Lutetium Aluminum Garnet (LuAG) is a garnet isostructure similar to Yttrium Aluminum Garnet (YAG). High quality Ho3+ doped YAG and LuAG transparent polycrystalline ceramics were fabricated successfully by a reactive sintering method under vacuum. The microstructures, absorption spectrum, fluorescence spectrum and the laser performances of Ho:YAG and Ho:LuAG ceramics were systematically investigated. The in-line transmittances of Ho:YAG ceramic in the visible and infrared region are higher than 82% and 84%. The absorption coefficient of 1.0 at.% Ho:LuAG is 0.88 cm−1 at 1906 nm and its absorption cross section is 0.62 × 10−20 cm2.
© 2014 Optical Society of America
As an eye safe wavelength, near 2.0 µm solid state lasers are becoming of increasing interest for biomedical applications and remote sensing applications [1,2]. In addition, around 2.0 µm is not in the strong absorption spectral band of carbon dioxide, so it can be used as a pump source for optical parametric oscillator operating in the mid-infrared region . It is well known that holmium ions are suitable for producing around 2.0 µm laser emission arising from the 5I7 → 5I8 transition of Ho3+ [4,5]. Ho3+ ions doped YAG and sesquioxide have been studied well for the infrared solid state laser materials [6–8]. Recently, Lu3Al5O12 materials were considered superior to the more conventional Y3Al5O12 in generating mid-infrared laser radiation [9–11]. Lutetium Aluminum Garnet (LuAG) is a garnet isostructure similar to YAG. It possesses higher density, higher crystal field, leading to large manifold splitting and low thermal occupation for the lower laser level . Nakao et al reported the first laser oscillation of an Yb:LuAGceramic thin-disk laser. With the multimode laser resona-tor setup, an output power of 101 W with a maximumoptical efficiency of 56% and a slope efficiency of 64%were obtained . Therefore, holmium doped LuAG materials have the potential to become the novel laser material for cooling-free laser systems in high-power regime.
Limited by the growth method, single crystal with large size and high doping concentration is difficult to obtain. Compared with single crystal, transparent ceramic have several advantages such as improved mechanical properties, increased range of composition, fabrication of large size and high doping concentration, mass production as well as lower cost [14–16]. Even though there is extensive literature on rare earth ions doped YAG transparent ceramics, there are few papers about LuAG transparent ceramics. The performances of Ho:LuAG transparent ceramics need further investigation. In addition, for laser gain media application, the optical quality of the fabricated ceramics is extremely important. By optimizing the manufacturing processes, high quality Ho:LuAG transparent laser ceramics with low loss are obtainable now. Consequently, Ho:LuAG transparent ceramics will be the optimum candidate laser material for the 2.0 µm laser applications in the future.
In this study, Ho:YAG and Ho:LuAG transparent ceramics were successfully fabricated by a reactive sintering method under vacuum. The microstructures, absorption spectrum, fluorescence spectrum and the laser performances of Ho:YAG and Ho:LuAG ceramics were investigated.
2. Experimental procedures
(1) Ceramic fabrication
The reactive sintering method was employed for the fabrication of Ho:YAG and Ho:LuAG ceramics and it was very similar to the method reported in the previous literatures [17,18]. Commercial α-Al2O3, Lu2O3, Ho2O3 powders and co-precipitated Y2O3 powder were selected as the starting materials. The primary particle size of α-Al2O3 powder was around 0.25 µm. The powder synthesis process for Y2O3 was similar to the method described by Zhang et al, and it is a coprecipitation process using ammonia as the precipitant . The Y2O3 precursor was calcined at 1000 °C for 3 h in air. The primary particle size of the synthesized Y2O3 powders was around 60–80 nm and the specific surface area was around 10.0 m2/g. The powders were weighed precisely according to the chemical stoichiometry composition, at Ho3+ doping concentrations of 0.8 at.%, 1.0 at.%, 1.5 at.%, 2.0 at.% Ho:YAG and 1.0 at.% Ho:LuAG respectively and mixed with 99.99% ethanol. The mixed slurry was then ball milled using a planetary milling machine for 15 h. The 0.5 wt.% tetraethyl orthosilicate (TEOS) was used to introduce SiO2 as the sintering aid. After milling, the powder mixtures were dried at 120 °C for 24 h in the oven and then sieved through the 200-mesh screen. After removing organic components by calcining at 800 °C for 3 h, the powders were dry pressed in a stainless steel die at 15 MPa. The green body pellets were further cold isostatically pressed (CIPed) at 200 MPa. After CIP, the relative density of the green body was around 53%. The green bodies of Ho:YAG and Ho:LuAG were sintered at 1780 °C and 1830 °C for 8 h in a high temperature vacuum sintering furnace under 10−3 Pa vacuum condition. The sintered pellets were then annealed in air at 1400 °C for 15 h to completely remove internal stress and eliminate the oxygen vacancies. The as-prepared ceramic pellets were mirror polished on both surfaces with different level of diamond slurries.
The microstructures of the transparent ceramics were observed by scanning electron microscopy (Model JSM-6360LV, JEOM, Tokyo, Japan). The optical transmittance of Ho:YAG and Ho:LuAG transparent ceramics were measured by a UV-VIS-NIR spectrophotometer (Lambda-950, PerkinElmer, USA). The photoluminescence (PL) spectra was measured at room temperature by spectrofluorometer (Fluorolog-3, Jobin Yvon, Edison, USA), equipped with a PbS detector. The slit was fixed at 2 nm and all the spectra were measured at room temperature.
(2) Laser experiment
Laser configuration used in this experiment is shown in Fig. 1.The dimension of Ho:YAG and Ho:LuAG slabs were 2mm × 3mm × 14 mm and 3mm × 3mm × 13.2 mm. A simple two-mirror resonator was adopted, it comprised a plane pump input coupler (IC) with high reflectivity (R>99.8%) at the lasing wavelength (2050-2250 nm) and high transmission (>95%) at 1850-1960 nm, and a concave output coupler (OC) of 100 mm radius-of-curvature (ROC). The output coupler had a transmission of either 6% at 2000-2250 nm and high reflectivity at pump wavelength to improve the total pump absorption efficiency. The physical cavity length is about 18 mm. Operating wavelength of the Tm:fiber laser was tuned to ~1907 nm.
3. Results and discussion
Many previous works have discussed the phase evolution during sintering process . In this study, the reaction sequence of LuAG was similar to YAG. The intermediate phases such as Y4Al2O9 (YAM) and YAlO3 (YAP) are commonly found during the reaction. Figure 2 shows the XRD patterns of Ho:YAG and Ho:LuAG transparent ceramics sintered at 1780 °C and 1830 °C for 8 h. All the peaks for the samples can be well indexed to the cubic garnet structure of YAG and LuAG. This indicated that the full transformation to YAG and LuAG occurred during the vacuum sintering. The XRD peaks of Ho:LuAG has a little shift because the lattice constant of LuAG is smaller than that of YAG.
The photo of the transparent Ho:YAG and Ho:LuAG ceramics and the in-line transmittances are shown in Fig. 3.The thickness of all samples were polished to be 3 mm. The letters under the ceramics can be clearly resolved. The in-line transmittances of Ho:YAG ceramics are 83.9% and 84.6% at 1000nm and 2500 nm. No obvious transmittances drop can be observed at UV and VIS wavelength region. The transmittances of all samples at 400 nm are higher than 82%, which means that few scattering center existed in the microstructures. This is consistence with the SEM results.
Figure 4 shows the SEM micrographs of Ho:YAG and Ho:LuAG transparent ceramics. The microstructures defects including grain boundary, grain size and residual pores are considered as the main factors affecting the transmittance . As shown in Fig. 4, the Ho:YAG ceramics are composed of grains of the average size ~10 μm. The Ho:LuAG transparent ceramics exhibited the homogeneous microstructures and the average grain size was more than 10 µm. Obviously, no grain-boundary phase or residual pores can be observed in the microstructure of Ho:YAG and Ho:LuAG samples. And the fracture mode of the ceramics were totally transgranular.
The room temperature absorption spectra of 1.0 at.% Ho:YAG and Ho:LuAG transparent ceramics is presented in Fig. 5.For Ho:LuAG and Ho:YAG ceramics, there is a same main absorption band in the region of 1800-2200 nm, which is corresponding to the transition from 5I8 → 5I7 energy levels of Ho3+ ions. The Ho:LuAG and Ho:YAG showed similar absorption curve. But the absorption peaks of Ho:LuAG transparent ceramic were shifted, compared with these of Ho:YAG ceramic. The strongest absorption peaks of Ho:LuAG and Ho:YAG were centred at 1906 nm and 1908 nm respectively. The absorption coefficients of 1.0 at.% Ho:LuAG and Ho:YAG ceramics are 0.88 cm−1 at 1906 nm and 0.89 cm−1 at 1908 nm. From the absorption coefficient, the absorption cross-section of the Ho:LuAG ceramic can be estimated from the following equations:
where αa is the absorption coefficient, N is the concentration of Ho3+ ions, NA is Avogadro’s number and M is atomic molar mass, ρ is the density and Cs is the actual molar concentration of Ho3+ in the sample. The peak absorption cross-section of Ho:LuAG is derived to be 0.62 × 10−20 cm2 at 1906 nm. The absorption cross section of Ho:YAG ceramic was 0.645 × 10−20 cm2 at 1908 nm. Walsh et al reported the absorption cross section of Ho:YAG and Ho:LuAG in 2006 . In that study, the absorption cross section of Ho:YAG and HoLuAG was calculated to be 1.0 × 10−20 cm2 at 1908 nm and 0.64 × 10−20 cm2 at 1906 nm.
The fluorescence spectrum of 1.0 at.% Ho:YAG and Ho:LuAG ceramics at room temperature is shown in Fig. 6..The main emission bands of Ho:YAG and Ho:LuAG are centered at 2093 nm and 2100 nm respectively, which corresponds to the 5I7 → 5I8 transition of Ho3+.
Laser performances of the 1.0 at.% Ho:YAG and Ho:LuAG transparent ceramic slabs were evaluated with an output coupler of 6% transmission. The laser output power as a function of incident pump power is shown in Fig. 7.For the 1.0 at.% Ho:YAG ceramic, a maximum output power of 2.97 W was obtained under an incident pump power of 11.3 W with a threshold of 0.8 W. The slop efficiency for 1.0 at.% Ho:YAG sample was 28.5%. By using the 1.0 at.% Ho:LuAG ceramic as laser media, the maximum output power of 0.7 W was obtained under the incident pump power of 6.9 W with a threshold of 3 W. The slope efficiencies for 1.0 at.% Ho:LuAG sample was 17.3%. It is interesting that the output laser wavelength centered at 2095 nm and 2100 nm.
High quality Ho:YAG and Ho:LuAG transparent ceramics were fabricated by a reactive sintering method under vacuum. The ceramic exhibited homogeneous grains and the average grain size was about 10 µm. The transmittance is above 84% at 2500 nm. The absorption coefficients of 1.0 at.% Ho:LuAG and Ho:YAG ceramics are 0.88 cm−1 at 1906 nm and 0.89 cm−1 at 1908 nm. The laser outputs were obtained successfully by using 1.0 at.% Ho:YAG and Ho:LuAG ceramics as laser medias.
This work was supported by Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD) and NSFC (51302115, 51402133, 61405081).
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