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2.0at.% Nd:YAG transparent ceramics were prepared by a solid-state reactive sintering method. The as-sintered specimens were annealed at different temperatures for different time. The ceramics with average grain size of 15μm are dense and homogeneous in structure. 1450°C is determined as the optimum annealing temperature and the annealing time longer than 5h is considered as the appropriate time by the optical spectra and laser performance. At this temperature and holding time, the concentration of color centers in the specimen is relatively low and the laser slope efficiency is the highest. It is concluded that less color centers and lower optical loss in Nd:YAG transparent ceramics are necessary to obtain the excellent laser performance.
© 2014 Optical Society of America
1. Introduction
Since the first ruby laser in the world was invented in 1960s [1], laser technologies have been developed for an immense number of applications. Neodymium-doped yttrium aluminum garnet (Nd:YAG) as a well-known laser host material has been widely used in military, medicine, manufacture and many other fields [2–6]. Compared with Nd:YAG single crystals fabricated by the Czochralski method, the Nd:YAG transparent ceramics have attracted much attention because of many advantages such as lower cost of fabrication, less limitation in size, shorter fabrication period and higher doping concentration. So a lot of work has been done for obtaining Nd:YAG ceramics with better optical properties approach or equal to Nd:YAG single crystals [7–11].
The Nd:YAG single crystals are always needed to be annealed in air to reduce the oxygen vacancies that are introduced during growth in reducing atmosphere [12–15]. Just as in single crystals, there are unavoidable defects in Nd:YAG ceramics after vacuum sintering, which degrade the optical properties significantly. Post-annealing is used to decrease the amount of defects. Previous work demonstrated the importance of annealing for Nd:YAG ceramics [16,17]. Stevenson et. al. [18] reported annealing in air at 1600°C for 10h reduced the number of color centers greatly and color center formation was controlled by oxidation-reduction of Fe2+/3+ impurities. Zhang et. al. [19] investigated the Nd:YAG ceramics annealed or even re-annealed in air or vacuum at 1200°C-1500°C for 20h and suggested air annealing at 1450°C for 20h was optimum. Other works focused on the fabrication and laser properties of Nd:YAG ceramics [20–22]. Lu et. al. [23] realized the cw laser output of 72W with a 1% Nd:YAG ceramic rod. Puzyrev et. al. [24] developed a new method for the preparation of nanosized precursors of an yttrium-aluminum garnet from yttrium and aluminum compounds of various natures. Ba et. al. [25] prepared the transparent YAG ceramics by the aqueous and non-aqueous based tape casting methods. However, the relationship between annealing and laser behavior of Nd:YAG ceramics was not investigated. And the physical principles of how the annealing has action upon the laser performance were not clear also.
In this paper, Nd:YAG transparent ceramics were fabricated by a solid-state reaction method with commercial oxide powers as starting materials. These vacuum-sintered Nd:YAG ceramics were annealed in air at different temperatures for various time. Then we observed the spectral characteristics and laser performance of the ceramic specimens. We discussed the relationship between annealing and laser performance in view of defects and impurities.
2. Experimental procedure
2.0at.% Nd:YAG ceramics were prepared by a solid-state reaction method. Commercially available high-purity powders of α-Al2O3 (99.98%, Alfa Aesar, USA), Y2O3 (99.999%, Alfa Aesar, USA) and Nd2O3 (99.99%, Alfa Aesar, USA) were used as starting materials. Tetraethoxysilane (TEOS, 99.999%, Alfa Aesar, USA) and magnesium oxide (MgO, 99%, Alfa Aesar, USA) were used as sintering aids. These powers were blended together with the stoichiometric ratio of (NdxY(1-x))3Al5O12 (x = 0.02) and mixed in ethanol for 12h. Then the slurry was dried and sieved through a 200-mesh screen. After calcining at 800°C, the powders were uniaxially pressed into a disk at about 15MPa and further pressed by CIP at 200MPa. The as-obtained green body was sintered under vacuum at 1750°C for 50h. The as-sintered specimen was cut into some pieces. These blocks were annealed at different temperatures for various hours and then processed into slabs with the dimension of 3mm × 3mm × 6mm.
The microstructures of the polished surface and fracture surface of the specimens were observed by the field emission scanning electron microscopy (FESEM, S-4800, Hitachi, Japan). Trace element analysis was performed on specimens using a glow discharge mass spectrometer (GDMS, Model VG9000, Thermal Elemental, England). Mirror-polished samples on both surfaces were used to measure transmittance and absorption spectra by a UV-VIS-NIR spectrometer (Cary-5000, Varian, USA). The emission spectra and fluorescence decay curves of post-annealed specimens at 1064nm were recorded using a spectrophotometer (Edinburgh, FL920), with a microsecond flash Xe lamp (Edinburgh, lF900) as the exciting source at 355 nm. The signals were detected with a NIR PMT (Hamamatsu, R5509).
The schematic of laser cavity is shown in Fig. 1. An end-pumped resonator was adopted with total cavity length of about 70mm. A fiber-couple 808nm semiconductor diode laser with beam waist diameter of about 0.4mm was used as the pumping source. The specimen was ensured a good thermal contact by water cooling with a copper heat sink. The output mirror has the transmittance of 10% at 1064nm.
3. Results and discussion
3.1 Analysis of specimen microstructure
Figure 2 shows the scanning electron microscope (SEM) micrographs of the polished and thermally etched surface and the fracture surface of Nd:YAG ceramics. It is noted that the specimen is dense and homogeneous in structure with the average grain size of about 15μm. There are almost no micro-pores and secondary phases at the inner grains or grain boundaries.
Fig. 2 SEM micrographs of the Nd:YAG ceramics sintered at 1750°C for 50h: (a) polished and thermally etched surface; (b) fracture surface.
3.2 Analysis of transmission and absorption spectra
Figure 3 shows the in-line transmittances of Nd:YAG ceramics annealed at different temperatures. It is illustrated that the transmittance at wavelengths shorter than 300nm is strongly dependent on the annealing temperatures. At 1250°C the transmittance at wavelengths longer than 300nm is very high because most of color centers are already eliminated. With the increase of annealing temperature from 1250°C to 1450°C, the absorption peaks at 254nm and 262nm become stronger. When the temperature is higher than 1450°C, the transmittance at the two peaks changes slightly.
Figure 4 shows the absorption spectra of the Nd:YAG ceramics annealed at 1450°C for different time. It is noted that the absorption of the unannealed specimen is very strong due to the large amount of color centers in the specimen. The effect of annealing to eliminate defects is obvious. Even when the holding time is only 2h, the absorption is greatly reduced at visual and near-infrared wavelengths except for those shorter than 300nm. As the holding time increases, more and more color centers are compensated. The absorption is already very weak at the wavelengths longer than 300nm when the holding time is more than 5h.
GDMS result shows that there is 3.3ppm iron in the ceramic specimens. We contributed the absorption peaks at 254nm and 262nm to the Fe3+ ion. It is similar to the absorption at 256nm in other work [26–30]. Vacuum sintering may introduce O- and color centers caused by oxygen vacancies associating with free electrons expressed as:
In this process, the Fe3+ ion existing originally in the specimen can capture free electrons and degrade to be Fe2+. During air annealing, the oxygen in the air can easily diffuse into the ceramics and compensate the oxygen vacancies. And the Fe2+ ion will lose an electron to be a Fe3+ ion. The reversible process can be illustrated as follows:
Though the concentration of Fe3+ ion is low, it still plays an important role in the optical properties of the specimens. For the specimen annealed at 1250°C, the absorptions at 254nm and 262nm are weaker than those annealed at other temperatures. The same phenomenon was also observed in the specimens unannealed or annealed for a short time. It is because that there are a few Fe2+ ions changing into Fe3+ ions. Higher temperature leads to faster diffusion rate. The longer annealing time contributes to the enough diffusion of oxygen atoms into the sites of oxygen vacancies. The air annealing oxidizes Fe2+ to Fe3+ and also fills oxygen vacancies providing oxygen ligands in the neighborhood of Fe3+, which enhances the charge transfer between iron and oxygen ions. Thus, the absorption bands at 254 and 262 nm, which are attributable to a Fe3+ charge-transfer band and made up of contributions from substitutional Fe3+ ions in octahedral and tetrahedral sites, became stronger.
3.3 Analysis of fluorescence spectra
By exciting at a wavelength of 355nm, the fluorescence emission spectra for all the annealed specimens in the range of 1040nm-1090nm were obtained and all the spectra are almost the same. Figure 5 shows the fluorescence emission spectrum of the Nd:YAG ceramics annealed at 1450°C for 10h. The photoluminescence peak is centered at about 1064nm. The fluorescence lifetimes of 4F3/2 in Nd3+ in the specimens annealed at different temperatures were obtained by fitting the decay curves shown in Fig. 6 into a single exponential function of Eq. (1). The decay curves of the specimens annealed at different time are also very similar. The fluorescence lifetime changes from 184.6μs to 191.8μs from the fitting results. So it seems the annealing temperature and holding time have no significant influence on fluorescence lifetime of Nd:YAG ceramics. On the other hand, the defects in specimens are not fluorescence absorption center neither nonradiative trapping.
where I is the fluorescence intensity, t is the decay time and τ is the fluorescence lifetime.3.4 Analysis of laser performance
Figure 7 shows the laser output power of the Nd:YAG ceramics annealed at different temperatures for 10h. Laser outputs with different efficiencies were realized in all the specimens. The highest slope efficiency of 37.37% and the highest laser output power of 2.29W were obtained for the specimen annealed at 1450°C. Quenching occurs in all specimens after obtaining highest output power. At high pumping power, the thermal lens effect is very strong. When the thermal lens develops to a certain extent, the laser cavity will be unstable resulting in the quenching. The laser slope efficiency has a good accordance with the transmittance of the specimen annealed at low temperature. Taking in account the fluorescence emission spectra and decay times of different samples are nearly equal, it is reasonable to explain the difference in slope efficiency with influence of annealing on transparency in IR- diapason. Nd:YAG ceramic annealed at 1450°C has the highest transparency in this area (Fig. 3), thus there is no wonder the slope efficiency of this sample is highest. The laser slope efficiency decreases when the annealing temperature is higher than 1450°C, and it may be because the small cavities developing at high annealing temperature and the existence of Y2Si2O7 phase lead to light scattering [31,32].
Fig. 7 The laser output power of the Nd:YAG ceramics annealed at different temperatures for 10h versus the incident pumping power.
Figure 8 shows the laser output power of the ceramic specimens annealed at 1450°C for different time. It is shown that the holding time also plays an important role in improving the laser performance of the specimens. The laser slope efficiencies of the specimens unannealed or annealed for a short time are relatively low. When the annealing time is more than 5h, the laser slope efficiency trends to stabilization. It is illustrated that the specimens maintain a better optical property when the annealing time is longer than 5h. Considering the energy consumption and fabrication efficiency, 8h is suggested as the best holding time in this work.
Fig. 8 The laser output power of the Nd:YAG ceramics annealed at 1450°C for different time versus the incident pumping power.
It shows the important influence of color centers in the ceramic specimens on the laser behavior, especially the laser slope efficiency. The reason can be summarized by following aspects. First, the existence of color centers increases the absorption to pumping light. This reduces the effective absorption of Nd3+ ions. Second, impurity color centers, especially Fe3+ ion color centers, enhance the non-radiative energy transfer of Nd3+ ion excited state and decrease the conversion quantum efficiency of Nd3+ ion luminescence. Finally, color centers can easily introduce thermal induced birefringence and thermal lens effect. It would increase the laser intracavity loss. The physical principles of influence of the Fe2+ ions and oxygen vacancies on the laser emission needs more detailed explanation.
However, the relation between fluorescence curves and laser output seems not close. It can be explained as follows: First, the influence of annealing temperature and time on fluorescence lifetime is slight. The current test condition couldn’t distinguish the tiny changes. Second, the most defects in ceramic specimens may be not fluorescence absorptive. So the influence of fluorescence spectra on laser behavior is not strong.
4. Conclusions
Nd:YAG transparent ceramics with high optical transparency were fabricated by a solid-state reactive sintering method. It is noted that annealing can eliminate color centers caused by oxygen vacancies significantly. The annealing temperature and time play a role in modulating the laser performance of Nd:YAG ceramics. It is suggested that the optimal annealing condition of 1450°C and 8h could improve the Nd:YAG ceramic laser efficiency. This work helps a better understanding of the importance of annealing to improve the laser performance. Further-more, we will investigate the detailed change of defects in specimens by other characterization methods.
Acknowledgments
This work was supported by the Major Program of National Natural Science Foundation of China (Grant No. 50990301),the Key Program of National Natural Science Foundation of China (Grant No. 91022035) and the Project for Young Scientists Fund of National Natural Science Foundation of China (Grant Nos. 51002172 and 51302298).
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