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This paper demonstrates that annealing can greatly improve the quality of Cr-sensitized Nd:LuAG (Cr,Nd:LuAG) ceramics. Additionally, the pumping efficiency of Nd:LuAG ceramics can be improved by co-doping with Cr3+ ions. The two broad and strong absorption bands around 600 nm (4A2- 4T2) and 440 nm (4A2- 4T1) caused by Cr3+ ions act as effective sensitizers to absorb and convert UV and visible light from a flash lamp. Furthermore, a near-infrared absorption band appears in the in-line transmittance spectra of Cr,Nd:LuAG ceramics. The XPS spectra show that this band is caused by Cr2+ ions. The effect of annealing is studied, and it is found that the near-infrared absorption could be eliminated by annealing in air at 1450 °C for 25 h. Furthermore, the effective fluorescence lifetime of Nd3+ ions in Cr co-doped LuAG ceramics increases from 211.28 μs to 553.80 μs when pumped at 440 nm.
© 2015 Optical Society of America
Corrections
29 September 2015: A correction was made to the author affiliations.
1. Introduction
High-performance laser materials are important for inertial confinement fusion (ICF). As the gain medium of ICF, such materials should have a moderate emission cross section, high thermal conductivity, excellent optical homogeneity, etc. Neodymium glass is a very important type of laser material, and at present, most laser systems use it as the gain medium. However, the pumping efficiency of Nd3+ in glass is generally only 2–3% [1–3], whereas laser diodes (LDs) can pump efficiently. However, because lamp pumping is more cost effective than LD pumping, it would be beneficial to improve the efficiency of lamp-pumped Nd3+ solid-state lasers. Moreover, the thermal conductivity of neodymium glass is only 0.83 W/(m·K). Thus, it is not suitable as a gain medium for ICF. Accordingly, it is crucial to explore new laser materials that possess a high pumping efficiency and a high thermal conductivity.
Transparent ceramics have many advantages [4,5] and have attracted significant attention since Ikesue et al. reported on fabricated Nd3+-doped YAG laser ceramics [6] with excellent optical quality. Lutetium aluminum garnet (LuAG) is another type of garnet with the same structure as YAG, and Nd3+-doped LuAG has many obvious advantages, such as a moderate emission cross section (~9 × 10−20 cm2) and a high thermal conductivity (9.6 W/(m·K)) [7,8]. Moreover, it has potential applications in ICF as a gain material [9]. The two broad absorption bands of Cr3+ ions match the emission spectra of Xe flash lamps and solar light [10,11]. Therefore, Cr3+ that is co-doped as a sensitizer can improve the efficiency of lamp-pumped Nd-doped lasers. Cr3+ and Nd3+-co-doped YAG (Cr,Nd:YAG) [12,13] and GSGG (Cr,Nd:GSGG) [14] laser systems have been developed, and performance-based advantages, such as the high pumping efficiency and long efficient lifetime of Cr3+ and Nd3+-co-doped materials, have been observed [15–17]. Compared with Nd3+-doped laser materials, Cr3+ and Nd3+-co-doped laser materials are more suitable for ICF.
High-quality laser ceramics can be fabricated by vacuum or hot isostatic pressing (HIP) sintering processes [18–20]. Unfortunately, there is a strong and wide near-infrared absorption band when Cr3+ and Nd3+ ions are co-doped in LuAG ceramics. In addition, the lifetime of Nd3+ ions decreases dramatically because of this absorption band. Annealing is an effective method to solve these problems. Therefore, it is necessary to comprehensively study the effect of annealing on the quality of laser ceramics.
The purpose of this study is to investigate the effect of annealing on the in-line transmittance spectra and effective fluorescence lifetimes of sintered Cr,Nd:LuAG laser ceramics, and to clarify the energy transfer process, especially in regard to the mechanism by which Cr3+ ions influence the fluorescence lifetime of Nd3+ ions in Cr, Nd:LuAG laser ceramics. In this study, the transformations of the effective fluorescence lifetime and the near-infrared absorption were analyzed by considering the fluorescence and absorption characteristics of Cr,Nd:LuAG laser ceramics before and after annealing.
2. Experimental
The Cr,Nd:LuAG laser ceramic samples were fabricated according to the vacuum sintering and HIP processes. The fabrication method consists of the following steps, as shown in Fig. 1.
High-purity powders of Cr2O3 (99.9%), Al2O3 (99.99%), Nd2O3 (99.99%), and Lu2O3 (99.99%) were used as raw materials, whereas tetraethyl orthosilicate (TEOS, 99.9%) and Sc2O3 (99.99%) were used as sintering aids. The raw materials were mixed uniformly by ball-milling with alcohol (99.99%) as a ball milling assistant for 12 h, dried for 5 h at 70 °C, sieved, dry-pressed at 3 MPa, and then cold-isostatic pressed (CIP) at 210 MPa.
The Cr,Nd:LuAG green bodies were first vacuum sintered at 1750 °C for only 5 h under a vacuum of 10−3 Pa, and then HIP sintered at 1750 °C and 200 MPa in an argon atmosphere for 5 h. After HIP sintering, the products were annealed at 1450 °C for 25 h in air to optimize the quality of the Cr,Nd:LuAG laser ceramic materials. The samples were mirror-polished on both surfaces, and in-line transmittance spectra were studied using UV/VIS/NIR spectrometry (Lambda 750, PerkinElmer, Inc., U.S.A.). The valence states of the Cr ions were studied using Kα X-ray photoelectron spectroscopy (XPS, Thermo Fisher Scientific, U.S.A.), whereas the fluorescence spectra were measured by using a triax-320-type spectrometer (Jobin-Yvon Co., France); the effective fluorescence lifetimes were subsequently fitted.
3. Results and discussion
Figure 2 shows in-line transmittance spectra of vacuum-sintered and both vacuum- and HIP-sintered Cr,Nd:LuAG ceramics. The in-line transmittance of HIP-sintered Cr,Nd:LuAG ceramics was greatly improved, and the letters under the specimen could be seen distinctly. The in-line transmittance of the specimen increased from 30.43% to 82.25% at 840 nm, and from 30.33% to 83.44% at 710 nm because the HIP method resulted in a reduced number of pores in the Cr,Nd:LuAG ceramic materials. As shown in Fig. 2, a strong absorption band around 1064 nm exists in the vacuum-sintered sample as well as the HIP-sintered sample. A comparison with the in-line transmittance spectrum of Nd:LuAG [7] suggests that the band localized near 1 μm can be attributed to chromium. The strong absorption band around 1 μm causes serious losses of laser output because the laser output wavelength of Nd:LuAG ceramics is 1064 nm. There are three possible valence states of Cr ions: Cr2+, Cr3+, and Cr4+. In the process of fabricating Cr,Nd:LuAG ceramics, the specimens were sintered in a vacuum and held in a tungsten mesh, which provided a reducing atmosphere. In addition, SiO2 was used as a sintering aid, and thus there were more electrons present than required for charge balance. Therefore, it was impossible to oxidize Cr3+ to Cr4+. The process of sintering the Cr,Nd:LuAG ceramics involves the following chemical reaction equations:
Fig. 2 In-line transmittance spectra of vacuum-sintered and hot isostatic pressing (HIP)-sintered Cr,Nd:LuAG ceramics. The thickness of both samples was 3.5 mm.
The XPS spectra of samples of HIP-sintered Nd,Cr:LuAG ceramics and annealed Nd,Cr:YAG ceramics are shown in Figs. 3 and 4, respectively. In Fig. 3, the Cr 2p3/2 spin–orbital splitting photoelectrons for Cr-doped Nd:LuAG ceramic materials located at binding energies of 586.0 eV and 576.5 eV could be assigned to Cr3+ [21]. Furthermore, the binding energy of chromium 2p3/2 was near 575.5 eV, indicating the Cr2+ state in HIP-sintered samples. Only the binding energy of chromium 2p3/2, which indicates the Cr3+ state, was observed in the annealed samples, as shown in Fig. 4. After annealing, the energy peak of the Cr2+ state of XPS for Cr,Nd:LuAG ceramics disappeared. Almost all Cr2+ produced in the process of sintering Cr,Nd:LuAG ceramics was oxidized to Cr3+ during the process of annealing in air. Thus, the near-infrared absorption was caused by the Cr2+ ions, and the Cr2+ ions can be eliminated by annealing.
Fig. 3 X-ray photoelectron spectroscopy (XPS) spectrum for Cr in hot isostatic pressing (HIP)-sintered Cr,Nd:LuAG ceramics.
Figure 5 shows photographs and in-line transmittance spectra of Cr,Nd:LuAG specimens before and after annealing. The color of the specimen before annealing was yellowish-green, which was caused by Cr2+ ions and oxygen vacancies. The differential spectrum in Fig. 5 shows that the absorption band between 200 nm and 400 nm was caused by oxygen vacancies, whereas the near-infrared absorption band was caused by Cr2+ ions. And after annealing, the color of the sample became different, due to the shift of absorption bands in visible range. As shown in Fig. 6, the absorption coefficient of HIPed sample from 380 to 435nm and around 500nm is stronger than that of annealed samples and annealed sample has a stronger absorption band than HIPed sample around 610nm, thus, the color of the samples is different.. Thus, for the Cr,Nd:LuAG ceramic specimen, after annealing in air at 1450 °C for 25 h, higher-transparency Cr,Nd:LuAG ceramics were obtained. More specifically, the in-line transmittance of the specimen increased from 82.25% to 83.44% at 840 nm, and from 53.13% to 84.38% at 1064 nm; moreover, the near-infrared absorption was completely eliminated as shown in Fig. 5. The chemical reaction equation during annealing can be described as follows:
Fig. 5 In-line transmittance spectra of hot isostatic pressing (HIP)-sintered Cr,Nd:LuAG ceramics before and after annealing in air. The thickness of both samples was 3.5 mm.
Fig. 6 Absorption spectra of HIP-sintered Cr,Nd:LuAG ceramics before and after annealing in air. (The thickness of both samples was 3.5 mm).
Figure 7 shows a comparison of the effective fluorescence lifetime of Nd ions(4F3/2- 4I11/2)in Cr,Nd:LuAG ceramic samples before and after annealing. When the sample was pumped at 590 nm, the effective fluorescence lifetime of the specimen before annealing was measured to be 94.70 µs, and the effective fluorescence lifetime increased to 378.03 µs by annealing the sample in air at 1450 °C for 25 h. The effective fluorescence lifetime of Nd:LuAG ceramics was only 222.34 µs. When the sample was pumped at 440 nm, the effective fluorescence lifetime of the sample before annealing was measured to be 139.46 µs, whereas the effective fluorescence lifetime increased to 553.80 µs after annealing the sample according to the process above. The effective fluorescence lifetime of the Nd:LuAG sample was only 211.28 µs. In addition, the effective fluorescence lifetimes of the annealed Cr,Nd:LuAG and Nd:LuAG ceramics were almost identical when the samples were excited at 808 nm. Thus, the absorption bands in the UV and visible regions (4A2- 4T1 and 4A2- 4T2) had a dominant effect on the effective fluorescence lifetime of Cr,Nd:LuAG laser ceramics.
The energy transfer process in Cr,Nd:YAG single crystals was reported by Taylor in 1967 [22], whereas the energy transfer process in Cr,Nd:YAG materials was reported by Y. Honda et al. in 2014 [1]. Cr,Nd:LuAG has broad absorption bands around 600 nm (4A2- 4T2) and 400 nm (4A2- 4T1). As shown in Fig. 8, when energy is absorbed by the absorption band in the UV region (4A2- 4T1), part of the energy is first transferred to the 4T2 or 2E level, and then transferred to the 4F3/2 band of Nd3+. At this stage, the upper-state population is transferred to the 4I11/2 state, and light is emitted at 1064 nm. The effective fluorescence lifetime τeff (2E→energy transfer→4F3/2→4I11/2) of the annealed Cr,Nd:LuAG ceramic was longer than that of the Nd:LuAG ceramic, in accordance with the long lifetime of Cr3+ ions (the 2E level) and the energy transfer time τTr. The lifetime of the 2E level is very long and the population can exist in the 2E and 4I11/2 states at the same time. Thus, when the excitation ceased, the population was still able to exist in the 4T2 or 2E state, and was transferred to the 4F3/2 band of Nd3+ over a long period of time. Furthermore, the energy transfer time τTr (2E→energy transfer→4F3/2), which was derived from Eq. (4) [1], Eq. (5) [1], and Eq. (4) was refashioned in the form of Eq. (6).
In this process, Y. Honda measured τCr(Nd) and τCr, calculated ηTr using Eq. (5), and reached the conclusion that ηTr is approximately 65% [1]. We calculated τTr to be about 650 µs according to Eq. (6). The effective fluorescence lifetime τeff includes the energy transfer time τTr and the direct lifetime τNd (4F3/2→4I11/2). Therefore, the effective fluorescence lifetime τeff of the Cr,Nd:LuAG ceramic is much longer than that of the Nd:LuAG ceramic.
4. Conclusions
High-quality Cr,Nd:LuAG laser ceramics were successfully fabricated by annealing sintered samples. The specimen that was first vacuum sintered at 1750 °C for only 5 h under a vacuum of 10−3 Pa and then HIP-sintered at 1750 °C in argon at 200 MPa for 5 h had reached the maximum density, but showed a strong and wide absorption band around 1064 nm. This strong absorption band caused serious losses in terms of laser output, and decreased the effective fluorescence lifetime of the sample. Annealing completely eliminated the near-infrared absorption, and the in-line transmittance of the specimen reached 84.38% near the lasing wavelength of 1064 nm, which is very close to the theoretical value for LuAG laser ceramics. Furthermore, the effective fluorescence lifetime of Cr,Nd:LuAG ceramics significantly increased to 553.8 µs when pumped at 440 nm.
Acknowledgment
The work is financially supported by the National Nature Science Founds of China (No.61378069, 51102257, 51302284), Shanghai City Star Program (No.14QB1400900, 14QB1402100).
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