperature sensitivity was calculated. The procedure of sensitivity calculation was the same as for glass samples co-doped with Er³⁺/Yb³⁺.

 

Fig. 7 Temperature sensitivities S for Er³⁺/Yb³⁺ co-doped lead silicate glasses. The results are compared to glass sample singly doped with Er³⁺.

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The temperature sensitivities S for lead silicate glasses singly doped with Er³⁺ ions (0.5Er) and doubly doped with Er³⁺/Yb³⁺ ions (0.5Er-2.5Yb) increase with increasing temperature. The maximum sensitivity S_MAX for 0.5Er system is equal to 26.4x10⁻⁴K⁻¹ at T = 590K [41] and its value is in a good agreement with the results (23x10⁻⁴K⁻¹ at T = 296K) obtained for similar Er³⁺-doped silicate glass based on SiO₂-B₂O₃-Na₂O [42]. Compared to Er³⁺ singly doped sample (0.5Er), the temperature sensitivities for Er³⁺/Yb³⁺ co-doped sample containing the same concentration of trivalent erbium (0.5Er-2.5Yb) are enhanced. Thus, the maximum sensitivity S_MAX increases from 26.4x10⁻⁴K⁻¹ at T = 590K (0.5Er) to 37.1x10⁻⁴K⁻¹ at T = 608K for glass sample in the presence of Yb³⁺ ions (0.5Er-2.5Yb). It can be also compared to those reported for other silicate based glasses co-doped with Er³⁺/Yb³⁺. Our value of S_MAX is well consistent with the results (31x10⁻⁴K⁻¹ at T = 550K) reported for SiO₂-B₂O₃-Na₂O-BaO glass [43] or (S_MAX = 33x10⁻⁴K⁻¹ at T = 296K) reported for SiO₂-B₂O₃-Na₂O glass [44]. It clearly suggests that the presence of Yb³⁺ ions give important contribution to green up-conversion luminescence of Er³⁺ and improve optical temperature sensitivity. Our further investigations for Er³⁺/Yb³⁺ co-doped glass samples with the same molar ratios (Er₂O₃:Yb₂O₃ = 1:5) give interesting experimental results. Based on up-conversion luminescence spectra measurements and calculated data, it was concluded that smaller concentrations of both Er³⁺ and Yb³⁺ ions are strongly recommended for optical temperature sensing. In fact, the maximum sensitivity S_MAX increases significantly from 37.1x10⁻⁴K⁻¹ at T = 608K (0.5Er-2.5Yb) to 49.6x10⁻⁴K⁻¹ at T = 570K (0.1Er-0.5Yb) with decreasing Er³⁺ and Yb³⁺ concentrations. It is also interesting to note that all maximum sensitivity values obtained for 0.5Er, 0.5Er-2.5Yb and 0.1Er-0.5Yb systems are located in high-temperature range. In contrast to the results presented above, the variation of sensitivity with temperature for 0.25Er-1.25Yb system is quite different. In such case, the temperature sensitivities S decrease significantly with increasing temperature and the highest value of S_MAX close to 58.5x10⁻⁴K⁻¹ was obtained at room temperature. This trend was also observed by us for lead-free fluorogermanate glasses co-doped with Er³⁺/Yb³⁺ ions, for which optical temperature sensing properties were examined as a function of ytterbium concentration [45]. Finally, we can conclude based on gathered data that Er³⁺/Yb³⁺ co-doped lead silicate glasses are promising for up-conversion luminescence temperature sensors. Their optical temperature sensing ability depend strongly on both Er³⁺ and Yb³⁺ ion concentrations.

4. Conclusion

In summary, the up-conversion luminescence properties of Er³⁺/Yb³⁺ co-doped lead silicate glasses have been examined as a function of temperature. The integrated luminescence intensities of bands corresponding to the ²H_11/2,⁴S_3/2 → ⁴I_15/2 (green) and ⁴F_9/2 → ⁴I_15/2 (red) transitions of trivalent Er³⁺ and red-to-green luminescence intensity ratios R/G (Er³⁺) reduce significantly with increasing temperature. To determine optical temperature sensing ability the fluorescence intensity ratio for band components of the green luminescence related to the ²H_11/2,⁴S_3/2 → ⁴I_15/2 transition of Er³⁺ was determined and the temperature sensitivity values were evaluated. It was found that the maximal temperature sensitivities for lead silicate glasses co-doped with Er³⁺/Yb³⁺ depend strongly on rare earth ion concentrations and their values S_MAX are close to 37.1x10⁻⁴K⁻¹ at T = 608K (0.5Er-2.5Yb), 49.6x10⁻⁴K⁻¹ at T = 570K (0.1Er-0.5Yb) and 58.5x10⁻⁴K⁻¹ at T = 300K (0.25Er-1.25Yb), respectively. It was suggested that Er³⁺/Yb³⁺ co-doped lead silicate glasses are promising for optical temperature sensing.