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Abstract
Raman spectra of Tm3+/Yb3+ co-doped Nb2O5-based glasses with different Yb3+ concentrations have been studied. The results reveal that Yb3+ ions have no effect on the maximum phonon energy. Moreover, this heavy-metal oxide glass has low phonon energy with a value of ∼ 800 cm−1, which is very helpful to obtain strong infrared luminescence. The absorption coefficient of OH- group is small, which is helpful for infrared luminescence. Nb2O5-based glasses have a high refractive index with the nd value over 2. Yb3+ is not good for the refractive properties on the whole. The abbe number can be calculated by using characteristic values. All the glasses with different Yb3+ concentrations show high a Abbe number with the value over 16. Nb2O5-based glasses perform excellent optical properties that satisfy well the requirements of commercial glasses. The amorphous phase is confirmed by XRD and SEM images. Additionally, EDS results indicate the homogeneous distribution of compositions in the glass. A broad and strong near infrared emission centered at ∼ 1850nm ascribed to 3F4→3H6 is obtained from Tm3+/Yb3+ co-doped Nb2O5-based glasses at 980 nm excitation. The emission intensity first increases and then decreases with Yb3+. When y = 0.03, the emission is the strongest. The luminescence process is the energy transfer from Yb3+ to Tm3+. The lifetime of the excited state for Tm3+/Yb3+ co-doped Nb2O5-based glasses (y = 0.03) is calculated to be 2.292 ms. Such long lifetime is very favorable to get strong luminescence.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Near 2.0 µm wavelength region has attracted much attention owing to wide applications in eye-safe medical surgery [1], remote sensing [2], coherent laser radar [3], military affairs [4], etc. Among rare earth ions, Tm3+ ions have abundant energy levels, which are often used to be activators to obtain 2.0 µm laser [5,6]. For photoluminescence materials, the excited laser source is very important for the application. 980 nm laser is well commercialized and cheap. Furthermore, more and more research is focusing on the infrared luminescence excited by 980 nm incident laser [7–9]. However, Tm3+ ions can’t absorb incident 980 nm photons for the lack of corresponding energy levels. Fortunately, Yb3+ ions have a single electronic excited-state which is corresponding to ∼ 980 nm [10]. Yb3+ ions can act an excellent sensitizer which absorbs incident 980 nm photons and then transfer the energy to Tm3+ ions [11]. A large amount of study reported the energy transfer between Yb3+ and Tm3+ [12–14]. Therefore, Tm3+/Yb3+ system is very prospective to emit near 2.0 µm laser at the excitation of 980 nm laser.
In order to obtain strong infrared emissions, the host matrix with low phonon energy, good transparency, high chemical and thermal stabilities, and good mechanical properties is required. Among all the Tm3+/Yb3+ co-doped hosts, heavy-metal oxide glasses show promising comprehensive performance which is very favorable to improve the applications. As a novel heavy-metal oxide glass, Nb2O5-based glass has some excellent performance such as low phonon energy, high transparency in infrared region, good mechanical properties, and high thermal stability. These make Nb2O5-based glass become an outstanding host for Tm3+/Yb3+ to emit infrared light. Moreover, this type of heavy-metal glasses shows excellent optical properties like high refractive index, which endows this glass with potential applications in fields requiring bifunction of optical properties and luminescence. Based on the above, Tm3+/Yb3+ co-doped Nb2O5-based glasses are very worth further study. And this special material will give new think for optical and near 2.0 µm laser fields. However, heavy-metal Nb2O5-based melt is very easy to crystal during the cooling process. It requires very fast cool rate to quench the melt to obtain bulk glasses. Conventional techniques are difficult to satisfy this condition. Hence, it’s very hard to prepare bulk Nb2O5-based glasses by traditional experimental machines.
According to the levitation phenomenon in space, aerodynamic levitation experimental technology has been developed for new materials. After study, the characters of aerodynamic levitation method can be concluded. It can constrain heterogeneous nucleation, obtain deep undercooling, and realize fast solidification for the melt [15–17]. In addition, aerodynamic levitation method can process materials with high melt temperature, prepare high-purity samples without contamination, compared with traditional high temperature melting methods. These characters make this new technology be good at vitrifying materials. Therefore, aerodynamic levitation method has been often employed to develop novel bulk glasses and metastable materials [18–20]. In the previous study, Nb2O5-based glasses have been successfully prepared by aerodynamic levitation [21–23]. Initial research confirms that this type of heavy-metal oxide glasses shows good optical and luminescence properties. The previous study about the luminescence mainly focuses on the upconversion emissions of rare earth ions in Nb2O5-based glasses [21]. The report about near 2.0 µm down-conversion luminescence from rare earth ions doped this heavy-metal oxide glass is few. It is necessary to pay attention to near 2.0 µm emission of Tm3+/Yb3+ co-doped Nb2O5-based glasses for the promising applications.
In this study, Raman spectra of Tm3+/Yb3+ co-doped Nb2O5-based glasses with different Yb3+ contents were studied. The absorption coefficient of OH- group was studied according to the transmittance spectra of the glasses. The dependence of refractive index on Yb3+ concentration was also analyzed. X-ray Diffraction (XRD) was used to confirm the amorphous structure of the samples. EDS was used to check the composition distribution. The down-conversion spectra were studied at the excitation of 980 nm laser. And the effect of Yb3+ was discussed. The luminescence process was analyzed. Lifetime of the excited state in Tm3+ ions was researched.
2. Experimental
The compositions of Tm3+/Yb3+ co-doped Nb2O5-based glasses are 0.65Nb2O5-(0.29-y)La2O3−0.01Tm2O3-yYb2O3−0.05Ta2O5 (y = 0, 0.03, 0.05). Aerodynamic levitation furnace equipped with CO2 laser was used to prepare the bulk glasses. The detail was introduced in the previous study [21]. At last, Nb2O5-based glass spheres with ∼ 3 mm diameter were successfully fabricated by aerodynamic levitation method. Then, the glass spheres were polished to be 1.5 mm thick wafers by two sides for later measurements.
Raman spectra of the glass samples were measured by laser micro Raman spectrometer (Thermo Scientific Raman DXR) at the incident laser of 532 nm. The transmittance curves were recorded by ultraviolet spectrophotometer (varian cary 5000) with the step of 1 nm. The refractive index values were characterized and fitted by spectroscopic ellipsometer (Uvisel 2). XRD patterns were tested by X-ray diffractometer (D8 ADVANCE). The surface morphology and composition were characterized by SEM (SU8220). The down-conversion luminescence spectra of Tm3+/Yb3+ co-doped Nb2O5-based glasses with different Yb3+ concentrations were measured by a spectrofluorometer (Edinburgh Instruments FLSP920) at the excitation of 980 nm laser. The measurement step is 1 nm. The lifetime of the excited state in Tm3+ ions was also characterized and fitted with the step of 2000 ns.
3. Results and discussion
To study the maximum phonon energy of Tm3+/Yb3+ co-doped Nb2O5-based glasses, Raman spectra were recorded. The results are presented in Fig. 1. The shape and position of the peaks perform no change with the increase of Yb3+ concentration. It can be concluded that the phonon energy of the glasses would not be affected by Yb3+ ions. From the results, the maximum phonon energy of Tm3+/Yb3+ co-doped Nb2O5-based glasses are estimated to be ∼ 800 cm−1, which is obviously lower than many of common luminescence glasses such as silicate glasses (1100 cm−1) [24], germinate glasses (845 cm−1) [25], and lanthanum tungsten tellurite glasses (920 cm−1) [26]. As a new oxide glass, Tm3+/Yb3+ co-doped Nb2O5-based glasses perform low phonon energy which is very helpful to achieve strong photoluminescence. Therefore, it’s meaningful to regard Nb2O5-based glasses as a host to obtain good infrared emission properties.
OH- group perform important effect on the infrared luminescence. To evaluate the luminescence properties of new glasses, it’s necessary to determine the absorption coefficient of OH- group. The transmittance spectra of Tm3+/Yb3+ co-doped glasses with different Yb3+ concentrations have been measured and are presented in Fig. 2. The thickness of the glasses is 1.5 mm. According to the results of transmittance curves, the absorption coefficient of OH- group can be estimated by this equation [27,28]: αOH=(1/d)ln(T0/T). Herein, d is the thickness of the glass, T0 is the transmittance of the host matrix, and T is the transmittance at the maximum absorption of the OH- band (∼ 2.9 µm). After calculation, the absorption coefficient αOH of the glasses with y = 0, 0.03, 0.05 can be determined to be ∼ 0.70, 0.49, 0.46 cm−1, which is similar to lanthanum gallate glasses (∼ 0.15 cm−1) prepared by aerodynamic levitation technique [27]. The small values of absorption coefficient imply that the present glasses have low OH- content which is helpful to obtain strong infrared luminescence. This also confirms that Nb2O5-based glasses are good host matrix.
Fig. 2. The transmittance spectra of Tm3+/Yb3+ co-doped Nb2O5-based glasses with different Yb3+ concentrations.
The refractive index curves have been used to study the optical properties of Tm3+/Yb3+ co-doped Nb2O5-based glasses. The dependence of refractive index value on Yb3+ ions is discussed. Figure 3 shows the relationship between refractive index and incident wavelength. From the curves, some characteristic values which are often used to evaluate the optical properties of glasses can be obtained. The refractive index values at the wavelength of 587.56, 486.10, and 656. 30 nm are regarded to be characteristic values. They are named as nd, nf, and nc, respectively. According to the results, nd, nf, and nc can be decided to be 2.2686, 2.3121, and 2.2537 (y = 0), while 2.0655, 2.1112, 2.0481 for y = 0.03 and 2.2216, 2.2611, 2.2085 for y = 0.05. Generally, nd is often used to evaluate the refractive properties of optical glasses. The value of nd is over than 2, indicating excellent refractive properties of the glasses. With the increase of Yb3+ concentration, the value of nd first decreases and then increases. The addition of Yb3+ ions plays negative role in the refractive index. When further increasing the Yb3+ content, the refractive index shows a recovering trend. On the whole, Yb3+ is not good for the refractive index of the glasses. Besides, wavelength dispersion can be also looked as one of important optical parameters for glasses. Optical dispersive ability is often characterized by Abbe number (vd). In general, large Abbe number means small dispersion. The values of Abbe number should be calculated to discuss the wavelength dispersive ability. According to the equation of vd=(nd−1)/(nf-nc), the values of Abbe number can be decided by using the three characteristic refractive index values. So vd of Tm3+/Yb3+ co-doped Nb2O5-based glasses can be calculated to be 21.7, 16.9, and 23.2, corresponding to y = 0, 0.03, 0.05, respectively. Considering the high refractive index, Tm3+/Yb3+ co-doped Nb2O5-based glasses show good wavelength dispersion with high Abbe number. For commercial optical glasses, the values of nd and vd often locate in the regions of 1.5-2.0 and 15-100 [29]. Tm3+/Yb3+ co-doped Nb2O5-based glasses satisfy well with the requirements of commercial optical glasses. Therefore, this type of glasses with high refractive index and good wavelength dispersion is an excellent optical material possessing prospective commercial applications. In addition, the effect of Yb3+ ions on the Abbe number is positive on the whole.
Fig. 3. The refractive index curves of Tm3+/Yb3+ co-doped Nb2O5-based glasses with different Yb3+ concentrations.
XRD patterns of the glasses are shown in Fig. 4. The results reveal the amorphous structure of the samples. Furthermore, the amorphous state of the sample has been confirmed by the surface image of SEM. From Fig. 5(a), it can be known that there are no crystals in the sample. So Tm3+/Yb3+ co-doped Nb2O5-based glasses were successfully obtained by aerodynamic levitation. Figure 5(b) is the EDS spectrum for composition analysis. The signals of La, Nb, Ta, O, Tm, and Yb have been detected in the glass. To reveal the distribution of rare earth ions, plane sweeping mode has been employed for the glass sample. Figure 5(c) and (d) show the distribution of Tm3+ and Yb3+ ions. From the results, Tm3+ and Yb3+ ions are distributed homogeneously in the glass. So Tm3+/Yb3+ co-doped Nb2O5-based glasses with homogeneous compositions have been prepared.
Fig. 5. (a) SEM image of the surface of Tm3+/Yb3+ co-doped Nb2O5-based glass, (b) EDS spectrum of compositions, the plane distribution of (c) Tm3+ ions and (d) Yb3+ ions.
At the excitation of 980 nm laser, the near infrared emission of Tm3+/Yb3+ co-doped Nb2O5-based glasses has been recorded. The results are presented in Fig. 6. A broad near infrared emission band centered at ∼ 1850nm has been achieved from the glasses. When y = 0.03, the full width at half maximum (FWHM) is evaluated to be ∼ 241 nm. The value of FWHM is ∼ 247 nm when y = 0.05. The infrared emission of the glasses is broad compared with some published research [30–32]. According to the down-conversion luminescence spectra, the near infrared emission intensity first increases and then decreases with the increase of Yb3+ ions. When y = 0.03, the emission is the strongest. It’s remarkable that near infrared emission can’t be obtained from the glass when y = 0. Tm3+ ions can’t absorb incident 980 nm photons directly due to lack of corresponding energy levels. If there is no Yb3+ ions in the glass, Tm3+ ions can’t be excited to generate near infrared emission. This can also confirm that Yb3+ is a very important sensitizer for Tm3+ ions to make full use of cheap 980 nm laser. Yb3+ ions absorb the pump power of 980 nm photons and then are excited from the ground state to upper state. The excited Yb3+ ions would transfer energy to Tm3+ ions by transiting back to the ground state. The excited Tm3+ ions can emit broad near infrared luminescence by the transition of 3F4→3H6. The sharp peaks around 1960nm in the spectra are resulted from frequency doubling phenomenon of incident 980 nm laser. According to the energy level structure of rare earth ions, near infrared luminescence mechanism has been studied. The schematic diagram of luminescence is presented in Fig. 7. Yb3+ ions in excited state transfer energy to Tm3+ ions, which are excited from the ground state to 3H5 levels. Then, the Tm3+ ions will transit to 3F4 state by nonradiative relaxation. In this way, 3F4 state can be populated. Near infrared luminescence at ∼ 1850nm will be emitted by the transition of 3F4→3H6 in Tm3+ ions.
Fig. 6. Down-conversion luminescence spectra of Tm3+/Yb3+ co-doped Nb2O5-based glasses at 980 nm excitation.
To judge the near infrared luminescence properties of Tm3+/Yb3+ co-doped Nb2O5-based glasses, the decay behavior has been studied. The decay curve is presented in Fig. 8 at the excitation of 980 nm laser. To obtain the value of the lifetime, the experimental data should be fitted. In general, single exponential function is often employed to fit the decay part of the curve. The present data has been well fitted by a single exponential function. From the result, we can know that the lifetime of 3F4 excited state in Tm3+/Yb3+ co-doped Nb2O5-based glasses (y = 0.03) can be calculated to be 2.292 ms. This is much longer than 0.25 ms of Tm3+ ions in silicate glasses [33] and 0.64 ms in bismuth silicate glasses [34]. The absorption coefficient of OH- group is small in Nb2O5-based glasses. This is favorable to decrease the quenching of infrared luminescence. Furthermore, Nb2O5-based glasses have low phonon energy. This can improve the lifetime of excited states. The above two factors result in long lifetime of 3F4 state of Tm3+ in Nb2O5-based glasses. So Tm3+ ions in Nb2O5-based glasses have long lifetime, which is very favorable to achieve strong near infrared emission. Down-conversion luminescence spectra reveal that broad and strong near 2.0 µm emission can be obtained from Tm3+/Yb3+ co-doped Nb2O5-based glasses (y = 0.03). This result also confirms that heavy-metal Nb2O5-based glasses are excellent host matrix which is helpful to realize powerful near 2.0 µm laser from Tm3+/Yb3+ system at the excitation of 980 nm.
Fig. 8. The lifetime curve of 3F4 excited state in Tm3+/Yb3+ co-doped Nb2O5-based glasses (y = 0.03) at the pump power of 980 nm laser.
4. Conclusions
Different Yb3+ concentrations have been doped into Tm3+/Yb3+ co-doped Nb2O5-based glasses by aerodynamic levitation. The Raman spectra results indicate that Yb3+ ions perform no effect on the maximum phonon energy of the glasses. Nb2O5-based glasses have very low phonon energy with the value of ∼ 800 cm−1, which is significantly lower than some common luminescence glasses. The absorption coefficient of OH− group of the glasses with y = 0, 0.03, 0.05 can be determined to be ∼ 0.70, 0.49, 0.46 cm−1, respectively. Therefore, Nb2O5-based glasses are favorable host to obtain strong near infrared emission. The dependence of refractive index on wavelength has been analyzed. Three characteristic refractive index values can be obtained from the curves. The nd first decreases and then increases with the increase of Yb3+ concentration. Yb3+ is not good for the refractive index of the glasses on the whole. The value of nd of all the glasses is over than 2. So, Nb2O5-based glasses have high refractive index. Abbe number should be calculated to judge the wavelength dispersive ability of the glasses by using the characteristic values. All the glasses perform high Abbe number while possessing high refractive index. Therefore, Nb2O5-based glasses show excellent optical properties which satisfy well with requirements of commercial glasses. XRD and SEM image confirm the amorphous phase of the sample. And EDS results reveal that the compositions are distributed homogeneously in the glasses. A broad and strong near infrared emission band centered at ∼ 1850nm has been obtained from Tm3+/Yb3+ co-doped Nb2O5-based glasses at the excitation of 980 nm laser. The emission intensity first increases and then decreases with Yb3+ concentration. The intensity is the strongest with y = 0.03. Near infrared emission can’t be obtained with y = 0, confirming Tm3+ ions can’t absorb 980 nm photons. The luminescence process is the energy transfer from Yb3+ to Tm3+. Near infrared emission is ascribed to the transition 3F4→3H6 in Tm3+ ions. The lifetime curve of the excited state for Tm3+/Yb3+ co-doped Nb2O5-based glasses (y = 0.03) has been measured. The lifetime is calculated to be 2.292 ms, indicating a long lifetime. This is very favorable to get strong down-conversion luminescence. Therefore, Tm3+/Yb3+ co-doped Nb2O5-based glasses perform good optical properties and strong near infrared photoluminescence.
Funding
National Natural Science Foundation of China (NSFC) (51472263, 51602330); Shanghai Sailing Program (16YF1413100).
Acknowledgements
This work was supported by National Nature Science Foundation of China (51602330, 51472263), and Shanghai Sailing Program (16YF1413100).
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