Large Faraday effect and structural properties of heavily Tb 3+ -doped borogermanate glasses: a potential precursor of magneto-optical fibers

New glass compositions containing high concentrations of Tb 3+ ions were developed aiming at the production of magneto-optical (MO) fibers. This work reports on the structural and MO properties of a new glass composition based on (100- x )(41GeO 2 25B 2 O 3 -4Al 2 O 3 -10Na 2 O-20BaO) – x Tb 4 O 7 . Morphological analysis (HR-TEM) of the sample with the highest concentration of Tb 3+ ions confirmed the homogeneous distribution of Tb 3+ ions and the absence of nanoclusters. All the samples presented excellent thermal stability against crystallization (ΔT >100 o C). An optical fiber was manufactured by a fiber drawing process. The UV-Vis spectra of the glasses showed Tb 3+ electronic transitions and optical windows varying from 0.4 to 1.6 μm. The magneto-optical properties and the paramagnetic behaviors of the glasses were investigated using Faraday rotation experiments. The Verdet constant (V B ) values were calculated at 500, 650, 880, 1050, 1330, and 1550 nm. The maximum V B values obtained at 650 and 1550 nm for the glass with x = 18 mol% were -128 and -17.6 rad T -1 m -1 , respectively. The V B values at 500 and 1550 nm for the optical fiber containing 8 mol% of Tb 4 O 7 were -110.2 and -9.5 rad T -1 m -1 , respectively, while the optical loss at around 880 nm was 6.4 dB m -1 . reports the synthesis and characterization of a new set of magneto-optical based on Tb 3+ -containing borogermanate glass compositions. The thermal, structural, morphological, spectroscopic, and optical properties were investigated using differential scanning calorimetry (DSC), X-ray powder diffraction (XRD), high-resolution transmission electron microscopy (HRTEM), and Raman, UV-Vis-NIR, luminescence, and M-Lines spectroscopy methods. The magneto-optical properties were evaluated by Faraday rotation, with (V B ) values measured at different in the Vis-NIR range. a


Introduction
Magneto-optical (MO) materials based on the Faraday effect have been increasingly studied for use in new technologies 1,2 . MO materials have been applied as modulators, as optical isolators, and as magneto-optical fiber sensors [3][4][5] . A wide variety of transparent MO glasses, crystals, and transparent glass-ceramics containing rare earth (RE) ions such as Tb 3+ , Dy 3+ , Pr 3+ , and Gd 3+ have been investigated and are considered promising materials for photonics and spintronics [6][7][8] .
The Faraday effect is defined by the rotation angle (θ) of a linearly polarized light beam, when the light travels through an optical path of known length (l), under the application of a longitudinal magnetic field (B) along the light propagation direction 9 . The MO performance of a material is evaluated and quantified by the magnitude of the Verdet constant (VB) value, and it may be maximized by the incorporation of paramagnetic species 10 .
Single crystals have larger VB values than magneto-optical glasses with similar composition. However, MO materials produced from glasses are more attractive than crystals, due to their lower cost, isotropy, and simple preparation procedures, in addition to great flexibility in obtaining materials with different shapes and lengths, such as fibers for applications in integrated devices 11 . Among the MO materials, especially attractive are paramagnetic glasses containing high concentrations of Tb 3+ ions and with large VB values in UV-Vis-NIR regions 12,13 .
The development of MO glass compositions able to support high RE ions contents and with high thermal stability represents an important step towards achieving successful fiber drawing processes without crystallization. Studies have reported the production of MO fibers based on Pr 3+ -doped aluminosilicate 14 , Eu-doped silica glass 15 , Gd2O3 NPs-doped aluminosilicate glass 16 , and Tb 3+ -doped silicate glasses 17 .
The main prerequisites in selection of a good candidate glass material for MO applications are large VB values and transparency in the visible and near-infrared (NIR) regions. In particular, for the NIR region (at 1550 nm), MO materials are good options for applications in telecommunication systems. Among all the paramagnetic ions, Tb 3+ is one of the most attractive 1,13,18 . It has the electronic configuration 4f 8 → 4f 7 5d 8,9 , providing good paramagnetic behavior and among the highest magnetic moments (μeff = 9.5-9.72) and susceptibilities (J = 6, g = 1.46) of all the rare earth ions 1 . Additionally, terbium gallium garnet single crystals (Tb3Ga5O12), known as TGG, are available commercially and are considered one of the most important bulk MO materials, with VB of -134 rad T -1 m -1 at 632 nm 19 .
Currently, heavy metal oxide (HMO) glasses based on borogermanates and borate glasses containing high Tb 3+ ions contents stand out, due to their large VB values in the red region of the visible spectrum 13 . Gao et al. 1 reported the MO properties of heavily Tb 3+ -doped materials with large Verdet constant values for use in fiber-integrated magneto-optics. In this case, the highest VB (at 632.8 nm) was -119 rad T -1 m -1 for glass containing 25 mol% of Tb2O3.
This work reports the synthesis and characterization of a new set of magneto-optical glasses based on Tb 3+ -containing borogermanate glass compositions. The thermal, structural, morphological, spectroscopic, and optical properties were investigated using differential scanning calorimetry (DSC), X-ray powder diffraction (XRD), high-resolution transmission electron microscopy (HRTEM), and Raman, UV-Vis-NIR, luminescence, and M-Lines spectroscopy methods. The magneto-optical properties were evaluated by Faraday rotation, with Verdet constant (VB) values measured at different wavelengths in the Vis-NIR range. In addition, a magneto-optical fiber was produced and characterized.

Results and discussion
Thermal, structural, and morphological analysis. Figure 1 shows the color evolution of the BGB-xTb glasses, as a function of the Tb 3+ content. The same color change has been reported for calcium aluminosilicate and borogermanate glasses containing high Tb 3+ contents, with the effects being attributed to the Tb 3+ -Tb 4+ redox process and the conditions of melting 1,22 .  were calculated for all the BGB-xTb samples. Table 1 summarizes the values of Tg, Tx, Tp, and ΔT, together with the density (g cm -3 ) and Tb 3+ ions density (ions cm -3 ) values. The Tb 3+ ions density was calculated using Equation 1: where, 3+ is the density of Tb 3+ ions, x is the mole fraction of Tb4O7, NA is the Avogadro constant, and M is the average molecular weight of the BGB-xTb composition.  As can be seen in Figure 2b, the Tg values increased as a function of the Tb 3+ content, ranging from 545 °C (BGB-4Tb) to 720 °C (BGB-16Tb), followed by a decrease for the most concentrated sample, suggesting the occurrence of structural changes. It has been shown that in borogermanate glasses, RE ions may act as glass modifiers by breaking Ge-O-Ge bonds and inducing the formation of boroxol rings 8,25 . However, for the highest RE content, the structural connectivity was lost, resulting in a decrease of Tg. The BGB-xTb glasses presented high ΔT values, with a maximum of 305 o C for the BGB-8Tb sample, so this sample was the one selected for fiber production. From Table 1, it can be seen that increasing the content of Tb 3+ led to densification of the matrix, as shown by the higher density values. It should be noted that higher density values are reflected in a higher refractive index, resulting in higher Verdet constants, as will be discussed below.   Fourier transform (FFT) (inset of Figure 4A) confirmed the absence of crystalline spots and the existence of long-range structural order, corroborating the XRD measurements. Figure 4(B) shows a high-angle annular dark-field (HAADF) image of the homogeneous structure of the BGB-18Tb glass (analyzed area) and the corresponding elemental EDS mapping (Ba-K, Tb-L, and O-K). Figure S1 shows the EDS spectrum of the BGB-18Tb glass.

Raman spectroscopy.
Raman spectra of all the BGB-xTb glasses are shown in Figure   S2(a). The Raman spectrum of the undoped glass (BGB-0Tb) is also plotted to facilitate discussion of the role of Tb 3+ ions in the glassy network. Raman spectra of the glass formers GeO2 (α-quartz-like) and B2O3 (vitreous) are shown in Figure S2(b).
The Raman spectra showed broad bands typical of glassy structures, assigned to a large distribution of bonds and angles, as well as several overlapping vibrational modes of the glass components. For these reasons, it was necessary to identify the contributions of the individual vibrational modes by deconvolution, involving the fitting of Gaussian peaks in different frequency regions of the spectra. Such Gaussian deconvolution has been described in previous spectroscopic studies of germanate, borate, and borogermanate glasses 8,20,25,[30][31][32][33][34][35][36] .
As shown in Figure 5  As shown in Figure 6(a), the UV cutoff for the undoped glass was at around 300 nm.
The red shift to around 600 nm (BGB-18Tb), observed after addition of Tb4O7, was the result of the intense absorption of the Tb 3+ transitions. However, the main origin of the red shift could be ascribed to the oxidation of Tb 3+ to Tb 4+ , characterized by the change of color from colorless, passing through pale yellow, and finally to dark brown, as the Tb 3+ content increased (see        In studies of the MO properties of RE-doped glasses, the VB values are generally expressed as a function of the RE ion density ( ). In this work, the Tb 3+ ion densities ( 3+ ) for all glasses were calculated using Equation (1) and are shown in Table 1.
The main wavelengths for applications of MO glasses are in the infrared range, between 1.05 and 1.33 µm 54 . Figure 8(d) shows the VB values obtained for all the glasses at 1.03, 1.33, and 1.55 μm. It should be highlighted that the maximum VB value at 1550 nm (in the telecommunications range) was -17.6 rad T -1 m -1 , which was 37-fold higher than the VB of silica glass (~0.471 rad T -1 m -1 ) 55 .
For practical purposes, the absorption of the glass in the spectral region employed should be minimized. As observed in this work, the optical window diminishes as a function of the terbium content, mainly due to the oxidation of Tb 3+ to Tb 4+ , which occurs at high temperature. However, this problem can be mitigated by the addition of reducing agents such as Ce2O3, as shown in Figure S4, which allow broadening of the optical window in the visible range. As can be seen, the addition of 0.5 mol% Ce2O3 was sufficient to maintain the reduced conditions necessary to avoid oxidation of Tb 3+ to Tb 4+ , without significantly affecting other thermal and structural properties. With this approach, it was possible to shift the absorption band from 0.75 to 0.55 m and obtain a glass that was light yellow in color, rather than dark brown, as shown in the inset in Figure S4.
Fabrication of magneto-optical glass fiber. Figure 9(a-b) shows photographs of the polished glass preform and the optical fiber obtained by applying the drawing process to the BGB-8Tb glass. Among all the BGB glasses analyzed, the BGB-8Tb glass presented the highest ΔT, so for this reason it was selected for production of the magneto-optical fiber. The BGB-8Tb fiber was coated with poly(methyl methacrylate) (PMMA) and the length of the fiber obtained was around 50 m (Figure 9(b)).  This value was the same as obtained for the corresponding bulk sample. Figure 9(d-e) shows photographs of the preform after the drawing process and the surface of the optical fiber, respectively. In neither case was there any evidence of crystallization on the surface after the drawing process.
Magneto-optical and optical fiber characterizations. Figure 10   Among the optical fibers reported in the literature, pure silica fibers are known to provide high performance, due to low attenuation in the NIR region 56 . On the other hand, silica fibers have very low VB values in the NIR region 11 . For example, VB of ~2.05 rad T -1 m -1 was found for an SMF at 830 nm 57 . In this work, the BGB-8Tb fiber presented VB of -32.2 rad T -1 m -1 at 880 nm, which was around 15-fold higher than obtained for the SMF 57 . In magnetooptical terms, the BGB-8Tb fiber has good potential for application in the so-called "first optical window" at around 820-900 nm, given its high VB value at 880 nm 58 .

Conclusions
This work reports the synthesis of transparent Tb 3+ -borogermanate MO glasses using the traditional melt-quenching method. The thermal, structural, morphological, spectroscopic, optical, and magnetic-optical properties of the glasses were investigated.
Structural changes in the glass network, following the addition of Tb4O7, were confirmed by  Table 1.
In the first step, vitreous boron oxide was obtained by thermal decomposition of H3BO3 at 500 °C for 30 min in a resistive furnace. The glass components were ground to fine powder and homogenized in an agate mortar. Batches were loaded into a platinum crucible and melted at between 1350 and 1500 °C (depending on the Tb4O7 content), for 2 h, under atmospheric conditions. The melt was cooled in a preheated stainless steel mold at 30 °C, below the glass transition temperature (Tg), and then annealed at the same temperature for 3 h, to minimize its mechanical stress, followed by slowly cooling to room temperature during 12 h. Pieces with thickness of 3 mm were obtained. As a final step, the samples were polished using silicon carbide (SiC) polishing papers, prior to the optical characterizations.
The glass preform based on the composition 92(41GeO2-25B2O3-4Al2O3-10Na2O- In the final step, the preform was mounted into the drawing tower and the fiber drawing process was started at 720 o C (Tg + 115 o C). During the drawing process, the BGB-8Tb fiber was coated with a low-index UV-cured poly(methyl methacrylate) (PMMA) polymer, in order to protect the magneto-optical fiber and improve its mechanical properties. Optical absorption and transmission spectra of the BGB-xTb glasses were obtained using a Varian Cary 500 dual-beam UV-Vis-NIR spectrophotometer, in the ranges from 200

Measurements and characterizations.
to 800 nm and from 0.25 to 3 μm, respectively. Linear refractive indexes for the BGB samples Similarly, Faraday rotation angle measurements for the BGB-8Tb fiber were performed at 500, 650, 880, 1050, 1330, and 1550 nm. The input and output fibers were cleaved using a 24X0-RCL cleaving machine and the optical path length (l) was 2 cm. The fiber was inserted in a holder and a 40x objective lens was used to focus the laser onto the fiber section. The Faraday rotation angles were measured in triplicate, using a polarizer with precision of ±2 o .
Optical fiber characterization. The cut-back fiber loss method was used to measure the optical attenuation of the BGB-8Tb fiber in the range from 350 to 1750 nm. The attenuation spectra were acquired using an optical spectrum analyzer (OSA) (Model AQ-6315A, Yokogawa) with wavelength resolution of 5 nm. To obtain flat surfaces, the input and output of the BGB-8Tb fiber were cleaved using a 24X0-RCL cleaving machine, after which the fiber was clamped into two SubMiniature version A (SMA) adaptors. For broadband measurement, the input fiber was clamped at the tungsten-halogen lamp housing and the output was connected to an OSA instrument. The cutback measurements were performed from the initial fiber length of 1.92 m to a final length of 21.8 cm. The output fiber was cut into different lengths using the cleaving machine and the output power was measured for each length, in order to obtain more accurate transmission losses data.

Data availability
All data regarding the work presented here are available upon reasonable request to the corresponding authors.