1. Introduction

Yb³⁺ ion is an essential dopant in glasses and fibers for IR-lasers due to its highly-efficient emission at ~1 µm. Owing to its unusual energy level scheme with only two manifolds separated by about 10000 cm⁻¹, Yb³⁺ offers interesting spectroscopic properties for optical applications such as absence of excited state absorption and relatively long emission lifetime. The lifetime of Yb³⁺ excited state ²F_5/2 is an important spectroscopic parameter because it determines the laser pump saturation intensity I_sat ~1/τ_IR which should be as low as possible [1].

To increase the IR-laser gain, Yb³⁺ concentration can be increased but results in the formation of clusters, especially in pure SiO₂ glasses where the network rigidity is not suitable to host a high content of rare earth ions. In glasses containing modifier ions, the local environment of Yb³⁺ ions is defined preferentially by Non-Bridging Oxygens. Their concentration impacts the Yb clusters content in oxide glasses as was demonstrated in aluminosilicate, phosphate and aluminoborosilicate glasses [2, 3].

Active fibers such as Yb³⁺ or Er³⁺ doped ones can be exposed to ionizing radiations in nuclear or space applications [4]. Besides quantifying the Radiation Induced Absorption (RIA) and analyzing the corresponding point defects, it is also necessary to estimate the influence of ionizing radiations on rare earth element spectroscopic properties in these fibers. Only a few papers have reported such studies up to now. It has been shown that ionizing radiations (electron, γ rays) can affect Yb³⁺ spectroscopic properties in aluminoborosilicate glasses such as the ²F_5/2 excited state lifetime and the cooperative emission intensity [5]. This was correlated to the formation of hole centers type defect under irradiation, in this case, Non-Bridging Oxygen Hole Centers (NBOHC) [5]. In Er-doped fibers, a similar decrease of ⁴I_13/2 lifetime has been observed under γ-irradiation [6, 7] and ion radiation [8]. This was attributed to non-radiative energy transfers from the ⁴I_13/2 level to radiation-induced point defects [6, 7].

In the present study, we investigate the evolution of Yb³⁺ exited state lifetime under ionizing radiations (electrons, γ rays) for a broad dose range (10²-10⁹ Gy) in a series of aluminosilicate glasses. The composition of these glasses was chosen to study the influence of Yb³⁺ clustering on spectroscopic changes. This was motivated by our previous observation of Yb³⁺ clustering impact on the thermal recovery of radiation induced paramagnetic point defects [9]. Here, we show that Yb³⁺ excited state lifetime decreases at a lower rate as a function of the irradiation dose in glasses with high clusters amount. A good agreement is also found with the variation of the defect concentration in these glasses. The same excited state lifetime evolution is also observed in Yb³⁺ doped phosphate glasses and Er³⁺ doped aluminosilicate glasses, suggesting that this behavior is independent on the particular rare earth-glass combination.

2. Experimental details

Four Yb-doped and two Er-containing aluminosilicate glass compositions were synthesized. For that, appropriate amounts of dried powders of SiO₂, Al₂O₃, Na₂CO₃ Yb₂O₃ or Er₂O₃ were mixed thoroughly to obtain a well-homogenized powder and slowly heated (during 10 hours) in a Pt-Au crucible from room temperature up to 1600°C (melting temperature) in a furnace under air during 4 hours. Slow heating facilitated the CO₂ release process taking place at ~800°C. In addition, it promoted higher homogeneity of the melt. Afterwards, glasses were quenched at room temperature and annealed at 580°C (below T_g) in order to reduce the residual thermal stresses. The nominal aluminosilicate glass compositions are presented in Table 1. Samples are labeled according to the Aluminum Saturation Index (ASI = Al/Na), which characterizes the Non-Bridging Oxygen concentration. Actual glass compositions were determined with microprobe analysis giving <1 mol. % of error compared to nominal compositions.

Table 1. Nominal Yb³⁺ and Er³⁺- doped Aluminosilicate Glass Compositions

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Phosphate glass samples were prepared starting from NH₄H₂PO₄, MgO, Na₂CO₃ and Yb₂O₃ (5 wt. %). The initial powders were mixed and melted in a silica crucible and then slowly heated from room temperature up to 900°C. As in the case of aluminosilicate glasses, slow heating favored CO₂ release and homogenization. The melted glass was quenched rapidly into a Pt-Au crucible on an electric plate at 300°C to avoid a high cooling rate, and immediately annealed at 350°C to reduce residual thermal stresses. The nominal phosphate glass compositions are presented in Table 2.

Table 2. Nominal Yb³⁺-doped Phosphate Glass Compositions

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Bulk glass samples (5 × 5 mm², 500-700 µm thickness) were continuously irradiated at SIRIUS electron accelerator (Laboratoire des Solides Irradiés, Palaiseau, France) with the 2.5 MeV electron beam with a dose rate close to 25 MGy/h. The sample holder was maintained around 40°C with a water-cooling system. The achieved integrated doses were 10⁵, 10⁶, 3∙10⁶, 10⁷, 10⁸ and 1.5·10⁹ Gy. Samples were also irradiated with γ-rays of 1.25 MeV from Co⁶⁰ source in “Centre National des Sciences et Technologies Nucléaires” in Tunisia (CNSTN). The dose rate in this case was 5.64 kGy/h. The achieved doses were 10⁴, 10⁵, 10⁶ and 3·10⁶ Gy. Two lower doses 100 Gy and 1000 Gy were achieved with γ-rays of 1.25 MeV from Co⁶⁰ source in “Institut de Radioprotection et de Sûreté Nucléaire” (IRSN, Fontenay-aux-Roses, France). The dose rate was 2 Gy/min. The error bar on the radiation dose after electron irradiation is considered as 10% because of the backscattered electrons that were not taken into account in the dose calculation”. Concerning gamma rays irradiation, at 100 Gy the error is 5.5%.

Yb³⁺ emission ²F_5/2→²F_7/2 in the IR-region was recorded on a HORIBA Jobin Yvon spectrofluorimeter under Xe lamp excitation at 975 nm with a 1200 gr/mm grating and a RG-830 filter. The lifetime of Yb³⁺ excited state ²F_5/2 was measured at 1000 nm with a time resolution of 100 µs. The lifetime of Er^{3+ 4}I_13/2 excited state was measured using pulsed laser excitation at 975 nm (Ekspla optical parametric oscillator NT342B) and detection by an InGaAs detector. Yb³⁺ cooperative luminescence was excited at 975 nm by a laser diode, dispersed by a Jobin-Yvon H25 spectrometer and detected by a photon counting photomultiplier tube. In order to perform accurate cooperative luminescence measurements, the samples were grounded into powders of well-controlled particle size in the range 100-125 µm [2].

Electron Paramagnetic Resonance (EPR) experiments were carried out on a Bruker X-band EMX spectrometer. Paramagnetic point defects were analyzed at room temperature and Yb³⁺ signal was recorded at 4K. The microwave frequency was 9.8 GHz. Spectra were normalized by attenuator gain and sample mass to allow quantitative comparisons. Radiation-induced point defects were also analyzed by time-resolved photoluminescence at room temperature. We used a frequency-doubled pulsed Nd:YAG laser for excitation and a Shamrock SR-303i spectrometer and an ANDOR ICCD camera for detection. The delay was 500 ns and the gate width was 100 µs.

3. Results and discussion

3.1 Yb³⁺ doped aluminosilicate glasses

3.1.1 Optical and EPR spectroscopy of non-irradiated samples

The lifetimes τ_IR of ²F_5/2 Yb³⁺ excited state in non-irradiated aluminosilicate glasses are displayed in Table 3. Although the concentration of Yb³⁺ was maintained at 5 wt. % of Yb₂O₃, we observed that τ_IR decreased from 2.14 ms in ASI03_Yb5 glass to 0.94 ms in ASI11_Yb5 glass. We attribute this to changes in Yb³⁺ environment in the different glasses, resulting in variations of the ²F_5/2→²F_7/2 transition oscillator strength [10].

Table 3. Yb^{3+ 2}F_5/2 Lifetimes in Non-irradiated Aluminosilicate Glasses

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Yb³⁺ environment changes are also clearly seen in the normalized emission spectra shown in Fig. 1. In particular, the peak located at 1006 nm in ASI03_Yb5 and ASI06_Yb5 shifts to 1027 nm in ASI09_Yb5 and ASI11_Yb5 glasses. This could reflect a larger Stark splitting in the latter glasses due to a stronger crystal field. These changes in Yb³⁺ environment were confirmed by EPR spectroscopy (see below).

 

Fig. 1 Normalized Yb³⁺ emission spectra in non-irradiated Yb³⁺ doped aluminosilicate glasses under 975 nm excitation.

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Yb³⁺ clustering was probed by cooperative luminescence [2]. Corresponding spectra for non-irradiated glasses are shown in Fig. 2. Spectra were normalized by the factor (α_D*τ_IR)², where α_D is the absorption coefficient calculated by integrating the product of the absorption spectrum of the sample by the emission spectrum of the diode (normalized at maximum). Since excitation absorption was low in the powder geometry, the cooperative emission probability, which reflects Yb³⁺ cluster concentration, was proportional to the area under the normalized cooperative emission spectra [3]. The cooperative emission probability increases strongly from ASI03_Yb5 glass to ASI11_Yb5 (Fig. 2) and the glass series can be divided into two groups: ASI03_Yb5-ASI06_Yb5 (low Yb³⁺ cluster amount) and ASI09_Yb5- ASI11_Yb5 (high Yb³⁺ cluster amount). The high clustering in ASI09 and ASI11 glass compositions can be explained by a lower amount of NBO due to an increasing Al³⁺ content that must be compensated by Na⁺ ions [2]. We checked by MAS NMR of ²⁷Al the fourfold coordination of Al³⁺ in those glasses [9] in order to exclude any modifications of Al³⁺ coordination number.

 

Fig. 2 Normalized cooperative luminescence spectra of non-irradiated Yb-doped aluminosilicate glasses.

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To further probe Yb³⁺ environment and clustering, EPR spectra were recorded at 4K (Fig. 3). The broad line in the whole range of magnetic field corresponds to well-diluted Yb³⁺ ions in glass [5, 11, 12]. In ASI09_Yb5 and ASI11_Yb5 glasses, the EPR signal is much lower despite the same amount of Yb³⁺ in all four glasses (Fig. 3, inset). This decrease indicates that part of Yb³⁺ ions is not detected, probably because of too short spin relaxation time T₁. This can be due to the formation of Yb³⁺ ion clusters [11], in agreement with the cooperative luminescence results. Besides, in both glasses, a shoulder at g = 12.8 can be seen, which we assign to Yb³⁺ clusters [16]. Indeed, the theoretical g-value for isolated Yb³⁺ ions does not exceed 2Λ·Μ where Μ is the maximum of the projection of the total angular momentum and Λ is the Landé factor. The ground multiplet of Yb³⁺ is ²F_7/2, corresponding to Λ = ⁸/₇ and Μ = ⁷/₂. Thus, g_max~8 for isolated Yb³⁺ ions [11], suggesting that the g = 12.8 shoulder corresponds to clusters.

 

Fig. 3 EPR spectra normalized at maximum signal of non-irradiated Yb-doped aluminosilicate glasses (T = 4K). Inset: normalization by mass and gain only.

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Yb³⁺ EPR spectra were analyzed in more details by assuming an axial site symmetry. Determining the exact parameters of the g-tensor is difficult in glasses because of inhomogeneous broadening. The value of g_┴ is estimated to 3.2 in ASI03_Yb5 and ASI06_Yb5 and 4.1 in ASI09_Yb5 and ASI11_Yb5 glasses. We did not perform simulations to obtain g_║ parameters but we estimate g_║ ~1.3-1.4 in ASI03_Yb5 and ASI06_Yb5 and g_║ ~1.1 in ASI09_Yb5 and ASI11_Yb5. The variation in g_┴ with glass composition is in agreement with the emission spectra and reflects different environments for Yb³⁺ ions in low and high ASI index glasses. It should also be noted that the g_┴ factors obtained in Yb-doped aluminoborosilicate (ABS) glasses with low clustering content were in the range 3.17 to 3.32 [12], close to the values found here for ASI03_Yb5 and ASI06_Yb5, in which clustering is low too.

3.1.2 Evolution of Yb^{3+ 2}F_5/2 lifetime under irradiation in aluminosilicate glasses

First, we studied the evolution of lifetimes after irradiation for delays up to 4 months. Despite irradiation, all fluorescence decays were well fitted by a single exponential. Figure 4 displays the obtained τ_IR values in irradiated glasses at 10⁵ Gy at different times after irradiation. The values before irradiation are labeled by larger symbols. In all samples, irradiation results in a clear decrease (40-50%) of the ²F_5/2 lifetime. Then, for glasses ASI09_Yb5 and ASI11_Yb5 there is almost no variation of τ_IR for delays up to 4 months. In opposition, for glasses ASI03_Yb5 and ASI06_Yb5, one can observe a slight increase of the τ_IR. In all glasses, the lifetime of Yb³⁺ excited state ²F_5/2 did not reach its pre-irradiation value on the time scale investigated.

 

Fig. 4 ²F_5/2 lifetimes of Yb³⁺ excited state recorded at different times after electron irradiation (10⁵ Gy dose). The non-irradiated glasses are shown with larger symbols.

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This behavior can be understood by considering that the ²F_5/2 level is quenched by the defects induced by the irradiation. Indeed, the concentration of paramagnetic point defects induced by irradiation decreases with time, with a slower rate in glasses with higher Yb³⁺ clustering [9]. This is explained by the ability of clustered Yb³⁺ ions to efficiently trap charges under a stable form during the irradiation process. As a consequence, point defects thermal recovery is slowed down, as its τ_IR increase.

We also analyzed the dose effect on τ_IR lifetime after 6 months to reach stable values. Figure 5 displays τ_IR as a function of lg(dose) obtained for both electron and gamma irradiation. First, we see that irradiation at 100 Gy has no impact on the ²F_5/2 lifetime in glasses with high or low Yb³⁺ cluster content. The first effect can be seen at 1000 Gy. This result is meaningful for space applications where cumulated doses reach 300 Gy in 10 years, although the dose rate is much lower than the one used in our experiments. Consequently a direct extrapolation of this result to space application is not straightforward.

 

Fig. 5 ²F_5/2 lifetimes for e^- and γ irradiated Yb³⁺-doped aluminosilicate glasses (measured 6 months after irradiation). Solid (open) symbols: electron (gamma) irradiation; larger symbols: non-irradiated values. Error bars are shown for ASI03_Yb5 glasses only for easier reading.

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For glasses with lower ASI parameters – ASI03_Yb5 and ASI06_Yb5 – in both the lower dose range (<10⁵Gy) and in the higher dose range (>3·10⁶Gy) clear linear decrease is seen (Fig. 5). However, little or no variation occurs in the 10⁵– 3·10⁶Gy dose range. In contrast, for the other two glass compositions with higher ASI parameter, the linear variation in lifetime τ_IR presents smaller slopes in the lower and stronger doses regions and there is little evidence for a plateau in the 10⁵– 3·10⁶ Gy dose range.

We also observed no difference in lifetimes between e¯- and γ-irradiated glasses (Fig. 5). This indicates that the concentration and nature of the defects involved in ²F_5/2 quenching are similar in both cases, in the limit of our observations (three dose levels).

The τ_IR lifetime evolution was further analyzed in ASI09 aluminosilicate glasses doped with various Yb³⁺ concentrations: 0.5, 5 and 8 wt. % (Fig. 6). At low Yb³⁺ concentration, the behavior of low clustering glasses ASI03 and ASI06 is reproduced, whereas at high Yb³⁺ concentration, the lifetime evolution follows that of ASI09 and ASI011. Taking into account that we checked that clustering is increasing with increasing Yb³⁺ concentration, we conclude that Yb³⁺ clustering, and not glass composition, modifies the evolution of τ_IR as a function of irradiation dose.

 

Fig. 6 Variation of the ²F_5/2 decay time value of Yb³⁺ as a function of the logarithmic dose. The irradiation has been performed with e¯-on aluminosilicate glasses ASI09_Ybx with different Yb concentration. Empty symbols are used for gamma irradiation

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3.1.3 Correlation between ²F_5/2 lifetime and radiation-induced point defects

In order to understand the ²F_5/2 lifetime variation, we measured the paramagnetic point defects nature and concentration, and their evolution under an annealing treatment. Figure 7 displays the total amount of paramagnetic point defects as a function of irradiation dose. This was determined from double integration of the EPR spectra in the g = 2 region [9]. A plateau (10⁵-3·10⁶Gy) is followed by an increase of defects with increasing dose, and then further followed by a decrease at the highest radiation exposure level. Moreover, glasses with the higher cluster content (ASI09_Yb5 and ASI11_Yb5) show a smaller variation of defects concentration with dose. This is in qualitative agreement with the limited ²F_5/2 lifetime variation with irradiation dose (Fig. 5). For glasses with lower clustering (ASI03_Yb5 and ASI06_Yb5), a similar correlation is observed.

 

Fig. 7 Total amount of paramagnetic point defects in irradiated Yb³⁺-doped aluminosilicate glasses.

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In order to examine the possible role of particular point defects in lifetime decrease, we carried out annealing treatments on two Yb³⁺-doped aluminosilicate glasses with opposite Yb³⁺ cluster content: ASI03_Yb5 and ASI09_Yb5. As a matter of fact, bleaching temperature is unique for each defect since it is associated to its energy activation, so an annealing treatment is a tool to separate defect contribution.

The total defect concentration of two Yb³⁺-doped aluminosilicate glasses as a function of annealing temperatures is presented in Fig. 8. The precise analysis of paramagnetic defect in those glasses was reported in details elsewhere [9]. In ASI03_Yb5, the lifetime of Yb³⁺ excited state ²F_5/2 is already back to its pre-irradiation value ~2.14 ms after annealing at 244 °C [Fig. 8(b)]. However, there are still defects in the glass, which are partly identified as HC type (HC1 and HC2) from the EPR spectrum [9]. Moreover, at the same temperature, we observed under 532 nm laser excitation, a broad band centered at 725 nm that we attribute to NBOHC [Fig. 8(b)]. Even if the position is deviated from 620 nm as in SiO₂ glass [13], we assume that the presence of Al can lead a red-shift the position band [14]. We conclude from these results that neither HC defects nor NBOHC defects are solely responsible for the decrease of the ²F_5/2 lifetime.

 

Fig. 8 ²F_5/2 lifetimes and total concentration of paramagnetic defects (left), and photoluminescence spectra of NBOHC (right) in ASI09_Yb5 (a) and ASI03_Yb5 (b) glasses obtained after annealing at various temperatures. Electron irradiation at 10⁸ Gy.

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Contrary to ASI03_Yb5, τ_IR in ASI09_Yb5 is not back to the pre-irradiation value after annealing at 400°C [Fig. 8(a)]. At this temperature, peroxy radicals were evidenced by EPR, whereas neither Al-OHC nor NBOHC defects were detected [Fig. 8(b)]. As the absorption band of peroxy radicals centered at 1.97 eV (630 nm) [13] extends to Yb³⁺ emission range, they could be responsible for the decrease of ²F_5/2 lifetime.

3.2 Extension to other systems

We first studied ²F_5/2 lifetime as a function of irradiation dose and cluster content in Yb³⁺ doped phosphate glasses. As in aluminosilicate glasses, one can observe a multi-step τ_IR variation in poly-phosphate glasses, in which the cluster amount is low (Fig. 9), opposed to a monotonically decrease in meta-phosphate glasses with higher Yb³⁺ clustering. The cluster concentration was determined by cooperative luminescence [2]. This result on phosphate glasses first confirms the role of clusters and second suggests that lifetime behavior is irrelevant to the glass composition and consequently on the type of point defect formed.

 

Fig. 9 ²F_5/2 lifetimes of Yb³⁺ doped- phosphate glasses.

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We also investigated Er³⁺ doped glasses, in order to further assess the role of clusters. Figure 10 displays the lifetime of ⁴I_13/2 excited state in ASI03 and ASI09 glasses as a function of lg(dose). In ASI03_Er05 glass with low Er³⁺ cluster content, the ⁴I_13/2 lifetime evolution is similar to that of glasses with low Yb³⁺ cluster content, whereas in ASI09_Er05 glass the lifetime variation is lower, as in high Yb³⁺ clustering glasses (see Fig. 5). This suggests that the de-excitation channels via point defect formation are similar for Yb³⁺ and Er³⁺ ions.

 

Fig. 10 ⁴I_13/2 lifetimes of Er³⁺ doped-aluminosilicate glasses.

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4. Conclusion

We have studied Yb³⁺ doped aluminosilicate glasses with varying clusters content, which was evidenced by cooperative luminescence and EPR spectroscopy. Yb³⁺ excited state lifetime was recorded as a function of irradiation dose in a 10²-1.5⋅10⁹ Gy range (electron and γ rays) and delays after a 10⁵ Gy dose irradiation. Ionizing radiations were found to decrease lifetimes with rates depending primarily on Yb³⁺ clustering and not glass composition. In particular, glasses with high clustering show a smaller decrease in lifetime with increasing radiation dose. This behavior was in agreement with the variation in paramagnetic defects concentration. The nature of quenching defects was studied by annealing, suggesting that peroxy radicals could have a larger effect than other types of hole centers. In addition, clustering was found to impact lifetimes in Yb³⁺ doped phosphate glasses as well as in Er³⁺ doped aluminosilicate ones. This suggests that this effect could be observed in a broad range of rare earth doped glasses.