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We report, for the first time, the laser oscillation from 2% Ho3+:Lu2O3 hot pressed ceramic. We have synthesized optical quality Lu2O3 nano-powders doped with concentrations as high as 5% Ho3+. The powders were synthesized by a co-precipitation method beginning with nitrates of holmium and lutetium. The nano-powders were hot pressed into optical quality ceramic discs. The optical transmission of the ceramic discs is excellent, nearly approaching the theoretical limit. The optical, spectral and morphological properties as well as the preliminary lasing performance from highly transparent ceramics are presented.
©2013 Optical Society of America
1. Introduction
Trivalent holmium has attracted a great attention as a laser dopant due to its multiple laser emissions from the visible to mid-IR including 5F4 + 5S2→5I8 transition at 0.5 μm, 5I6→5I8 transition at 1.2 μm, and 5I7→5I8 transition near 2 μm [1]. Among them, the emission due to the transition 5I7→5I8 at 2μm is of particular interest because of its use in eye-safe systems, remote sensing, and medical applications involving coagulative cutting and welding [2]. Currently Ho3+doped single crystal yttrium aluminum garnet (YAG), is the most extensively studied and widely used material [3–8]. However, YAG is not the best host material for high-power laser systems due to its relatively low thermal conductivity and high thermal expansion. The sesquioxides such as Sc2O3, Y2O3, and Lu2O3 are very promising host materials for high-power laser applications, mainly due to their higher thermal conductivity and the high absorption and emission cross-sections of the trivalent rare-earth [9]. Among them, Lu2O3 stands out as an excellent laser host because its thermal conductivity is predicted to be insensitive to dopant concentration due to its negligible phonon scattering. Our recent report, on the spectroscopic and calorimetric study of non-radiative losses in two micron pumped holmium doped laser materials, also revealed that the Lu2O3 is the best host material for holmium [10]. In fact, 2% Ho doped Lu2O3 exhibited far lower losses than YAG and even outperformed YLiF4. Taken together with higher 2µm cross sections, this makes Lu2O3 a promising host for Holmium lasers. In fact, Koopmann et al [11] reported the lasing oscillation from Ho3+:Lu2O3 single crystals. They measured 19W output power using 1.9μm diode pump laser. They also demonstrated 5.2W output power and 54% slope efficiency using Tm fiber laser pump.
Attempts to demonstrate lasing from ceramics failed mainly due poor quality of the transparent ceramics and resulted in a very low transparency which is not suitable for lasing performance. Galceran et al [12] have reported the synthesis of 0.5% and 10% Ho3+:Lu2O3 powders and fabricated semi-transparent ceramics by vacuum sintering process. The maximum transparency they observed was less than 60% at the 1.9 µm range, and thus was not suitable for laser operation. Recently, we have reported a record high lasing performance from highly Yb-doped Lu2O3 ceramics [13–16]. This was accomplished by developments in high purity powder synthesis and low temperature scalable sintering technology. In fact, ceramic lasers are very attractive as solid-state lasers due to their low cost, easy fabrication, and good mechanical and optical properties with respect to crystal materials. Furthermore, ceramics significantly improve the thermal shock parameter and resistance to laser damage allowing high power laser operation [17]. In this study, we report a preliminary result on the laser oscillation from various doped Ho3+:Lu2O3 ceramics hot pressed using high purity powder synthesized using co-precipitation method.
2. Experimental methods
Lu2O3 powders doped with Ho3+ in various doping concentration (0.1%, 1%, 2%, and 5%) were synthesized by the co-precipitation method reported earlier using high purity nitrate precursors obtained by recrystallization [13]. In brief, the mixed crystal was dissolved in de-ionized H2O and co-precipitated in H2O/ammonium hydroxide solution. A white precipitate started to form and the reaction mixture was stirred for 1 hour and cooled to room temperature. The wet precursor powder was dried and subsequently calcined in air. For ceramic fabrication, the powder was mixed with a sintering aid, hot pressed, and then hot isostatically pressed (HIP) for full transparency. The crystal phase, morphology and purity of the powder and ceramic were characterized using x-ray diffractometry (XRD), scanning electron microscopy (SEM) and energy dispersive x-ray spectroscopy (EDX), and glow discharge mass spectroscopy (GDMS). Absorption measurements were performed on polished ceramics using a Fourier-transform IR spectrometer. The fluorescence lifetimes of the ceramics and the powders used to fabricate them were measured by pumping at 1942 nm using a low-power pulsed diode laser. To minimize absorption and reemission (resonant radiative energy transfer), which can produce artificially longer lifetimes, all measurements were performed on fine powder and ceramics that had been ground to ~250 μm particle size. Small 3 mm diameter samples of 2% Ho3+:Lu2O3 ceramic with 2 mm thickness were obtained by core drilling from the large 25 mm diameter samples and polishing both surfaces to a high optical quality (<2 nm rms surface roughness).
3. Results and discussion
3.1 Chemical and morphological properties of the Ho:Lu2O3 powder
Table 1 summarizes the GDMS chemical analysis results of 2% Ho-doped Lu2O3 powder and corresponding ceramic. Only the elements higher than 1ppm are listed. It can be seen that the powder has high purity level. Only a few nonvolatile elements such as Al and Si and a few volatiles such as S and Cl remain in the oxide and ceramic. And some volatile elements such as S and Cl are further reduced due to etching and removal of impurities during sintering process. This result is consistent with our previous report on Yb:Lu2O3 where most of these impurities are not even detected from the ceramics obtained after hot pressing the powder since it is either volatilized or removed by the aid of sintering agent Lithium Fluoride [13,18].
Table 1. Glow Discharge Mass Spectroscopy (GDMS) Chemical Analysis Results of the Synthesized 2% Ho:Lu2O3 Powder and Ceramic
In Fig. 1(a), XRD patterns are given for samples of Lu2O3 doped with 0.1, 2, and 5% Ho3+. The patterns indicate phase-pure Lu2O3 with the cubic structure and can be indexed to JCPDS 43-1021. EDX results confirmed the presence of Lu, Ho, and O in the powder. The particle size was determined by measuring the full width at half maximum and using the Scherer equation. All samples had a crystallite size of 12-13nm. In Fig. 1(b), we present an SEM image of the Ho3+:Lu2O3 powder. It can be seen that the powder is fluffy and composed of soft agglomerates well below sub micron size, limited by the resolution of the SEM. The surface area of these powders is typically in the range of 30-40 m2/g.
3.2 Ho3+:Lu2O3 ceramics
Figure 2(a) shows photographs of the 0.1%, 1.0%, 2%, and 5% Ho3+:Lu2O3 ceramics fabricated using high purity powders synthesized using the method described earlier. The color of the ceramics become darker as the Ho3+ content is increased. Powders of freshly synthesized high purity Ho3+:Lu2O3 were densified by hot pressing. For ceramic fabrication, the powder was first mechanically mixed or uniformly coated with small amount of sintering aid that was eliminated by evaporation prior to full densification. The sintering aid containing powder was placed in a graphite die and hot pressed at 1300-1700°C. Samples were 99% of theoretical density. At this point the samples were transparent, but there was visible scattering due to residual porosity that would not have allowed lasing. Samples were then hot isostatically pressed (HIP) and optically polished. As discussed, the excellent optical quality of these samples is a good indication of high quality of powder and our well established sintering process. Figure 2(b) shows the grains of chemically etched 2% Ho3+:Lu2O3 ceramic. The average grain size of the ceramic is 40~50μm and it is consistent with our previous report based on Yb:Lu2O3 ceramics [13].
Fig. 2 (a) Ho3+:Lu2O3 ceramic fabricated using NRL powder. (0.1%, 1%, 2% and 5% Ho3+ concentration from left to right) (b) Optical microscope picture of 2% Ho3+:Lu2O3 ceramic showing average grain size of 40~50μm.
3.3 Optical transmission, fluorescence lifetime, and refractive indices of Ho3+:Lu2O3 ceramics
Figure 3 shows the optical transmission plot of the polished 2% Ho-doped Lu2O3 ceramic (2.2mm thick) fabricated using the powder synthesized by the method described earlier. Insert shows the corresponding ceramic. It is seen that the transmission is near 80% through the entire range from the visible to 2.4 µm, which is near the theoretical limit as calculated using the measured refractive indices. This is an indication of the excellent quality of the transparent ceramic. The absorption peaks for Holmium are clearly evident. The absorption peaks in Fig. 3 are the optical transition bands of the trivalent holmium including 5I8-5I7 (1775–2175 nm), 5I6-5I8 (1100–1250), and 5I8→5F4 + 5S2 (525–580 nm). Figure 4 shows the fluorescence lifetime plot for a ceramic sample of 2% Ho-doped Lu2O3. The lifetime was found to be ~10 msec and that was similar to the one for powder. This is a long lifetime and it indicates that there is no quenching in the ceramic. This is similar to the lifetime found by Koopman et al. [12]. This bodes well for making a laser since it implies that the ceramization process did not adversely impact the rare earth ion sites. We also measured the refractive indices of transparent Ho-doped Lu2O3 ceramics of various doping concentrations using ellipsometry (J. Woollam and Co.). Figure 5 shows the plot of refractive indices vs. wavelength at various Ho doping concentrations. The refractive index increases with Ho3+ addition, presumably due to the higher polarizability of the Ho3+ ion compared with the Lu3+ ion. For example, the refractive indices of Lu2O3, 2% Ho doped Lu2O3, and 5% Ho doped Lu2O3 are 1.8917, 1.8922, and 1.8957, respectively, at a wavelength of 2 μm.
Fig. 3 Optical transmission plot of the polished 2% Ho-doped Lu2O3 ceramic (2.2 mm thick). Insert shows the corresponding ceramic. Theoretical transmission was calculated using refractive indices in Fig. 5.
Fig. 5 Plot of refractive indices of transparent Ho-doped Lu2O3 ceramics of various doping concentrations measured using ellipsometry.
3.4 Lasing performance
We report, for the first time, the lasing oscillation from 2% Ho3+:Lu2O3 ceramic fabricated using hot press/HIP method. Figure 6 shows a plot of laser output power at 2124nm obtained using pulsed pumping at 1942 nm on uniformly doped 2% Ho3+:Lu2O3 ceramic samples which were about 2 mm thick and 3 mm in diameter. One surface was coated with a dichroic coating with high reflectivity (>99.9%) at the laser wavelength of 2140 nm and high transmission at the pump wavelength of 1942 nm. An antireflective coating for 1942 nm was applied to the sample’s other surface. Samples were tested in a short 2µm laser resonator defined by the high reflector back coating and a 99% reflection mirror with 1 meter center of curvature. A thulium fiber laser operating at 1942nm was focused to longitudinally pump the sample. The pump was operated in a low repetition rate free-running pulsed mode. Pulse durations were varied between 1 to 10 ms and peak powers were varied between 3 to 50W. The f/10 pump beam produced a 500µm excitation spot with a peak intensity of 26kW/cm2. Laser operation was monitored with a bandpass filtered Thorlabs InGaAs photodiode and Gentek pyro-electric energy head. Laser wavelength was measured using a Oriel 77250 monochrometer. Our preliminary results show that output power up to 182mW was demonstrated with a slope efficiency of 1%. Lasing performance is relatively poor and we speculate this is mainly due to the scattering caused by trace amount of phase/morphological impurities present in the ceramic. These efficiencies can be increased with improved processing. We are currently investigating to further improve the lasing performance and the details of the results will be published elsewhere.
4. Conclusion
We have demonstrated, for the first time, the laser oscillation from 2% Ho3+:Lu2O3 ceramics fabricated by hot press/HIP process using high purity powder synthesized by co-precipitation method. Output power up to 182mW and a slope efficiency of 1% were observed. The relatively poor performance might be due to trace amount of phase/morphological impurities present in the ceramic. A comparison of the fluorescence lifetime for a sample of 2% Ho-Lu2O3 powder and ceramic indicated that there was no quenching in the ceramic. The refractive indices of Lu2O3, 2% Ho doped Lu2O3, and 5% Ho doped Lu2O3 ceramics were 1.8917, 1.8922, and 1.8957, respectively, at a wavelength of 2 μm.
Acknowledgments
The authors would like to acknowledge the financial support provided by the Joint Technology Office for High Energy Lasers (JTO-HEL).
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