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Optical characterization of Tm3+ doped Bi2O3-GeO2-Ga2O3 glasses in absence and presence of BaF2

In this paper, Two new Bi2O3-GeO2-Ga2O3 glasses (one presence of BaF2) doped with 1mol% Tm2O3 were prepared by melt-quenching technique. Differential thermal analysis (DTA), the absorption, Raman, IR spectra and fluorescence spectra were measured. The Judd–Ofelt intensity parameters, emission cross section, absorption cross section, and gain coefficient of Tm3+ ions were comparatively investigated. After the BaF2 introduced, the glass showed a better thermal stability, lower phonon energy and weaker OH− absorption coefficient, meanwhile, a larger ~1.8 μm emission cross section σem (7.56 × 10−21 cm2) and a longer fluorescence lifetime τmea (2.25 ms) corresponding to the Tm3+: 4F3 → 3H6 transition were obtained, which is due to the addition of fluoride in glass could reduce the quenching rate of hydroxyls and raise the cross-relaxation (3H6 + 3H4 → 3F4 + 3F4) rate. Our results suggest that the Tm3+ doped Bi2O3-GeO2-Ga2O3 glass with BaF2 might be potential to the application in efficient ~1.8 μm lasers system.

The OH − groups may quench 3 F 4 → 3 H 6 emissions of Tm 3+ ions and reduce emission efficiency 5 . But hydroxyl and the fluorine ions are isoelectronic and their ionic size was similar; hydroxyl ions could easily be removed by fluoride during melting 27 . Therefore, 1 mol% Tm 3+ -doped bismuth-germanium-gallate glasses in absence and presence of BaF 2 were studied for ~1.8 μ m emission.
Differential thermal analysis (DTA) was performed using a SETARAM TAG24 analyser, for characteristic temperatures (the temperature of glass transition T g , temperature of onset crystallization T x and temperature of peak crystallization T p ). Density and refractive index of samples was obtained by Archimedes method and spectroscopic ellipsometer method, respectively. The absorption spectrum was recorded using a spectrophotometer (Perkin Elmer Lambda9). The near-infrared emission spectra and luminescence lifetime were measured by FLSP920 (Edinburgh instruments Ltd., UK) under 808 nm laser diode pumped. Raman spectra were monitored with a FT Raman spectrophotometer (Nicolet Module). All measurements were carried out at room temperature.

Results and Discussions
Thermal property. Figure 1 shows the DTA curve of the studied glass, and the values of T g , T x and T p in Tm 3+ -doped BGN and BGF samples are indicated. The difference between the glass transition temperature T g and the onset crystallization temperature T x , ΔT = T x − T g , has been frequently used as a rough estimate of glass formation ability or glass thermal stability. It can be seen that the values of T g is decreased from 520 °C to 495 °C as the Na 2 O is replaced by BaF 2 in BGF glass. However, it is still higher than of fluoride 28 , tellurite 29 glasses, this results show that the glasses have good thermal shock resistance performance under the condition of high power pump. Generally, the ΔT of the glass sample should be higher than 100 °C to obtain a better thermal stability and to avoid crystallization during the optical fiber drawing process 30,31 . After the addition of BaF 2 , the thermal stability (ΔT) of Bi 2 O 3 -GeO 2 -Ga 2 O 3 glass is increased quite significantly. The value of ΔT for BGF sample is 110 °C,which is higher than of BGN (59 °C), indicating that the BGF sample has better thermal stability against crystallization for ~1.8 μ m emission.
Absorption and IR transmittance spectra. Figure 2 shows the absorption spectra of the Tm 3+ doped BGN and BGF samples under room temperature. All absorption bands belong to transition of Tm 3+ ions from ground state to higher levels are labeled in Fig. 2. As expected, BGN and BGF samples have similar absorption peaks, and the 3 H 6 -1 G 4 transition has not appeared, due to the UV cut-off wavelength of bismuthate glasses is redshift. Strong absorption around 790 nm indicates that these glasses can be excited efficiently by 808 nm LD. As shown in Fig. 3, BGF sample shows better IR transmittance than BGN sample. The absorption band ranging from 2700 to 3700 cm −1 is due to stretching vibrations of free OH − groups. Hydroxyl and the fluorine ions are isoelectronic and their ionic size is similar 28 , hydroxyl ions can easily be removed by fluoride during melting through the reaction OH − + F − → HF + O 2− . The OH − absorption coefficient in the glass can be calculated by the IR transmission spectra, which is given by 31 Judd-Ofelt analysis. According to absorption spectra ( Fig. 1), Judd-Ofelt (J-O) theory has been applied to determine the important spectroscopic and laser parameters of Tm 3+ ion. In this paper, J-O intensity parameters Ω t (t = 2, 4, and 6) are calculated and radiative transitions within 4f n configuration of Tm 3+ is analyzed, the value of them list in Table 1. The value Ω 2 of BGF are lower than those of BGN, however, they are still much larger than that of silicate 31 , tellurite 32 , fluoride 33 and germanate 34 glasses. As known Ω 2 is related with the covalency between rare earth ions and ligands anions and reflects the asymmetry of local environment at Tm 3+ site in the glass hosts.
Large Ω 2 means stronger covalency between the rare-earth ions and ligand anions, while the Ω 6 has a relation with the overlap integrals of 4f and 5d orbits 26 . Large value of Ω 6 exhibits the large value of emission bandwidth and spontaneous radiative probability of rare earth 31 . Values of Ω 4 and Ω 6 also provide some information on the rigidity and viscosity of hosts.  As shown in Table 2, spontaneous emission probability (A) for Tm 3+ can also be calculated by using J-O theory, which is related with the J-O parameters and the refractive-index of host glass. Total spontaneous emission probability (∑ A) of Tm 3+ : 3 F 4 level in BGN glass (454.8 s −1 ) is higher than that in BGF glass (406.38 s −1 ), so is the A rad of transition Tm 3+ : 3    Emission properties. Figure 4 shows the ~1.47 μ m and ~1.8 μ m emission spectra in BGN and BGF samples under 808 LD pumped. After the BaF 2 introduced, peak intensity of the ~1.8 μ m emission in BGF is 2 times higher than that in BGN, while the intensity of ~1.47 emission is only a little change between two samples. As shown in the insert Fig. 4, the large intensity ratio of ~1800 nm to ~1470 nm (I 1800 /I 1470 ) is related to the cross-relaxation (CR, 3 H 6 + 3 H 4 → 3 F 4 + 3 F 4 ) 36 .
With the introduction of BaF 2 , the maximum phonon energy of glass hosts lower accordingly, which can be seen from the measured Raman spectra shown in Fig. 5, the maximum phonon energy of BGN and BGF samples can be presumed about 746 cm −1 and 730 cm −1 , respectively. The Raman scattering band higher than 700 cm −1 is mainly caused by the vibration of the tetrahedron group, the peak bond located in 756 cm −1 and 846 cm −1 , correspond to the structure unit vibration of Ge-O and Ga-O, respectively 34 . For BGF sample, lower phonon energy is also a key factor for stronger ~1.8 μ m emissions.
∫ σ λ π λ λ λ λ λ = A cn em rad 4 2 where λ is the wavelength, A rad is the spontaneous emission probability calculated by J-O theory, I(λ) is the fluorescence intensity, n is the refractive index of the glass, and c is the light speed. It is noted that σ em mainly related to ~1.8 μ m emission spectrum and radiative transition probability of Tm 3+ : 3 F 4 → 3 H 6 , which is a normalized line-shape function, respectively. According to Eq. (3), the stimulated emission cross-sections (σ em ) of ~1800 nm calculated are shown in Fig. 6. It can be determined that σ em of BGF sample performs a maximum 7.56 × 10 −21 cm 2 at 1865 nm, which is higher than that of BGN sample (7.01 × 10 −21 cm 2 , centered at 1865 nm).  For BGN and BGF samples, the values of the maximum stimulated emission cross-section at the wavelength of 1865 nm, which are larger than that of the fluorophosphate glasses 37 , silicate glasses 2,16,31 and germanate glasses 38 , due to high refractive index, high J-O parameters and good emission, and are beneficial to ~1.8 μ m laser action of Tm 3+ ions.
Cross-relaxation process. Because of the cross-relaxation transfer process ( 3 H 6 + 3 H 4 → 3 F 4 + 3 F 4 ) is beneficial for the ~1800 nm emission 5 . It is necessary to study the cross-relaxation process between Tm 3+ ions. According to the theory of Dexter and Forster, the cross-relaxation rate can be calculated by the integral overlap of absorption cross-sections and emission cross-sections 33 , which belongs to a dipole-dipole interaction. The microscopic transfer probability can be expressed by 34 where R is the distance between donor and acceptor, C D−A is the transfer constant defined as follows 15 , where R c is the critical radius of the interaction and τ D is the intrinsic lifetime of the donor-excited level. The transfer constant can be obtained according to Eq. (4) when phonons participate in the process to balance the energy gap 5 .    where 1/τ cal is the spontaneous radiative probability A rad , W MPR is the nonradiative multiphonon relaxation rate, W OH −1 is the nonradiative transition probability due to the energy transfer to OH − impurities and W ET represents an additional nonradiative loss mechanism due to the energy transfer between the RE ions. In this study, the concentrations of Tm 3+ ions for BGN and BGF are the same, this third process can be neglected.
The multiphonon relaxation W MPR can be expressed 43 : where W 0 is an experimentally determined parameter which is independent of the particular RE ion. ∆E is the energy gap between the 3 F 4 and 3 H 6 levels. g is the electron-phonon coupling strength parameter, and ћω max is the highest phonon energy obtained from Raman spectra and p = ∆E/ћω max . Multiphonon decay depends on the number of phonons required to bridge the energy gap to the next lower lying manifold. The higher the ћω max is, the larger the multiphonon relaxation is. W OH − is proportional to the concentration of Tm 3+ ions and the measured absorption coefficient of OH − groups 42,44 . For BGF sample, after BaF 2 introduced, the α OH − shows a significantly decrease, W OH −1 is expected to decrease which results in a reduced nonradiative transition rate. Thus the lifetime is much longer while the quantum efficiency is higher in BGF. Generally, the relatively longer radiation lifetime is beneficial to reduce the laser oscillation threshold 45 .

Conclusion
In conclusion, we reported on ~1.8 μ m emission in Tm 3+ -doped Bi 2 O 3 -GeO 2 -Ga 2 O 3 glasses in absence and presence of BaF 2 . The addition of BaF 2 not only influences the network of glass, but also effectively reduces the content of hydroxyls and maximum phonon energy. For BGF sample, it shows a better thermal stability, and a stronger ~1.8 μ m emission than that in BGN sample. It is also found that BGF glass possesses relatively large ~1.8 μ m emission cross-section σ em (7.56 × 10 −21 cm 2 ), measured fluorescence lifetime τ mea (2.25 ms) and figure of merit gain σ em × τ rad (14.69 × 10 −21 cm 2 ms) corresponding to the Tm 3+ : 3 F 4 → 3 H 6 transition. Our results suggest that introduced the BaF 2 into the glass network structure, which paves a way to enhance the ~1.8 μ m emission properties and improve the fluorescence lifetime of Tm 3+ : 3 F 4 in Tm 3+ doped bismuthate glass.