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Laser oscillation was demonstrated using a 1 at.% Nd3+-doped Lu2O3 (Nd3+:Lu2O3) transparent ceramic produced by spark plasma sintering. Nd2O3, Lu2O3, and LiF commercial powders were mixed by ball milling and were sintered at 1723 K using a two-step sintering profile. After the transparent Nd3+:Lu2O3 ceramic was annealed in air, its transmittance at 1076 nm reached 81.8%, which was close to the theoretical value for Lu2O3 (82.2%). The absorption cross-section at 806 nm was 1.29 × 10−20 cm2, and the fluorescence decay time at 1076 nm was 229 μs. The laser oscillation of Nd3+:Lu2O3 ceramic for the transition from 4F3/2 to 4I11/2—specifically, at 1076.7 and 1080.8 nm—was simultaneously obtained, with a laser output of 0.21 W and slope efficiency of 14%.
© 2014 Optical Society of America
1. Introduction
Transparent polycrystalline ceramics for laser applications have been studied worldwide after Ikesue et al. demonstrated laser oscillations in a Nd3+-doped yttrium aluminum garnet (Nd3+:YAG) ceramic in 1995 [1]. Although YAG has been widely studied and used as a host material for solid-state lasers, materials possessing higher thermal conductivity and stability are required to enable scale-up to a high-power laser [2]. In recent years, lutetium oxide (Lu2O3) has attracted attention because of its thermal conductivity, which is higher than that of YAG and other sesquioxides (e.g., Y2O3 and Sc2O3) [3]. However, Lu2O3 is difficult to produce by conventional single-crystal synthesis methods because of its high melting point. Therefore, powder densification processes are promising routes to the fabrication of highly transparent Lu2O3 polycrystalline ceramics.
Laser oscillation has been obtained using transparent Lu2O3 ceramics fabricated by several powder densification processes. Lu et al. reported a laser oscillation at 1080 nm in Nd3+:Lu2O3 ceramic, with an output of 0.01 W and a slope efficiency of 12% [4]. Transparent Yb3+:Lu2O3 ceramics were widely studied with different Yb concentration for laser oscillations at different wavelengths and modes [5–10]. Laser oscillation in Lu2O3 doped with Ho3+ and Tm3+ ceramics were also investigated [11–13]. Among the doping ions, Nd ions has emission bands, for example, 917, 1076, 1080, and 1359 nm in Lu2O3, which can be widely used for laser applications on remote sensing, spectroscopy, and optical communications [14,15]. Nd-doped composite laser ceramics are promising in energy and automobile fields as laser ignition [16,17]. On the other hand, the transparent ceramics used in the literature were primarily fabricated by pressureless vacuum sintering processes and hot pressing combined with hot isostatic pressing using powders prepared by a wet chemical method. The drawbacks of these approaches are the prolonged sintering process and the use of homemade powders.
Among sintering techniques, spark plasma sintering (SPS) is a promising method for the fabrication of transparent ceramics. SPS is a fast densification process where sintering is activated by combination of uniaxial pressure and large electric current. A powder compact is heated rapidly by Joule heating of the graphite mold. An applied pressure promotes pore elimination through lattice and grain boundary diffusion. Therefore, SPS is advantageous to consolidate ceramics in a short time and at a low temperature with limited grain growth, resulting in an excellent transparency [18]. We have reported the preparation of transparent Lu2O3 ceramics by SPS using commercially available powders [19] and Boulesteix et al. recently reported highly transparent Nd3+:Lu2O3 fabricated by SPS using shaped precipitation powder by slip-casting [20]. However, there is rarely laser performance reported on Lu2O3 prepared by SPS. In this paper, we prepare highly transparent Nd3+:Lu2O3 ceramic by SPS from commercially available powders and demonstrate its laser oscillation.
2. Experimental procedures
Lu2O3 (Shin-Etsu Rare Earth, Tokyo, Japan; 99.99% purity), Nd2O3 (Wako Pure Chemical, Tokyo, Japan; 99.9% purity), and LiF (Wako Pure Chemical, Tokyo, Japan; 99.9% purity) commercial powders were used as starting materials. These powders were mixed in a Lu:Nd molar ratio of 99:1 with 0.2 wt.% LiF by ball milling using zirconia balls in ethanol for 12 h. The milled slurry was dried at 333 K for 24 h, and the obtained powder was ground and passed through a 200-mesh sieve. The powder was calcined at 1273 K in air for 7.2 ks, and then sintered using an SPS apparatus (SPS-210 LX, Fuji Electronic Industrial, Japan) in vacuum. A two-step profile of heating rate and pressure was used during sintering [19,21]. The sintering temperature was increased to 873 K in 180 s and to 1373 K in 300 s, and then held at 1373 K for 300 s. The temperature was further increased to 1723 K at 0.17 K s−1 and maintained at 1723 K for 2.7 ks. A pressure of 10 MPa was preloaded from room temperature to 1373 K and was subsequently increased to 100 MPa above 1373 K. After sintering, the specimen was annealed in air for 21.6 ks. Both sides of the sample were mirror-polished; the thickness of the sample was 1 mm.
The crystal phase was identified by X-ray diffraction (XRD, RAD-2C, Rigaku, Japan); the instrument was equipped with a graphite-monochromated Cu-Kα radiation source (0.154 nm) operated at 30 kV and 15 mA. Samples were scanned over the 2θ range of 10°–80°. The density was measured using the Archimedes method in distilled water. The sintered body was thermally etched at 1573 K in air for 3.6 ks. A field-emission scanning electron microscope (FESEM, JSM-7500F, JEOL, Japan) and a scanning electron microscope (SEM, S-3100H, Hitachi, Japan) were used to observe the ball-milled powder and the thermally etched surface of the sintered body. The average grain size was determined from the linear intercept length using SEM micrographs (under the assumption of the grain size to be 1.56 times the mean intercept) with at least 250 grains counted [22]. The in-line transmittance was measured using a spectrophotometer (UV-3101PC, Shimadzu, Japan) in the wavelength range 190–2500 nm. The fluorescence spectrum excited by an 808-nm laser diode was recorded by a spectrofluorometer (Fluorolog-3, Jobin Yvon, France) at room temperature. The fluorescence decay curve was measured at the wavelength of 1076 nm using the spectrofluorometer and a digital oscilloscope (TDS 3020, Tektronix, USA).
A schematic diagram of the experimental setup for laser oscillation is shown in Fig. 1. The pump source is a fiber-coupled diode laser (808 nm) with a core diameter of 100 μm and a numerical aperture of 0.22 (LIMO, Germany). The pump radiation was collimated and focused to a spot with a diameter of 130 μm. The input mirror was flat, and the laser output coupler was a dielectric mirror with a radius of curvature of 50 mm and reflectivity of 94.5% at 1076 nm. Transparent Nd3+:Lu2O3 ceramic cut into a 3 mm × 3 mm × 1 mm specimen was wrapped with indium film and mounted on a water-cooled copper holder, whose temperature was maintained at 290 K.
3. Results and discussion
Figure 2 shows the XRD pattern and FESEM image of the Nd3+:Lu2O3 powder. The XRD pattern can be indexed on the basis of cubic Lu2O3 (JCPDS #43-1021) (Fig. 2(a)). The powder was spherical and the average particle size was 80 nm (Fig. 2(b)).
Figure 3 shows the microstructure of the Nd3+:Lu2O3 ceramic sintered by SPS. The thermally etched surface showed polyhedral-shaped equiaxed grains with an average grain size of 8 μm. Pores were rarely observed, which indicated that the sample was highly dense. The microstructure was consistent with the relative density (99.5%) measured by the Archimedes method. The fracture surface was intergranular mode incorporated with transgranular mode.
Figure 4 shows the transmittance spectra of the transparent Nd3+:Lu2O3 ceramic before and after annealing. The transmittance of the annealed Nd3+:Lu2O3 ceramic increased at wavelength in the visible and near-infrared ranges (i.e. longer than 370 nm). For example, the transmittance at 1076 nm was 79.8% before annealing, whereas it reached 81.8% after annealing; this value approached the theoretical value (82.2%) based on the reported refractive index of Lu2O3 [23]. The inset in Fig. 4 shows photographs of the transparent Nd3+:Lu2O3 ceramic before and after annealing. The specimen before annealing exhibited a gray color, whereas it changed to blue after annealing; this blue color was attributed to the light absorption by the Nd3+ ions. Similar changes in color and improved transparency of transparent ceramics prepared by SPS have been reported for Lu2O3 [24], Al2O3 [25], and ZrO2 [26]. Oxygen vacancies might form because of the graphite mold and the reduced atmosphere during sintering and be compensated after annealing, resulting in color change and enhancement of transparency.
Fig. 4 Transmittance spectra of Nd3+:Lu2O3 ceramic before and after annealing. The theoretical transmission of Lu2O3 was calculated from the data in [23]. The inset shows photographs of the specimens before (left) and after (right) annealing.
Figure 5 shows absorption cross-section spectrum of the transparent Nd3+:Lu2O3 ceramic after annealing. Strong absorption bands in the near-infrared range were located at 806 and 822 nm, which were assigned to the transitions from ground state (4I9/2) to 2H9/2 and 4F5/2 excited states. The absorption cross-sections of these two bands for the specimen after annealing were 1.29 × 10−20 cm2 and 1.77 × 10−20 cm2, respectively. This result is accordance with that in the Nd3+:Lu2O3 single crystal (1.97 × 10−20 cm2 at 806 nm for 1 at.% Nd3+) [15].
Figure 6(a) shows the emission spectrum of the Nd3+:Lu2O3 ceramic pumped by a diode laser (808 nm) at room temperature. The emission bands in the 870–970 nm, 1040–1160 nm, and 1300–1500 nm ranges corresponded to the transitions from 4F3/2 to 4I9/2, 4I11/2, and 4I13/2 states of Nd3+, respectively. The strongest emission peaks were located at 1076 and 1080 nm and were attributed to the 4F3/2 → 4I11/2 transition. The fluorescence spectrum of the Nd3+: Lu2O3 ceramic prepared in this study is the same as that of the Nd3+:Lu2O3 transparent ceramic [4] and single crystal [15]. Figure 6(b) shows the fluorescence decay curve at 1076 nm of the Nd3+:Lu2O3 ceramic. It can be fitted by first-order exponential formula and the resulting decay time was 229 μs, which is similar to that in the Nd3+:YAG ceramics (224–238 μs for 1 at.% Nd3+) [27–29] and greater than that in the Nd3+:Y2O3 ceramic (158 μs for 1 at.% Nd3+) [30]. The decay time declined to 29 μs for the Nd3+Lu2O3 ceramic at a higher Nd3+ concentration (5 at.%).
Fig. 6 (a) Emission spectrum of Nd3+:Lu2O3 ceramic pumped by an 808-nm diode laser and (b) fluorescence decay curve at 1076 nm of Nd3+:Lu2O3 ceramic.
Figure 7 shows the laser oscillation of the transparent Nd3+:Lu2O3 ceramic prepared by SPS before and after annealing. Two lines located at 1076 and 1080.8 nm were attributed to simultaneous laser oscillations because of their quite similar fluorescence emission intensity, as shown in Fig. 6 [4,15]. For the specimen before annealing, the threshold absorbed pump power was 1.47 W and the maximum output power was 0.03 W. After annealing, the threshold absorbed pump power decreased to 0.35 W and the obtained maximum output power increased to 0.21 W at an absorbed pump power of 1.74 W. The slope and optical–optical efficiencies were 14% and 12%, respectively. The slope efficiency in the present study is comparable to that of Nd3+:Lu2O3 ceramic (0.15 at.% Nd3+) [4], which is 12%, and an Nd3+:Lu2O3 single crystal (1 at.% Nd3+), which is 17.3% [15]. This preliminary laser performance confirms that SPS is a promising method for the preparation of laser-quality transparent ceramics. SPS has a scalability potential. Frage et al. prepared larger transparent YAG ceramics 20 mm in diameter and 5 mm in thickness by SPS [31]. For industrial implementation, a large-scale SPS apparatus for mass production system has been developed [32]. The scalability of SPS has been studied in both experimental and analytical aspects [33–36].
4. Conclusions
Highly transparent Nd3+:Lu2O3 ceramic was fabricated by SPS using commercial powders. The transmittance at 1076 nm was 79.6% before annealing and reached 81.8% after annealing in air, which was close to the theoretical value for Lu2O3 (82.2%). Laser oscillation was demonstrated for the transparent Nd3+:Lu2O3 ceramic. A maximum output power of 0.21 W with a slope efficiency of 14% was obtained.
Acknowledgments
This research was supported in part by the Research Fellowships of the Japan Society for the Promotion of Science for Young Scientists. This research was also supported in part by the Global COE Program of the Materials Integration, Tohoku University, and in part by the Grant-in-Aid for Exploratory Research, JSPS (#24655186).
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