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Transparent polycrystalline NdxGd3-xAl3Ga2O12 (Nd:GAGG) ceramics were prepared via solid-state reactive sintering from a mixture of commercial Gd2O3, Al2O3, Ga2O3, Nd2O3 and 0.05wt.% ZrO2 powders at 1650 °C. Fully dense ceramics were obtained for all samples with different Nd2O3 doping level. Uniform microstructures with average grain size of about 20 μm were observed in the ceramics, while no secondary phase was detected. Optimum transmittance of as high as 77% in near-infrared bands, which is close to its theoretical value of 81.1%, was achieved in the 0.8 at.% Nd:GAGG ceramics. The spectroscopic properties of the Nd:GAGG transparent ceramics were also investigated. Absorption cross-section of the 0.8 at.% Nd:GAGG ceramic at 808 nm is about 4.1 × 10−20 cm2. The emission cross-section at 1063 nm is about 1.8 × 10−19 cm2.
© 2016 Optical Society of America
1. Introduction
In recent years, the applications of diode-pumped solid-state lasers (DPSSLs) [1–5] are very attractive in many fields including scientific research and industry utilizations. Among all laser crystals, Nd:GGG is considered an excellent host material due to its refractory nature, good thermal conductivity, and high Nd-ion doping concentration (higher than 4 at.%) [6–10]. Diode-pumped high power Nd:GGG lasers have attracted much attention and been widely used recently [3,11,12]. However, the high price as well as the serious evaporation and decomposition of Ga2O3 during the crystal growth process limit the wide spread application of the Nd:GGG crystal [15]. With certain gallium (Ga) ions substituted by aluminum (Al) ions in Nd:GGG crystal, another new laser single crystal Nd:Gd3AlxGa5-xO12 (x = 0.94) (Nd:GAGG) has been grown by Czochralski method [14,15]. Its laser performance has been reported [13,15–17] and the results show that Nd:GAGG could be a promising laser medium.
Compared with single crystal, ceramics have a lot of advantages such as lower cost, much higher activator doping level, uniformity and superior machine-ability. However, to the best of our knowledge, few reports have been found in literature concerning the preparation of transparent Nd:GAGG ceramics. Fortunately, its cubic structure makes it possible to prepare fully transparent and uniform laser ceramics. In 1990s, Ikesue successfully developed transparent Nd:YAG ceramics, with its performance comparable to single-crystal laser oscillation [18], proving that ceramic materials could overcome both the technical and economical problems of melt-growth single crystals. After that, transparent ceramics became promising substitute for single crystal.
In this work, we report, for the first time to our knowledge, a relatively simple method to fabricate this new transparent ceramic derived from the well-known Nd:GGG crystal, with a particular interest toward its optical properties. To decrease the decomposition of Ga2O3 during sintering, the Nd:GAGG ceramics were prepared in oxygen atmosphere at normal pressure. Besides, comparing with vacuum sintering and hot isostatic pressing (HIP) sintering, oxygen sintering is time and cost saving since the prepared samples do not need to be post-annealed in air for a long time. A simple, solid-state reaction method was used in this fabrication. Nd:GAGG ceramics with excellent optical properties were developed successfully in the present work. Results shown in this paper can contribute to the controllable synthesis of high quality Nd:GAGG ceramics and further exploration of the applications of the garnet based laser matrix material.
2. Experimental
Commercial Gd2O3 (GanZhou QianDong Rare Earths Group Co. Ltd. China, 5μm), α-Al2O3 (Aladdin Industrial Inc. America, 200nm), Ga2O3 (Chalco Henan Aluminum Fabrication CO. LTD, China, 2μm) and Nd2O3 (Aladdin Industrial Inc. America) powders with 99.99% purity were used as the starting materials. The powders were weighed according to the composition of NdxGd3-xAl3Ga2O12(Nd:GAGG) (x = 0, 0.024, 0.054, 0.135, respectively) with 0.05wt.% ZrO2 (Aladdin Industrial Inc. America, 50nm) as the sintering aid. Mixed powders were ball milled in alcohol at the rotation speed of 300 rpm for 12 h. After that, the mixtures were dried, calcined and dry-pressed into 25 mm diameter pellets and cold isostatically pressed under a pressure of 250 MPa. For the forming process, no binder was added. The powder compacts were sintered at 1650°C for 20 hrs in flowing dry oxygen at 0.6 L/min to obtain the transparent ceramics. After the sintering process, no further annealing treatment was applied on these samples. Finally the specimens of ~20 mm in diameter were mirror-polished on both surfaces into 1 mm thick pellet and then thermally etched at 1300°C for 30 min for grain size measurement and other characterizations. The detailed experiment conditions can be found in our previous work on transparent Ce:GAGG ceramics [19].
The phase of the samples was identified by X-ray diffraction (XRD, Model D8 Advance, Bruker AXS Co. Germany) using CuKααradiation in the range of 2θ = 10°-90°. The density was determined by the Archimedes method. Morphologies of powders and ceramics were examined by transmission electron microscope (TEM, Tecnai F20, FEI Co. USA) and scanning electron microscopy (SEM, Quanta FEG 250, FEI Co. USA). Grain sizes of the sintered samples were obtained by the linear intercept method (200 grains counted) [20]. The average grain size was calculated by multiplying the average linear intercept distance by 1.56. The distribution of Al, Ga, and Gd was observed by energy-dispersive spectroscopy (EDS). The optical transmittance was measured at room temperature by a spectrometer (Lambda 950, Perkin Elmer Co. USA. spectral resolution is 1nm, sample thickness is 1mm). Emission spectra and lifetime were measured at room temperature by a fluorescence spectrometer (Fluorolog-3, HORIBA, Ltd. Japan. spectral resolution is 1nm, sample thickness is 1mm). The thermal conductivity of our ceramics were measured by the laser flash method, the average value is about 5.7 W/mK, which is approximately five times higher than that of Nd-doped glass, such as LHG-8 (1.20 W/mK).
3. Results and discussion
Figure 1 shows the micrographs of the starting powders before and after ball milling. The commercial Gd2O3 and Ga2O3 particles are relatively coarse with large particle size (>1 μm), while the commercial Al2O3 powders are nearly spherical with primary particle size of about 0.2 μm. As shown in Fig. 1(d) and 1(e), those particles are homogeneously mixed after ball milling.
Fig. 1 Microstructures of starting powders of (a) Al2O3, (b) Gd2O3, (c) Ga2O3, and (d),(e) 0.8at.% Nd:GAGG after ball milling for 12 h.
Figure 2(a) shows the XRD patterns of 0at.%-4.5at.% Nd doped GAGG ceramics. All observed peaks from 10° to 90° can be indexed to the standard Gd3Al3Ga2O12 diffractions pattern (PDF# 46-0448). No other phases were detected in the XRD pattern, indicating that full transformation to GAGG phase from its raw material occurred despite the significantly large size of Gd2O3 and Ga2O3 powders. However, it can be noted that the positions of the XRD peaks shift to higher 2θ values (especially for the 0at. % Nd: GAGG ceramic sample) compared to the standard GAGG PDF card. As 0.05wt.% ZrO2 was added as the sintering aid, a small amount of Zr4+ (R = 72 pm) may replaced the Gd3+ (R = 94 pm), which caused the decreases of the lattice parameter. On the other hand, with an increase in the Neodymium content, the positions of the XRD peaks shift to lower 2θ values, as observed in Fig. 2(b). Because the ionic radii of Nd3+ (99.5pm) is larger than that of Gd3+ (94pm), the doping of Nd3+ increases the lattice constant. The calculated lattice constants are exhibited in Fig. 3 Clearly, the cell parameter linearly increases along with more Nd3+ incorporation and obeys the Vegard’s law, implying the formation of homogeneous solid solutions.
Fig. 2 (a) XRD patterns of 0at.%-4.5at.% Nd3+ concentration GAGG ceramics and Fig. 2(b) Expanded view of the 2θ diffraction peak between 30°and 35°.
Grain growth process is an important process to remove the residual pores to achieve dense structure [21]. Fig. 4 shows the microstructures of the 0.8 at.% and 4.5 at.% Nd doped GAGG ceramic specimens sintered at 1650°C for 20 hrs. The thermal-etched surface of the polished Nd:GAGG ceramic shows that the samples are fully consolidated with almost no visible pores. Abnormal grain growth is not observed either. Besides, no secondary phase is observed either at the grain boundaries or inside the grains. The average grain size slightly decreased with the increased Nd3+ concentration. Such a phenomenon was also observed in Nd:YAG ceramics [22]. The average grain size of about 20 μm is much smaller than the Nd:YAG ceramics reported by Ikesue A et al. [22] and Sang-Ho Lee et al. [21]. Figure 5 shows the EDS analysis of the 0.8 at.% Nd:GAGG sample sintered at 1650°C for 20 hrs. It is proven that Al, Ga, and Gd are homogeneously distributed throughout the specimen. SEM and EDS analyses proved that the microstructure of the Nd:GAGG ceramics is homogeneous.
Fig. 4 The microstructures of the 0.8 at.% and 4.5 at.% Nd:GAGG specimens sintered at 1650 °C for 20 hrs.
The transmittances of 0, 0.8, 1.8 and 4.5 at.% Nd doped GAGG ceramic samples sintered at 1650 °C for 20 hrs are shown in Fig. 6(a). According to the Lambert-Beer law, the transmissivity (I/I0) is given by: I/I0 = (1-R)2exp(-μx), where R is the reflectivity of the material, μ is the absorption coefficient, x is the thickness of sintered body. In the present study, we assume that μ is close to zero, the transmittance of an ideal GAGG single crystal is: I/I0 = (1-R)2, the reflectivity is given by R = (nGAGG–nair)2/(nGAGG + nair)2, where nGAGG is the refractive index of the GAGG materials, and nair is the refractive index of air. At about 1000nm, nGAGG≈1.92 [14], nair is taken as 1 [23]. Hence, the theoretical transmittance of the Nd:GAGG material is 81.1%. In present work, the highest transmittance of the sample at 1063 nm is obtained in 0.8 at.% Nd doped GAGG ceramic, which is shown in Fig. 6(b). The highest transmittance value of about 77% is close to the theoretical transmittance. More importantly, it is quite encouraging that in contrast to Nd:YAG ceramics, this material is fabricated at much lower temperature in oxygen at normal pressure rather than vacuum sintering or post hot isostatic pressing. As shown in Fig. 6(b), the transmittance at 1063 nm increases at first and then decreases with Nd concentration increase from 0 at.% to 4.5 at.%. It is well-known that the sintering aids play an important role in the exploitation of optical transparent ceramics. In this study, 0.05wt.% ZrO2 was added as the sintering aid, and the results indicate that Nd2O3 may also play a role of sintering aids. As the ionic radii of Nd3+ (99.5pm) is larger than that of Gd3+ (94pm), further lattice distortion and structural disorder might be possible factors which affect the sintering process. Transparent Nd:GAGG specimens of about 20 mm diameter by 1mm thickness and containing 0, 0.8, 1.8, and 4.5 at.% Nd3+ are shown in Fig. 7(a). As the Nd3+ concentration increases, the color of specimens becomes more violet. The standing sample of 0.8 at.% Nd:GAGG ceramic demonstrates its excellent in-line transparency, see Fig. 7(b).
Fig. 6 (a) The transmittances of 0, 0.8, 1.8 and 4.5 at.% Nd:GAGG samples sintered at 1650 °C for 20 hrs and (b) Relationship between Nd concentration and transmittances at 1063nm of Nd:GAGG ceramics.
Fig. 7 (a) Transparent Nd:GAGG specimens with 0, 0.8, 1.8, and 4.5 at.% Nd3+ and (b) 0.8 at.% Nd:GAGG specimen, which is standed on the paper. The specimens were sintered at 1650°C for 20 hrs in oxygen and the size was about 20 mm diameter by 1mm thickness.
Figure 8 shows the absorption coefficient (α) of the 0.8 and 4.5 at.% Nd:GAGG ceramics with thickness of 1 mm in the wavelength range of 400-1000 nm. This absorption coefficient was determined from transmittance measurements. The strongest absorption band locates around 800 nm, which is associated with the transition of electrons from the ground state (4I9/2) to the 4F5/2 + 2H9/2 energy level [24]. The absorption coefficient of the 4.5 at.% Nd:GAGG ceramic is much larger than that of 0.8 at.% one because of its higher Nd3+ concentration. The absorption cross-section (σabs) of Nd3+ can be calculated by Lambert’ Law [25]: σabs = α/C, where C is the active-ion concentration per unit volume. The absorption cross-section of the 0.8 at.% Nd:GAGG ceramic at 808 nm (the strongest peak) is about 4.1 × 10−20 cm2, which is close to the 5 at.% Nd:GGG single crystal of about 4.35 × 10−20 cm2. The relationship between Nd3+ concentration and absorption intensities of 4F5/2 + 2H9/2 bands is shown in Fig. 9. These bands are important for laser oscillation. Nd:GAGG ceramics can be pumped dominantly by a laser diode of 808 nm. And the absorption intensity increased monotonically as Nd3+ concentration increased. The solubility of Nd3+ in our GAGG ceramics is larger than the GAGG single crystal which was grown in an RF-heated Czchralski apparatus [13,16].
Fig. 8 The absorption spectra of the 0.8 and 4.5 at.% Nd:GAGG ceramics in the wavelength range of 200-2000nm.
Figure 10 shows the fluorescence spectrum of the 0.8 at.% Nd:GAGG ceramic excitated by 808 nm at room temperature. Three emission bands observed at 850-950, 1040-1130, and 1310-1440 nm are corresponding to the 4F3/2→4I9/2, 4I11/2, and 4I13/2 transitions of Nd3+ ions. The strongest emission peak located at 1063 nm can be assigned to the 4F3/2→4I11/2 transition. The relationship between Nd3+ concentration and fluorescence intensity of Nd:GAGG ceramics around 1063 nm is shown in Fig. 11. The fluorescence intensity of the samples increases, up to a Nd3+ concentration of about 1.8 at.%. However, the fluorescence intensity starts to decrease due to the quenching effect for a Nd concentration around 2 at.%. The absorption efficiencies for 0.8at.%, 1.8at.% and 4.5at.% samples are 26.1%, 39.7% and 56.8%, respectively. QE (Quantum Efficiency) is usually used to evaluate the level of photoluminescent materials. The internal QE (ηint) is called as fluorescence quantum yield (ΦF), i.e. emission efficiency, which is the ratio of emitted photons by the number of only absorbed photons: ηint = Nem/Nabs. The external QE (ηext) is the ratio of emitted photons by the number of incident photons: ηext = Nem/Ninc. Their relationship is ηext = Aηint, where A is the absorbance. The emission efficiency is consisted with the external QE, it can be estimated qualitatively by the change of the fluorescence intensity and exhibits a same dependence with the fluorescence intensity on the doping concentrations, as shown in Fig. 11.
Fig. 11 Relationship between Nd3+ concentration and fluorescence intensities of Nd:GAGG ceramics around 1063nm and absorption efficiency change curve with Nd3+ concentration.
It is very important for “laser ceramics” to identify the emission lifetime τe. Figure 12 shows the decay curve of 0.8at.% sample and the fitting result of τe = 218.5μs. By means of Judd-Ofelt method [26], we can easily obtain the emission cross-section at 1063 nm:
Where λ = 1063nm is the wavelength of fluorescent emission, c = 3 × 108m/s is the speed of light, n = 1.92 is the refractive index of the GAGG materials, Δλ = 12nm (determined by numerical integration method) is the effective half-width of the fluorescence emission band, τe is the emission lifetime. The result is shown that σe(1063nm) = 1.8 × 10−19 cm2.4. Conclusions
Transparent ceramics of 0, 0.8, 1.8 and 4.5 at.% Nd:GAGG can be prepared from commercially available powders by a solid-state reactive sintering method in oxygen at normal pressure. Specimens sintered at 1650 °C for 20 hrs were fully consolidated with a grain size of about 20 μm regardless of the Nd concentration. Few pores and no secondary phase impurities were observed in the Nd:GAGG ceramic microstructure. The transmittance in near-infrared band is about 77% for the 0.8 at.% Nd:GAGG ceramics at 1063 nm, which is close to its theoretical value. The spectroscopic properties of the Nd:GAGG transparent ceramics were also investigated. The absorption cross-section at 808 nm is about 4.1 × 10−20 cm2. The emission cross-section at 1063 nm is about 1.8 × 10−19 cm2. These results can contribute to the controllable synthesis of high quality Nd:GAGG ceramics and further exploration of their laser applications.
Acknowledgments
We appreciated the kind discussion with Prof. Takunori Taira. And this work is financially supported by the National Natural Science Foundation of China (NSFC 51502308, 11404351), Ningbo Municipal Natural Science Foundation (2014A610014) and Ningbo Science and Technology Innovation Team (Grant No. 2014B82004) and the Zhejiang Provincial Science Fund for Distinguished Young Scholars (LR16E020001).
References and links
1. Y. Ishida and K. Naganuma, “Compact diode-pumped all-solid-state femtosecond Cr4+:YAG laser,” Opt. Lett. 21(1), 51–53 (1996). [CrossRef] [PubMed]
2. Q. Liu, F. Lu, M. Gong, C. Li, and D. Ma, “15 W output power diode-pumped solid-state lasers at 515 nm,” Laser Phys. Lett. 4(1), 30–32 (2007). [CrossRef]
3. C. Zuo, B. Zhang, J. He, H. Huang, X. Dong, J. Xu, Z. Jia, C. Dong, and X. Tao, “13.2 W cw output generated by a diode-end-pumped Nd: GGG laser,” Laser Phys. Lett. 5(8), 582–584 (2008). [CrossRef]
4. T. Taira, “RE3+-ion-doped YAG ceramic lasers,” Selected Topics in Quantum Electron. IEEE J. 13(3), 798–809 (2007).
5. Y. Sato and T. Taira, “Temperature dependencies of stimulated emission cross section for Nd-doped solid-state laser materials,” Opt. Mater. Express 2(8), 1076–1087 (2012). [CrossRef]
6. K. Yoshida, H. Yoshida, and Y. Kato, “Characterization of high average power Nd: GGG slab lasers,” IEEE J. Quantum Electron. 24(6), 1188–1192 (1988). [CrossRef]
7. H. Hayakawa, K. Maeda, T. Ishikawa, T. Yokoyama, and Y. Fujii, “High average power Nd: Gd3Ga5O12 slab laser,” Jpn. J. Appl. Phys. 26(1010A), L1623–L1625 (1987). [CrossRef]
8. B. Labranche, W. Qun, and P. Galarneau, “Diode-pumped-cw and quasi-cw Nd: GGG (Ca, Mg, Zr) laser,” Proc. SPIE 2041, 326–331 (1994). [CrossRef]
9. Z. Jia, X. Tao, C. Dong, X. Cheng, W. Zhang, F. Xu, and M. Jiang, “Study on crystal growth of large size Nd3+: Gd3Ga5O12 (Nd3+: GGG) by Czochralski method,” J. Cryst. Growth 292(2), 386–390 (2006). [CrossRef]
10. L. Qin, D. Tang, G. Xie, H. Luo, C. Dong, Z. Jia, H. Yu, and X. Tao, “Diode-end-pumped passively mode-locked Nd: GGG laser with a semiconductor saturable mirror,” Opt. Commun. 281(18), 4762–4764 (2008). [CrossRef]
11. R. Mahajan, A. Shah, S. Pal, and A. Kumar, “Analytical study for investigating the behaviour of Nd-doped Glass, YAG and GGG under the heat capacity mode of operation,” Opt. Laser Technol. 39(7), 1406–1412 (2007). [CrossRef]
12. L. Qin, D. Tang, G. Xie, C. Dong, Z. Jia, and X. Tao, “High-power continuous wave and passively Q-switched laser operations of a Nd: GGG crystal,” Laser Phys. Lett. 5(2), 100–103 (2008). [CrossRef]
13. Y. Zhi, C. Dong, J. Zhang, Z. Jia, B. Zhang, Y. Zhang, S. Wang, J. He, and X. Tao, “Continuous-wave and passively Q-switched laser performance of LD-end-pumped 1062 nm Nd:GAGG laser,” Opt. Express 18(8), 7584–7589 (2010). [CrossRef] [PubMed]
14. Y. Kuwano, S. Saito, and U. Hase, “Crystal growth and optical properties of Nd: GGAG,” J. Cryst. Growth 92(1), 17–22 (1988). [CrossRef]
15. J. Zhang, X. Tao, C. Dong, Z. Jia, H. Yu, Y. Zhang, Y. Zhi, and M. Jiang, “Crystal growth, optical properties, and CW laser operation at 1.06 μ m of Nd: GAGG crystals,” Laser Phys. Lett. 6(5), 355–358 (2009). [CrossRef]
16. B. Zhang, J. Yang, J. He, H. Huang, S. Liu, J. Xu, F. Liu, Y. Zhi, and X. Tao, “Diode-end-pumped passively Q-switched 1.33 μm Nd:Gd3AlxGa5-xO12 laser with V3+: YAG saturable absorber,” Opt. Express 18(12), 12052–12058 (2010). [CrossRef] [PubMed]
17. A. Agnesi, F. Pirzio, G. Reali, A. Arcangeli, M. Tonelli, Z. Jia, and X. Tao, “Multi-wavelength Nd: GAGG picosecond laser,” Opt. Mater. 32(9), 1130–1133 (2010). [CrossRef]
18. A. Ikesue, T. Kinoshita, K. Kamata, and K. Yoshida, “Fabrication and Optical Properties of High-Performance Polycrystalline Nd: YAG Ceramics for Solid-State Lasers,” J. Am. Ceram. Soc. 78(4), 1033–1040 (1995). [CrossRef]
19. X. Chen, H. Qin, Y. Zhang, Z. Luo, J. Jiang, and H. Jiang, “Preparation and Optical Properties of Transparent (Ce, Gd)3Al3Ga2O12 Ceramics,” J. Am. Ceram. Soc. 98(8), 2352–2356 (2015). [CrossRef]
20. M. I. Mendelson, “Average grain size in polycrystalline ceramics,” J. Am. Ceram. Soc. 52(8), 443–446 (1969). [CrossRef]
21. S. H. Lee, S. Kochawattana, G. L. Messing, J. Q. Dumm, G. Quarles, and V. Castillo, “Solid-State Reactive Sintering of Transparent Polycrystalline Nd: YAG Ceramics,” J. Am. Ceram. Soc. 89(6), 1945–1950 (2006). [CrossRef]
22. A. Ikesue, K. Kamata, and K. Yoshida, “Effects of neodymium concentration on optical characteristics of polycrystalline Nd: YAG laser materials,” J. Am. Ceram. Soc. 79(7), 1921–1926 (1996). [CrossRef]
23. D. C. Harris, Materials for Infrared Windows and Domes: Properties and Performance (SPIE Optical Engineering Press, 1999), pp. 195–199.
24. A. Arcangeli, S. Bigotta, M. Tonelli, Z. Jia, J. Zhang, and X. Tao, “Spectroscopic analysis of Nd3+: Gd3AlxGa5-xO12 garnet crystal,” J. Opt. Soc. Am. B 28(6), 1475–1480 (2011). [CrossRef]
25. J. D. Ingle, Jr. and S. R. Crouch, Spectrochemical Analysis (Prentice Hall, 1988).
26. K. Wu, L. Hao, H. Zhang, H. Yu, H. Cong, and J. Wang, “Growth and characterization of Nd:Lu3ScxGa5−xO12 series laser crystals,” Opt. Commun. 284(21), 5192–5198 (2011). [CrossRef]
