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Yb3+ ion doped highly transparent Lu3Al5O12 (LuAG) ceramics were fabricated by a pressure-less sintering in H2 atmosphere starting from nanocrystalline powder derived from a wet chemical processing. A fully transparent Yb:LuAG ceramic was densified under 1750 °C for 10 hours with a linear optical transmittance of 83.1% at the wavelength of 800 nm. The average grain size of obtained transparent ceramics was only 1μm with nearly uniform distribution of the nanocrystals. A 0.91 W continuous wave laser output was achieved with a 3at.%Yb:LuAG ceramic at room temperature, when pumped with a 940 nm single-emitter laser diode. The absorbed pump power threshold was only about 0.29 W, and the best optical to optical efficiency was 58%.
© 2017 Optical Society of America
1. Introduction
Compared with YAG host material, ytterbium ions doped lutetium aluminum garnet (Yb:Lu3Al5O12, Yb:LuAG) is characterized by favorable properties as high Zeff, high density, high doping concentrations and more outstanding thermal conductivity [1–4]. Because of the above advantages, the Yb:LuAG materials were widely used in solid state lasers, such as ceramics and crystals [5–7].
Nowadays, there are two main routes to fabricate laser grade transparent ceramics. One is the solid state reaction vacuum sintering, represented by Dr. A. Ikesue, World Lab. In this route, the high purity raw sesquioxide powders are accurately weighted according to the stoichiometric ratio, and then ball milled, formed and sintered in vacuum. The first Nd:YAG transparent ceramic of laser quality was fabricated in 1995 [8]. Since then solid state reaction vacuum sintering was wildly used in YAG and LuAG transparent ceramics fabrication [9,10]. Jian Zhang et al. reported the first Yb:LuAG ceramic laser fabricated by using solid state reactive vacuum sintering method in 2012 [5,11]. In the experiments, 0.5wt.% tetraethyl orthosilicate (TEOS, Sigma-Aldrich, 99.999%) was used to introduce SiO2 as sintering aids. The maximum laser output of as fabricated 5at.%Yb:LuAG ceramic reached 7 W. The other is the nanocrystalline vacuum sintering route represented by Konoshima Chemical Co., Ltd. In this route, wet chemical derived nanocrystalline powders are synthesized, and then ball milled, formed and sintered in vacuum [12]. Highly transparent ceramic and laser performances were reported in many kinds of material systems by this method [13,14]. Hiroaki Nakao et al. reported the first CW Yb3+:Lu3Al5O12 ceramic laser using nanocrystalline vacuum sintering technique in 2012 [15]. The efficient CW laser operation was obtained with maximum output power of 2.14 W. However, there are not too many details about Konoshima’s nanocrystalline vacuum sintering technology or other researches on the fabrication of Yb:LuAG ceramics by wet-chemical processing were reported, as the fabrication of laser grade Yb:LuAG ceramics still remains a challenge [5].
In this paper, we reported a mixed precipitant derived powder (MPP) and pressure-less sintering in H2 atmosphere (PLSH) technology to fabricate high optical quality LuAG ceramic. Laser grade Yb:LuAG ceramics were fabricated by this processing for the first time to our knowledge. Fully transparent Yb:LuAG ceramics were densified under 1750°C for 10 hours with a linear optical transmittance of 83.1% at the wavelength of 800nm. The microstructures of as synthesized MPP powder and PLSH ceramic were characterized by SEM and TEM. The average grain size of the transparent ceramic was only 1μm measured by line interception method, which was much finer than solid state reaction vacuum sintering route (tens of micron meters) [9] or the nanocrystalline vacuum sintering route (several micron meters) [16]. The laser performance of as fabricated 3at.%Yb:LuAG transparent ceramic was reported as well.
2. Material and experiment
(1) Powder synthesis
The Yb doped LuAG powders were synthesized by a wet chemical approach, using ammonium hydroxide (NH4OH) and ammonium hydrogen carbonate (NH4HCO3) as the mixed precipitator. The starting chemicals used were commercial high purity powders without further purification: commercial lutetium oxide (Lu2O3, 5N), ytterbium oxide (Yb2O3, 5N), nitric acid (HNO3, GR), aluminum nitrate (Al(NO3)3, AR), ammonium hydrogen carbonate (NH4HCO3, AR) and ammonium hydroxide (NH4OH, AR). Aqueous solution of (Lu3+, Yb3+, Al3+) were prepared by dissolving these solid powders into hot HNO3 or deionized water, according to the atomic composition of (Yb0.03Lu0.97)3Al5O12. Finally, 3at.%Yb-doped LuAG precursors were synthesized by a wet chemical co-precipitant processing, using reverse-strike precipitation by adding the aqueous solution containing Lu3+, Yb3+ and Al3+ into mixed precipitator of NH4HCO3 and NH4OH. After aged, filtered, washed with deionized water and alcohol for several times, the as synthesized precursors were calcined in a muffle furnace to obtain the polycrystalline Yb:LuAG powders.
(2) Ceramic fabrication
As calcined Yb:LuAG powders were preformed into green compacts in a stainless steel die, and then the green compacts were cold isostatic pressed (CIP) under 200 MPa. The relative green body density was around ~50% by measuring the weight and diameter. At last, the CIPed green bodies were sintered in a furnace with a tungsten-mesh heater at 1750 °C for 10 hours in flowing H2 atmosphere. In the process of sintering, the heating rates used in this experiment were 10°C/min from room temperature to 1300 °C and 2 °C/min from 1300 °C to 1750 °C. After the sintering was finished, the ceramic samples were optical polished on both sides for the upcoming measurements.
(3) Measurements
Phase composition identification was performed with X-ray diffractometer (D\max-2550, Rigaku, Japan) equipped with graphite monochromatized Cu Kα radiation (λ = 1.5406 Å, 40 kV/200 mA) in the range of 2θ = 10-70°. The microstructure of the LuAG powder and the sintered transparent ceramics were observed by field emission scanning electron microscopy (XL-30, Philips, Netherlands) and transmission electron microscopy (JEM2100F, JEOL, Japan). The linear optical transmittance of Yb:LuAG ceramics was measured over the wavelength region of 200-1200 nm on a UV/Vis/NIR spectrophotometer (Agilent Cary5000, Palo Alto, USA).
3. Results and discussion
Figures 1(a), 1(b) and 1(c) show the SEM micrographs of the Yb:LuAG powders calcined at different temperatures. SEM observations demonstrate that the average primary particle size of Yb:LuAG polycrystalline powders is 50 nm, 100 nm and 150 nm, when the calcination temperature was 1100 °C, 1200 °C, and 1300 °C separately. Some agglomerations still exist in Fig. 1(a) and the particle profile of Yb:LuAG powders calcined at 1100 °C is not as clear as that of the other two powders calcined at higher temperatures as shown in Fig. 1(b) and 1(c). Figure 1(d) displays the XRD patterns of Yb:LuAG powders calcined at different temperatures for 2 h. All of these XRD patterns exhibit single cubic garnet phase and no other impurity phases were detected. The characteristic peaks are in good agreement with the standard PDF cards (LuAG JCPDS: 73-1368). There are no differences between these patterns except for the XRD intensity.
Fig. 1 SEM of Yb:LuAG polycrystal powders calcined at various temperatures (a) 1100°C/2h, (b) 1200°C/2h, (c) 1300°C/2h, and (d) the comparison of X-ray diffraction patterns calcined at various temperatures.
Transmission electron microscopy observation provides further insight into the size and morphology details of synthetic Yb:LuAG powders. Figures 2(a) and 2(b) display TEM micrographs of as-prepared Yb:LuAG powders calcined at 1200 °C and 1300 °C, respectively. The primary particles observed from the TEM are about 100 nm and 150 nm separately. Both of the two powders show clear crystalline in grain profiles. Powder homogeneity is a key parameter on ceramic sintering. While the calcination temperature is increased to 1300 °C, the adjacent particles slightly connected with necks grow to be a whole large particle in local areas, marked by red circle in Fig. 2(b). Therefore, the powder calcination temperature is maintained at 1200 °C for 2 hours.
Figure 3 shows the optical transmittance of the 3at.% Yb:LuAG ceramic, polished on both sides (2.5 mm in thickness) on the wavelengths range from 200 to 1200 nm. Inset is the appearance of the Yb:LuAG transparent ceramic sintered in reducing H2 at 1750 °C for 10 hours. From the transmittance curve, it can be seen that the linear transmittance increases from 80.6% to 83.1% in the visible wavelength range of 400-800 nm, and the maximum transmittance reaches 83.5% at 1200 nm wavelength. The absorption bands located at 913, 933, 965 and 1026 nm wavelengths originated from the absorptions of Yb-ions in the measurement wavelength. The high linear optical transmittance indicates that the nanocrystalline pressure-less sintering in H2 (NC-PLSH) route is a promising technique for preparing Yb:LuAG transparent ceramics with high optical quality.
Fig. 3 Inline optical transmittance curves of optical polished 3at.%Yb:LuAG ceramic. (Inset: Appearance of the Yb:LuAG transparent ceramics before and after polishing).
Figures 4(a) and 4(b) illustrate the SEM micrograph of the fractured surfaces and thermally etched surface of as fabricated Yb:LuAG transparent ceramic. Figure 4(c) and (d) are the corresponding magnification photographs. The average crystalline size of the transparent Yb:LuAG ceramic is about 1μm, measured by line interception method. The morphology in Fig. 4(a) and 4(c) proves that the fracture mode of Yb:LuAG transparent ceramics is intercrystalline fracture. No obvious micro pores and impurities are observed at the grain boundary or within grains as shown in Fig. 4.
Fig. 4 (a) Scanning electron micrograph (SEM) image of the fracture surface of Yb:LuAG transparent ceramic; (b) SEM image of the polished and thermally etched surface; (c) magnification microscopy of fractures; and (d) magnification microscopy of the etched surface.
Continuous wave laser is demonstrated by pumping the plane-paralleled 3at.%Yb:LuAG transparent ceramic as gain medium. The laser experiments are carried out at room temperature without actively cooling the working elements. The output power is measured with a Thorlabs PM200 power meter. The laser emitting spectra of the lasers are measured with an Anritsu optical spectral analyzer (MS9740A). The pump source used is a 940 nm single-emitter laser diode, and the emitting cross section of single-emitter laser diode is 1 × 50 μm2. Two lens with 8 mm focal length are used to collimate and focus pump beam on Yb:LuAG ceramic. The incident pump beam area after optical coupling system is about 80 × 80 μm2. To reduce the intracavity loss, one surface of Yb:LuAG ceramic is coated with anti-reflection at 940 nm and high reflection at 1030 nm to act as rear cavity mirror, and the other surface of Yb:LuAG ceramic is coated with anti-reflection at 1030 nm. Three plane-parallel mirrors with reflectivity of 80%, 90% and 98% are used as output couplers.
Figure 5 shows that the absorbed pump power thresholds are 0.29 W, 0.32 W and 0.37 W for ROC = 98%, 90% and 80%, respectively. The absorbed pump power thresholds are 0.29 W, 0.32 W and 0.37 W for ROC = 98%, 90% and 80%, respectively. The output power increases linearly with the absorbed pump power for different output couplings when the absorbed pump power is well above the absorbed pump power threshold. The best laser performance is achieved with ROC = 90%. The maximum output power of 0.91W is obtained at the available maximum absorbed pump power of 1.55 W for ROC = 90%, corresponding to an optical-optical efficiency of 58%.
4. Conclusions
Yb doped LuAG nanocrystalline powder was synthesized by a mixed precipitant derived powder (MPP). The powders showed good sinterability after calcined at 1200 °C for 2 hours, corresponding to a primary particle size of about 100 nm. Laser grade Yb:LuAG transparent ceramics (83% inline transmittance@800 nm) were obtained at 1750 °C for 10 hours by nanocrystalline pressure-less sintering in H2 (NC-PLSH). The average crystal grain size was only 1μm measured by line interception method, which was much finer than that of the results reported previously.
CW Laser performances of as fabricated 3at.%Yb:LuAG transparent ceramic were demonstrated by pumping the gain media with a 940 nm single-emitter laser diode. A 0.91 W continuous wave laser output was realized without water cooling. The absorbed pump power threshold was only about 0.29 W and the best optical to optical efficiency was 58%. In the following experiment, the optimization of the pumping source and cooling conditions of Yb:LuAG transparent ceramics fabricated by NC-PLSH technology will be demonstrated to get more powerful and higher efficient laser output.
Funding
National Natural Science Foundation of China (Grant No.51572175).
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