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Abstract
We report on the optical and magnetic properties of Tm2O3-doped calcium aluminosilicate glasses with dopant concentrations of up to 7 mol%. These materials provide a rare case in which high magnetic susceptibility, low Faraday rotation, Tm3+-related infrared photoluminescence and the ability to produce optical fibers are combined. From emission intensity and decay curves of the 3H4→3F4 and 3F4→3H6 transitions, we find cross-relaxation already for 0.5 mol% of Tm2O3 doping, indicating notable Tm2O3 clustering. This facilitates antiferromagnetic interaction and results in high magnetic susceptibility. Substitution of Al2O3 by Tm2O3 induces a more asymmetric local structural environment around Tm3+ species and enhances the diamagnetic contribution to Faraday rotation as opposed to the other rare-earth ions.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Aluminosilicate glasses are relatively inexpensive and comparably stable in terms of thermal behavior and susceptibility to chemical corrosion; they exhibit excellent mechanical properties and a low coefficient of thermal expansion. For these reasons, they are used in many industrial and technological fields [1–4]. Starting from the binary of SiO2-Al2O3, the addition of alkali and alkaline earth oxides widens the glass-forming region [5,6], whereby the structural role of alumina remains a complex subject [7–9]. In addition to alkali and alkaline earth compounds, aluminosilicate melts can dissolve large amounts various other oxide species, including all rare earth oxides. Together with their ability for fiber manufacture, compatibility with pure silica claddings, and a capacity for splicing into silica-based fiber networks [10], this has made them an interesting class of glasses also for optical and photonic applications. Specifically, thulium oxide (Tm2O3) doped bulk or fiber glasses have been widely investigated in this context [11–13], for example, for application in Doppler radar wind sensing [14], advanced manufacture [15] or free-space communications [16] at a wavelength of ∼ 2 µm. However, the development of Tm3+-based devices has been complicated by a variety of factors, including fluorescence quenching, dopant clustering and the identification of optimal dopant concentrations in a given glass matrix. Cross-relaxation of Tm3+ species is a representative phenomenon which can be used as a probe to extract information on activator distribution. Besides helping to understand the structure and properties of the Thulium-doped glasses at hand, such investigation also allows conclusions on the general behavior of other rare-earth ion species and applications, for example, cooperative upconversion of Yb3+ and cross-relaxation of Nd3+ [13,17].
The addition of paramagnetic species such as Tm2O3 also affects the magnetic susceptibility, magnetic exchange behavior, and magneto-optical properties of a glass. In particular, the magneto-optical properties, most prominently, Faraday rotation, have been investigated in glasses with high atom concentration of paramagnetic Tb3+. For the notoriously low extents of achievable Faraday rotation, on the other side, studies on the magnetic properties of Tm2O3-containing glasses remain comparably scarce [18].
Here, we report on the magnetic, magneto-optical and spectroscopic properties of percalcic glasses from the system of CaO-Al2O3-SiO2 with increasing Tm3+-for-Al3+ substitution up to a Tm2O3 concentration of 7 mol%
2. Materials and methods
2.1 Glass preparation
For the present study, we used glass samples from the same batches as already reported in Ref. [19]. They cover the percalcic compositional range of 25CaO-(15-x)Al2O3-60SiO2-xTm2O3 (with x = 0-7; labeled as CASTn, where n is two times the mol% of Tm2O3) and were produced by conventional melt-quenching. Homogeneously mixed batches of 100 g were melted in a Pt/Rh10 crucible at 1650 °C for 2 h and subsequently poured onto an iron plate that had been preheated to 800 °C. To relieve thermal stresses, the obtained glass slabs were annealed near their Tg of ∼810 °C in a precision temperature-controlled annealing furnace. Samples for spectroscopic measurements were cut and polished to 10 mm × 10 mm × 2 mm.
2.2 Characterization
Some basic physical data (density, mechanical properties and vibrational spectra) of the glasses were already reported in Ref. [19]. In addition to these data, the glass transition temperature Tg was determined by DSC (Netzsch STA 449 F1 Jupiter). For this, a disk-shaped sample of about 30 mg was mounted in Pt/Rh10 crucibles and heated up to 1000 °C at a rate of 20 °C/mm in N2 atmosphere. Tg values were extracted from the onset of the glass transition. The uncertainty of the measurement was estimated to be ± 2°C based on the specifications given by the manufacturer of the instrument. UV-Vis-NIR absorption spectra were recorded on a Cary 5000 (Agilent) double-beam spectrophotometer with a spectral resolution of 1.0 nm. Static luminescence spectra and lifetime were recorded on a FL920S spectrofluorometer (Edinburgh instruments Ltd., UK), using an 808 nm laser diode as the excitation source and InGaAs for detection. The refractive index was measured by the V-prism method. All these measurements were performed at room temperature. The magnetic properties of the samples were analyzed on powder specimens that were fixed on a non-magnetic polymer sample holder. Magnetic susceptibility under zero-field cooling and field cooling conditions was measured from 2 to 300 K in a DC magnetic field of 100 Oe by a superconducting quantum interference device magnetometer (MPMS-XL, Quantum Design). The Faraday rotation was analyzed at room temperature (25°C) over the spectral range of 350 to 850 nm using a magneto-optical analyzer (K-250, JASCO) with 15 kOe. The change in the polarization angle was detected with a precision of 0.005°.
3. Results and discussion
3.1 General
The obtained glass samples were transparent and did not exhibit any visible inclusions, bubbles or batch residuals. The absence of a crystalline phase was further confirmed by X-ray diffraction [19]. However, some striae could be observed by the naked eye even after annealing. The concentration of Tm2O3 affected glass density and the thermal characteristics (Table 1): the glass density increased gradually with Tm2O3 substitution from 2.65 to 3.45 g/cm3. Tg tended to decrease slightly (from 821 to 817°C) as Tm2O3 was increased. Furthermore, replacing alumina Al2O3 (Λ = 0.60) [20] by Tm2O3 (Λ = 0.944) [20] increased the optical basicity of the glass.
Table 1. Physical properties of CAST glasses. Density data ρ are from Ref. [19].
3.2 Optical properties
The recorded absorption spectra of CAST glasses at room temperature (Fig. 1a) exhibit bands in the VIS-NIR range (346-372, 445-494, 628-728, 728-839, 1063-1301 and 1457-2000 nm) that are attributed to the excited higher levels 1D2, 1G4, 3F2,3, 3H4, 3H5, and 3F4 from the ground state, 3H6. Because of the intrinsic band gap absorption in the host glass, energy states higher than 1D2 are not observed. The absorption coefficient shows a linear dependence on Tm3+ concentration (Fig.1b). The shape and peak positions of each transition for CAST glass samples are similar to those in other Tm3+-doped glasses [21].
Fig. 1. (a) Absorption spectra of CAST glasses, (b) Concentration dependence of the 3H6-3F4 absorption coefficient and (c) schematic energy level diagram of Tm3+. Under 808 nm excitation (red arrow), two main emissions (1475 nm and 1880nm emissions) and cross relaxation (bold solid arrow) occur. (d) Judd-Ofelt intensity parameter $\varOmega$2 obtained from absorption data (Table 2) and absorption intensity ratio of the hypersensitive transition 3H6→3F4 to the magnetic dipole transition 3H6→3H5. The datapoint for the sample containing 1 mol% of Tm2O3 is taken as an outlier (shown in parentheses).
Table 2. Judd–Ofelt intensity parameters Ωt (×10−20 cm2, t = 2, 4, 6), absorption cross-section σabs (×10−21 cm2), emission cross-section σem (×10−21 cm2), radiative lifetime (τrad ms), measured lifetime (τmeas ms), and product of σemτrad (×10−20 cm2ms) for 3F4→3H6 of Tm3+ in various glasses.
The parity-forbidden f-f transitions of rare earth ions can be observed due to selection rules such as the electron vibration coupling between the ligands and the center species [22]. Furthermore, asymmetry of the coordination environment can affect the spectroscopic properties. Calculation of Judd-Ofelt intensity parameters based on absorption spectra is typically employed to interpret the local environment of the dopant [23]. Here, Judd–Ofelt parameters $\varOmega$t (t = 2,4,6) (Table 2) were obtained using a least-square method to correlate the oscillator strengths. While changes are not big, their values are gradually increasing with increasing Tm2O3 (Fig. 1(d)). The evolution of Judd–Ofelt intensity parameters is a result of variations in the local environment of Tm3+ in the glasses: Judd-Ofelt intensity parameters provide information of the influence of the host on the radiative transition probabilities [23]. The $\varOmega$2 intensity parameter is a semi-empirical metric for the asymmetry of the local crystal field, and is often related to the specific characteristics of the 4f and 5d states. With increasing Tm2O3 concentration, the larger value of $\varOmega$2 indicates that the Tm3+ ions occupy increasingly asymmetric or distorted sites involving highly covalent chemical bonds [24] (Table 2, Fig. 1(d)). In the percalcic range of the host glass, the excess of CaO leads interruption of the network of corner-sharing AlO4 and SiO4 structural units through the formation of non-bridging oxygen species (NBO). Substitution of Tm2O3 for Al2O3 leads to further network depolymerization [25]. A similar trend is visible in the hypersensitive transition of 3H6→3F4 (Fig. 1(d)): its intensity and shape reflect the local environment surrounding Tm3+ ions [26,27]. Normalizing its absorption intensity over that of the magnetic dipole transition at 1210 nm (3H6→3H5) reveals an increasing value with increasing Tm2O3 concentration, thus, increasing site asymmetry.
The fluorescence spectra (Fig. 2) of CAST glass samples with different Tm2O3 concentrations exhibit two main peaks at 1470 nm (3H4→3F4) and 1881 nm (3F4→3H6), and a weaker band at 1225 nm (3H5→3H6). For the 3H4→3F4 transition (1470 nm), the emission intensity of the 1470 nm band is lower than that of the 3F4→3H6 transition at 1881 nm because of the high non-radiative decay rate caused by high phonon energy (∼ 1005 cm−1, maximum Raman peak). A high doping concentration of thulium enhances the 3F4→3H6 transition (1881 nm) while suppressing the 3H4→3F4 transition (1470 nm) due to an increase in the magnitude of cross-relaxation (3H4, 3H6 - 3F4, 3F4) as the average distance between Tm3+ ions decreases [30]. The lifetime of the 3H4 state decreases as the doping concentration increases. The corresponding emission decay follows a non-exponential function with a fast decay component. The intensity ratio of (3H4→3F4)/(3F4→3H6) evolves correspondingly (Fig. 2b inset). The degree of cross-relaxation is very sensitive to the distribution of Tm3+ ions, for example, when clustering occurs [31]. The statistical inter-ionic distance is ∼ 16 Å in 0.5 mol% Tm2O3-doped glass (CAST1, Table 1, Fig. 2c inset); only for the highest dopant concentration, it approaches the expected critical value (e.g., ∼ 7.9 Å in YVO4 [32]) at which the energy transfer probability between donor and acceptor ions becomes equal to the intrinsic decay rate of donor ions [30]. Noteworthy, the statistical distance does not necessarily represent the physical distance between neighboring activator ions and, thus, the probability of cluster formation [33,34]. Altogether, the observed spectral features suggest that Tm3+ is clustering, at least to some extent [17]. This is probably primarily due to the limited solubility of Tm2O3 in the given glass matrix [35].
Fig. 2. (a) Photoluminescence spectra of CAST glasses and (b-c) normalized decay curves of Tm3+ for (b) 3H4, (c) 3F4 states. In (b), dashed and solid lines denote a fast and a slow decay component, respectively. The emission intensity ratio of (3H4→3F4)/(3F4→3H6) is given in the inset of (b); the statistical inter-ionic distance as a function of the volume concentration of Tm3+ is provided in the inset of (c).
For the 3F4→3H6 transition (1881 nm), the fluorescence intensity reaches a maximum at ∼ 1 mol% of Tm2O3 (CAST2). This is attributed to an increase in the cross-relaxation probability which causes an increase in the population of 3F4 while decreasing that of 3H4. Further increase in the concentration of Tm3+ causes decreasing emission intensity as a result of concentration quenching. However, the observed emission lifetimes decrease continuously from 477 µs to 192 µs (x = 4). Here, the increase in the concentration of Tm3+ causes enhanced probability of cross-relaxation and energy migration; the emission from 3F4→3H6 shows a large increase in intensity, but the one from 3H4 is strongly reduced. Thus, the decay process first intrinsic transitions, followed by cross-relaxation when two Tm3+ ions are close to each other. The shape of the decay curves indicates 3H4→3F4 energy relaxation [36]. For CAST1, the radiative decay time of the Tm3+ emission was calculated from the Judd-Ofelt analysis at 7.2 ms, which is longer than the measured value (477 µs); therefore, some other factors may contribute to the transfer of energy, in particular, the presence of OH groups. However, the lifetime of 3F4→3H6 is still longer than what has been observed in other glasses, e.g., silica (420 µs) [21].
3.3 Magnetic properties
In all glasses containing Tm3+, paramagnetic behavior is observed from 2K to 300K; the pristine glass exhibits diamagnetic behavior (Fig. 3a). To investigate the magnetic interactions between the Tm3+ ions, the inverse susceptibility is considered using the Curie-Weiss law,
where ${\chi _0}$ is the temperature-independent contribution (7.0 × 10−6 emu/Oe·g), CM is the molar Curie constant, T [K] is the temperature, θw [K] is the Weiss temperature. The inverse susceptibility increases linearly as the temperature increase from 100K to 300K (Fig. 3b), but a weak non-linearity occurred below 100 K even in 0.5 mol% Tm2O3-doped glass (CAST1). θw decreased as the Tm2O3 concentration was increased (Fig. 3c). From the slope of the inverse magnetic susceptibility versus temperature, the effective magnetic moment per ion µeff was obtained after a correction for the diamagnetism of the glass matrix (Fig. 3c). The experimental values are slightly smaller than those which are expected for free Tm3+ ions (7.57 µB) [37]. These smaller moments indicate the presence of aggregate Tm3+ species. The negative θw can be tentatively ascribed to antiferromagnetic superexchange by covalent coupling with mediating ligands in Tm3+-O-Tm3+ bonds [38,39]. These observations correspond well to decay data (Fig. 2b–2c).Fig. 3. Magnetic susceptibility (a) and reciprocal magnetic susceptibility (b) as functions of temperature for various Tm2O3 contents. Values of the theoretical magnetization were obtained from the ideal effective magnetic moment of the free ion. (c) Weiss temperature and effective magnetic moments as a functions of Tm2O3 concentration in CAST glasses.
3.4 Magneto-optical properties
The wavelength-dependent Verdet constants, V, estimated from the Faraday rotation angle θ, external magnetic field H and sample thickness l, V = θ/(Hl) of the 7 mol% Tm2O3 doped glass (CAST14) was calculated as 19.82 rad/(T·m) at 400 nm and 11.12 rad/(T·m) at 500 nm, respectively. These are relatively low values [40]. The magneto-optical effect in the visible region is mainly due to the intra-ionic parity allowed electric dipole transition between 4f-5d configurations. The magnitude of V tends to increase as the wavelength decreases. Anomalies of Faraday rotation are observed at wavelengths near the f-f transitions of Tm3+ (1D2→ 3H6 at 358 nm (out of range in Fig. 4a), 1G4 → 3H6 at 471 nm, 1G4 → 3F4 at 682 nm, 3H4 → 3F4 at 789 nm); 1G4-3F4 at 682 nm has a symmetric shape whereas the other peaks are asymmetric [41,42]. Similar phenomena can be observed on Pr3+ ions in borate glasses and Dy3+ ions in aluminosilicate glasses [41]. The sign (or direction) of the angle of rotation is conventionally described as diamagnetic if θ is positive and paramagnetic if θ is negative [43]. Then, V can be expressed as V = Vm+Vr+VTm, where Vm is the contribution from the diamagnetic properties of the glass matrix, Vr is the diamagnetic contribution induced by replacement of Al2O3 by Tm2O3 and VTm is the contribution by the paramagnetism of the Tm3+ ions. In the present case, the measured V is always positive with increasing concentration of Tm2O3; this is in contrast to other rare-earth ions such as Ce, Pr, Tb, Dy and Ho (Fig. 3). It reveals that the paramagnetism of Tm3+ is compensated by the diamagnetic effects of substitution. Positive V has also been observed in Sm, Gd, Yb and Y aluminates and gallates [44–47]. The case of Yb3+ in phosphate glass is particularly unique because of enhanced positive paramagnetic rotation with temperature in contrast to the mix of a small paramagnetic and a larger diamagnetic term [43,48]: in such distinct cases, paramagnetic ions have been reported with exhibit opposite direction of rotation despite their paramagnetic susceptibility [47–49]. In the present case, however, the paramagnetic contribution of Tm3+ is too low for unambiguous quantification in these terms [49]. The replacement of Al2O3 by Tm2O3 increases the diamagnetic Faraday effect related to Vr presumably because Al3+ (0.53 Å) with a smaller ionic radius is replaced by Tm3+ (1.020 Å) with a larger ionic radius. A larger atom or ion can contribute more efficiently to the diamagnetism of a material. For comparison, the Tm3+ ion has an almost similar or a little higher positive Verdet constant [45,46] at compared to that of La3+ (1.172 Å) with lager ionic radius and no unpaired electron [50]. Evidently, the replacement effect can no longer be ignored: theparamagnetic rotation of Tm3+ is small in relation to the diamagnetic contributions of the matrix and of the replacement effect. Therefore, we now focus on the total (diamagnetic) Verdet constant instead of on the paramagnetic contribution of the Tm3+ ion. The diamagnetic Verdet constant is described by the classical Becquerel theory, V = [γeλ/(2mc2)](dn/dλ), in which γ is a correction factor, e and m are the charge and mass of the free electron, c is the speed of light, and dn/dλ is the dispersion of refractive index. Using a single-term Sellmeir equation, V can be approximated through [51]
where a and b are fitting parameters, and λ0 is a mean resonance wavelength obtained from the dispersion curve of the refractive index. For λ >> λ0, V ∼ 1/λ2 [51,52]. In the present case, the mean resonance wavelength was ∼ 146 nm, obtained from the dispersion with a V-block refractometer. In Fig. 4b, an example of fitting Eq. 2 is shown for CAST8 with a = 505.43 × 10−9 [T−1] and b = 0.256 × 10−20 [m2·T−1] using 12 different wavelengths excluding the f-f transition. All experimental datasets were similarly well reproduced by Eq. 2 in this way.Fig. 4. (a) Variation of Verdet constant with wavelength, (b) data fit for CAST8 according to Eq. 2, and (c) wavelength dependences of the magneto-optical figure of merit for CAST14.
As a practical figure of merit FoM, the ratio between V and the absorption coefficient V/a [rad·T−1] is typically used to evaluate Faraday rotation performance. An increase in Tm2O3 induces an increase in the baseline of absorption. Furthermore, FoM of the CAST glasses is strongly limited by the low V value, and by the f-f transitions at 358 (1D2-1G4), 484 (3H6-1G4), 682 (3H6-3F2,3), and 798 (3H6-3H4) nm (Fig. 4c). Despite their high magnetic sensitivity, CAST glasses exhibit low magneto-optical activity. The highest FoM of ∼ 0.645 rad·T−1 is found for 7 mol% of Tm2O3-doping (CAST14) at ∼ 401 nm (potentially compatible with a UV diode)..
4. Conclusion
We investigated optical and magnetic properties of Tm2O3-doped percalcic aluminosilicate glasses. Substituting Al2O3 by Tm2O3 increased the local structural asymmetry around the Tm3+ centers, reflected in the second Juddy-Ofelt intensity parameter and in the intensity of the hypersensitive transitions. Cross-relaxation between neighbor ions and anti-ferromagnetic interaction were observed already at a dopant concentration as low as 0.5 mol%, for which the statistical Tm-Tm distance is ∼ 16 Å. On the basis of dynamic spectroscopic data, this was related to the formation of clusters or aggregates, also consistent with the observed anti-ferromagnetism: negative Weiss temperature and an effective magnetic moment below that of the free ion indicate anti-ferromagnetic superexchange across Tm3+-O-Tm3+. All glasses had a low, positive Faraday rotation, assumedly dominated by a diamagnetic replacement effect when Tm3+ substitutes for Al3+. The material presents a rare example of potential optical fiber capability combined with high magnetic susceptibility and low Faraday rotation.
Funding
H2020 European Research Council (681652); CAS “Light of West China” Program (2017447).
Disclosures
The authors declare that there are no conflicts of interest related to this article.
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