We report a broad comparative analysis of the spectroscopic and laser properties of solid solution Lutetium-Yttrium Aluminum Garnet (LuYAG, (LuxY1-x)3Al5O12) ceramics doped with Yb. The investigation was mainly aimed to assess the impact of the Lu/Y ratio on the Yb optical and laser properties. Therefore we analyzed a set of samples with different Y/Lu balance, namely 25/75, 50/50 and 75/25, with 15% Yb doping. We found that the Yb absorption and emission spectra changed from YAG to LuAG when gradually increasing in Lu content. Regarding the laser emission, remarkable results were achieved with all samples. Maximum output power was 8.2 W, 7.3 W and 8.7 W for Y/Lu balance 25/75, 50/50 and 75/25 respectively, at 1030 nm; the slope efficiency and the optical-to-optical efficiencies approached or exceeded 60% and 50% respectively. The tuning range was investigated using an intracavity ZnSe prism. The broadest tuning range (998 nm to 1063 nm) was obtained with Y/Lu balance 75/25, whereas the emission of the other two samples extended from 1000 nm to 1058 nm. To the best of our knowledge, this is the first comparative analysis of Yb:LuYAG ceramics or crystals as laser host across such a broad range of Y/Lu ratios.
© 2016 Optical Society of America
Solid state laser materials with good thermo-mechanical properties  are very attractive to develop high power laser systems. The active media are subjected to severe stresses due to the increase of the local temperature as well as the temperature gradient, which can cause additional losses through several mechanisms, e.g. thermal population of the lower level of the laser transition and thermal lens effects.
In consideration of these problems, in the recent years several investigations have focused on Yb3+ doped crystals and polycrystalline matrices containing Lutetium, Lu3+, as Lu2O3 [1-6] or LuAG [7-9]. As a matter of fact, the Lu3+-ion mass is close to that of Yb3+, therefore it reduces the scattering at the mass defect of phonons responsible for the heat transportation. As a results, Lu based hosts can admit a high level of Yb doping, without suffering a sharp decrease of thermal conductivity (12.2 W/m·K for Lu2O3  and 8.8 W/m·K in undoped LuAG ), as it occurs for instance in YAG. This makes these hosts more suitable to withstand high levels of thermal load, which otherwise would negatively affect the laser performance by reducing the output power, the slope efficiency and the spatial beam quality.
On the other hand, the development of the above hosts (in the crystal or ceramic forms) is hampered by some drawbacks. One is the excessive cost of the high purity Lu2O3 powder used for the fabrication of both the ceramics and the crystals; moreover the high melting point (above 2400 °C for Lu2O3, 2010 °C for LuAG [11,12]) requires the use of special ovens and crucibles for the growth of crystals.
To overcome these problems, the use of solid solutions of LuAG and YAG, namely(LuxY1-x)3Al5O12 (Lutetium-Yttrium Aluminum Garnet, LuYAG) was recently proposed [13-20], as crystals and polycrystalline ceramics. Due to the mixed composition, they require a lower quantity of Lu3+than LuAG and Lu2O3; they have a lower melting point but at the same time they show a thermal conductivity (7.8 W/m·K ) comparable to the LuAG although lower than Lu2O3 . Moreover, some of their properties can be fine-tuned by adjusting the Lu/Y ratio . Until now, laser properties of Yb:LuYAG crystal were studied only for Lu/Y = 50/50 . The growth of large size LuYAG crystals with Yb doping presents some issues. Indeed, the segregation coefficient of Yb and Y is different form unity (see [13,20]), determining a variation of the crystal composition during the growth. This problem can be overcome by ceramic fabrication processes, which allow to obtain a better uniformity of the resulting samples.
Recently, we have demonstrated the laser action for Yb:LuYAG ceramics with Y/Lu = 50/50 . A preliminary assessment of the spectroscopic and laser emission properties of the formulation with Y/Lu = 75/25 was also reported in .
In this paper we report an extensive spectroscopic and laser characterization of a set of Yb0.15:LuxY1-xAG (x = 0.25, 0.50, 0.75) ceramic samples with 15at.% Yb doping. The comparison of their performances allowed us to study how the different ratios between the Lu3+ and Y3+ influence the laser behavior of the ceramics themselves; the measurements are carried out using the same experimental set-up to reduce any influence arising due to the use of different experimental schemes.
2. Ceramic fabrication and spectroscopic properties
2.1 Ceramic production
The fabrication of the samples was carried out using the technique already exposed in  and is reported here for completeness.
Yb:LuxY1-xAG (x = 0.25, 0.50, 0.75) ceramics doped with 15at.% of Yb were fabricated by the solid-state reactive sintering method. The starting materials are high purity commercial powders, i.e. α-Al2O3, Lu2O3, Y2O3, Yb2O3, which were mixed in stoichiometric ratio and ball milled in a corundum bottle for 10 h with Al2O3 balls having a diameter of 10 mm, in ethanol. Tetraethoxysilane (TEOS) and MgO were used as sintering aids. The homogenized suspension was dried while the obtained granulate was uniaxially pressed into 20 mm diameter pellets at 20 MPa and cold isostatically pressed at 200 MPa. Sintering was conducted at 1850°C for 30 h in a tungsten mesh-heated vacuum furnace under 5 × 10−4 Pa vacuum during holding. The oxygen vacancies were removed by annealing the specimens at 1500°C for 10 h in air. The resulting samples have a diameter of about 16 mm and a thickness of about 4 mm.
Figures 1(a)-1(c) reports the FESEM micrographs of the fracture surfaces of the ceramics; the grain boundaries appear clean and there are almost no pores and secondary phases at the grain boundaries or inner grains. For polished and thermally etched Yb0.15:(Lu0.75Y0.25)3Al5O12 ceramics, see Fig. 1(d) the average grain size is about 25μm.
For sake of clarity hereafter the three samples will be indicated only by mentioning the percent of Lu3+ present in the ceramic samples, i.e. Lu0.25, Lu0.50 and Lu00.75 instead of Yb0.15:(Lu0.25Y0.75)3AG, Yb0.15:(Lu0.50Y0.50)3AG and Yb0.15:(Lu0.75Y0.25)3AG.
2.2 Spectroscopic measurements
The absorption spectra, see Fig. 2, were acquired by means of Shimadzu spectrometer 3101PC at room temperature. From these measurements we also determined the baseline transmission of the samples, which resulted 82.9% for the Lu0.25 sample, 80.7% for the Lu0.50 sample and 80.9% for the Lu0.75 sample, around 750 nm. The deviations from the theoretical baseline transmission (~83.7%) are about 0.8%, 2.8% and 3% respectively, due to scattering on the internal defects.
For comparison, we also added to the graph the spectra of 15% Yb:YAG and 15% Yb:LuAG ceramics, prepared with the same procedure described above. Addressed to the Yb3+ 2F7/2-2F5/2 transitions, they show a large absorption band, from 915 nm to 943 nm, suitable for diode-laser pumping, and a narrow peak at 968 nm, ascribed to the zero-phonon absorption line.
It can be seen that for increasing Lu3+ concentration the absorption in the band 920-936 nm increases slightly while the absorption peak at 940 nm shows a small decrease; the zero line moves from 968 nm to 967 nm. From the lattice parameter data by Kuwano et al  the cation site density can be estimated as 1.39 × 1022 cm−3 for Lu0.25, 1.40 × 1022 cm−3 for Lu0.50 and 1.41 × 1022 cm−3 for Lu0.75. The spectra acquired in the UV-region show the Yb3+ edge of the charge transfer transition (CTT) below 230 nm. Slight shift of the CTT edge towards shorter wavelength with increasing content of Lu3+ could be tentatively explained by the decreasing covalency in the Lu-O bonds compared to the Lu-Y ones; this determines a slight downward motion of oxygen energy levels which constitute the very top of the valence band. The absorption shoulder around 250-270 nm is not related to CTT of Yb3+ as can be seen from the photoluminescence excitation spectra (PLE) of CTT emission in Fig. 3. This shoulder is due to 4f-5d transition of Tb3+ trace impurity which comes from Lu2O3 raw powder . Photoluminescence excitation (PLE) and emission (PL) spectra were measured with a custom made 5000M spectrofluorometer from Horiba Jobin Yvon at room temperature.
For the measurement of the lifetime of the upper laser level of Yb3+ we used the pinhole method. This method (originally proposed by Kühn et al. ) allows the measurement of the upper level lifetime, in transitions whose absorption and emission spectra are partially superimposed, as in the present case. In this way it is possible to avoid the effect of the radiation trapping, which increases significantly the fluorescence decay time with respect to the upper level lifetime. More details on the theoretical framework and the experimental apparatus can be found elsewhere [22,23]. We found 890 μs, 870 μs and 880 μs for Lu3+ concentration of 25%, 50% and 75% respectively, with 15% Yb doping. It must be noticed that on another sample with 50% Lu3+ and 5% Yb concentration the lifetime resulted longer (935 μs). In the case of 50% Lu3+ concentration therefore the lifetime observed on the 15% Yb doped sample is affected by concentration quenching effects. It is then probable that concentration quenching effects are also occurring in the other samples under test.
In order to calculate the emission cross section (σe), see Fig. 4, we acquired the fluorescence spectra, with the same method reported in . A small transparent window was cut and polished on the side of the samples. The samples were longitudinally pumped by a semiconductor laser emitting at 936 nm. The pump beam was focused in the sample to a diameter of 300μm. The beam path was set parallel to the small window and grazing the internal side of the window itself. The fluorescence light was collected perpendicularly to both the pumping axis and the windows surface. We ensured that the excited volume was at the surface of the sample by maximizing the fluorescence signal near the zero phonon line, as reported in . In this way the thickness of unpumped sample volume crossed by the fluorescence emission before the detection was minimized, to avoid reabsorption effects. The spectra were acquired by a fiber coupled grating spectrometer equipped with a gated CCD array with a spectral resolution of 1.5 nm. The residual pump radiation was rejected by chopping the excitation signal and activating the detection just after the end of the excitation.
The emission cross section spectrum was calculated using the β-τ method . For this evaluation (which requires the value of the upper level lifetime) we did not use the values of the upper level lifetime reported above, which are probably affected by concentration quenching; we rather adopted a value of 958 μsec for all the samples, resulting from the average of the radiative lifetime of Yb in low doped YAG  and in 10% doped LuAG . For comparison, we also added to Fig. 4 the emission cross section spectra of 15% Yb:YAG and 15% Yb:LuAG ceramics, acquired with the same experimental procedure. For the calculation of the emission cross section spectra of these samples we used the values of radiative lifetime drawn from literature [26,27].
3. Laser performance
3.1 Laser set-up
Figure 5 reports the laser cavity used to test the laser behavior of all ceramics, which have a doping level of 15at.% with a thickness of 1.41 mm. The cavity is made by three mirrors; EM is the end-mirror (flat), FM is the spherical folding mirror (curvature radius 100 mm) and OC is the flat output coupler mirror. The separation between EM and FM is 55 mm, and between FM and OC is 165 mm. The total cavity length is 220 mm. The TEM00 mode size was calculated as 54.2 μm in the cavity folding plane and 55.2 μm in the perpendicular plane. DM is a dichroic mirror (high transmission at the pump wavelength, high reflection at the laser wavelength) used to cut the residual pump beam. The uncoated sample is soldered with Indium on a copper heat sink, which is cooled by water at 18°C. The sample is placed near to the EM and it is oriented so as to re-inject the Fresnel reflections into the cavity itself. The sample is longitudinally pumped. The pump source is a fiber coupled laser diode emitting at 936 nm (fiber diameter 200 μm, NA 0.22) with a bandwidth of about 4 nm FWHM. The intensity distribution of the pump in the focal plane is almost Gaussian with a spot radius around 150 μm of radius at 1/e2. The samples were pumped in quasi-Continuous Wave, QCW as well as in Continuous Wave, CW. In QCW the Duty Factor was set at DF = 20% and the repetition rate was 10 Hz. The maximum pump power incident on the samples was 22 W. The measurements of the absorption (ABS) were carried out in both pumping regime in order to measure the absorbed pump power when laser action is switched on. The latter is obtained by measuring the residual pump power transmitted through the ceramic which is collected by a converging lens (L) and focused on the power meter M2.
The laser was tuned by placing a home-made ZnSe tuning prism with an apex angle of about 41.8° (set at minimum deviation angle) into a cavity arm between FM and OC. The laser emission is tuned by tilting the OC-mirror around an axis perpendicular to the prism dispersion plane, and correspondingly adjusting the slit position. The emission wavelength was measured with a fiber coupled, 60 cm focal length spectrometer equipped with a multichannel detector, which has a spectral resolution of 0.4 nm.
3.2 Laser measurements and results
Before analyzing the experimental data, it is worth to underline that the experiments were carried out on three ceramic samples having the same thickness, with the same cavity scheme, pumping system as well as detection equipments. Moreover, we used the same protocol to estimate the slope efficiency (ηS) starting from the measurements of the laser output power (Pout) acquired at different levels of the pump power.
Figure 6 reports the laser output power versus the absorption pump power measured by using several OCs with different transmission (from T = 2.2% to T = 57.6%). The samples are pumped in QCW. The results are summarized in Table 1.
All ceramics show good laser performances delivering up to 8.7 W with a slope efficiency ηS as high as 65.6% and an optical-to-optical efficiency (ηO) as high as 55.8%. However their behavior shows some dependence on the Lu3+ ions concentration. First, the unsaturated absorption ABS increases from 74.8% to 83.6% by increasing the concentration of Lu3+. This is due to the fact that the value of the absorption cross section (averaged in the emission band of the pump diode) slightly increases for increasing Lu content (from about 5.8 × 10−21 cm2 in Lu0.25 to about 6.1 × 10−21 cm2 in Lu0.75), as it can be seen in Fig. 2. Secondly, the sample where Lu3+ and Y3+ are present in the same percent shows the worst performance. Comparable output powers are achieved with Lu0.25 and Lu0.75. The highest slope efficiency is reached with Lu0.25 which represents the best result reported in literature . Moreover, this is the only sample capable to emit at 1048 nm in free running, when the cavity has low losses (T = 2.2%). Conversely, for the other output couplers the concentration of Lu3+ does not play a role on the laser wavelength and on laser threshold (Pth) as both remain unchanged.
With all the output couplers, all the samples emitted on a single line at the wavelength specified above. The typical line width was about 1.4 nm FWHM.
The slope efficiency values reported in Fig. 6 were analyzed with the Caird formula  for the assessment of the nonsaturable cavity roundtrip losses. We obtained a value of 1.2%, 7.9% and 6.6% respectively for the samples Lu0.25, Lu0.50 and Lu0.75.
Figure 7 reports the data obtained pumping the samples in CW. Also in this case, the best performance is achieved with Lu0.25 while the worst one with the sample Lu0.50. All ceramics emit at 1030 nm. The main difference with respect to the results obtained in QCW, besides the lower values of the output power and slope efficiency which are addressed to the thermal effects, is the value of the OC transmission which maximizes the performance of the samples. The maximum injected pump power was limited at the level where the laser output showed a deviation from the expected linear dependence from the pump power, which indicates the occurrence of deleterious thermal effects.
The beam quality factor was measured with a beam diagnostic system based on a CCD camera (Thorlabs BC 106 VIS), in CW lasing conditions. Pumping level was set to 5.6 W of absorbed power for all the samples, in order to make a comparison in the same conditions of thermal load. The values of M2 resulted 2.4 for the sample Lu0.25, 1.9 for the Lu0.50 sample and 2.2 for the sample Lu0.75.
Table 2 reports the laser output, the output wavelength, the slope efficiency, the optical-to-optical efficiency and the ABS in CW with the three samples.
Finally, we explored the tuning range. Figure 8 reports the normalized values of the laser output power as a function of the emission wavelength. It allowed to highlight the difference among the samples in terms of the tunability and FWHM of the curves. The maximum value of the used pump power was fixed in order to obtain from each sample a comparable output power at the main emission peak, around 1030 nm. The better performance are achieved by Lu0.25 which spans from 998 nm to 1063 nm. The maximum output powers for all samples were around 0.66 W.
4. Discussion and conclusions
The presented study of this new laser host material had a twofold aim: (i) the in-depth investigation of the spectroscopic characteristics, to assess the impact of the mixed composition; (ii) the investigation on the laser emission properties to enlighten if the presence of Lu3+ can impart to the host the characteristics needed in high power laser systems.
The high quality of the prepared optical ceramics, almost free of secondary phases and scattering pores has been demonstrated by FESEM photographs. All the three samples under test have shown high internal transmission, close to the theoretical limit. Dominating features in the optical absorption and emission spectra in near infrared spectral region are due to 4f-4f transitions of Yb3+ ion, which is further completed by its charge transfer absorption and emission transitions in UV spectral region. The CTT moves to longer wavelengths for decreasing Lu concentration. Trace contamination by Tb3+ ions are evidenced by their 4f-5d absorption transition within 250-270 nm, and further confirmed by the analysis of the photoluminescence emission and excitation spectra.
The absorption spectra of the Yb3+ 2F7/2-2F5/2 transition were only slightly influenced by the Y/Lu balance in the composition, with the main peak of the absorption spectrum (in the interval 920-945 nm) becoming slightly higher and narrower for decreasing Lu concentration.
From the comparison with literature data and from the spectra shown in Fig. 2 it appears that for increasing Lu content the absorption spectrum changes gradually between those of the end compositions Yb:YAG and Yb:LuAG. In particular the main absorption peak gradually shift from red (941 nm in YAG see Brenier et al. ) to blue (939 nm in LuAG, our measurements). Moreover, for increasing Lu concentration the main peak shows a small splitting, with an increasing secondary peak at 936 nm, which is not featured by YAG. The peak absorption cross section increases for decreasing Lu content, and it remains between the value of Yb:LuAG (7.2x10−21cm2 ) and Yb:YAG (8.2x10−21cm2 ). Concerning the emission spectra, they have a main peak around 1030 nm, whose position does not change appreciably with the Y/Lu balance, see Fig. 4. The position of this peak is the same found in YAG and LuAG. The peak value increases slightly for increasing Lu concentration, from 2.80x10−20cm2 for Lu0.25 up to 2.96 x10−20cm2 for Lu0.75. In this case the comparison with literature data is complicated by the relatively large spread of values (see for instance [27,29]).
From the spectroscopic analysis we can then conclude that the mixed composition of such a solid solution ceramic host has only a small impact on the Yb spectroscopic properties. This is probably due to the fact that the lattice structure of the two end compositions (LuAG and YAG) are only slightly different (as reported by Euler et al. , and confirmed in ), as well as and the absorption and emission spectra of the Yb3+ in these two hosts, resulting in only a small variation of the crystal field experienced by Yb3+ for the mixed compositions.
The laser performance was analyzed both in a free-running laser cavity and in a tunable laser cavity. All the compositions under test have shown very high slope efficiencies under QCW pumping regime, with the best result for the Lu0.25 sample. The Caird analysis on the slope efficiencies suggests that the differences in the efficiencies are due to the different level of internal losses due to scattering, with a good correlation between the cavity internal losses and the baseline transmission with the different samples. In other words, the better performance of the Lu0.25 with respect to the other two samples is related mainly to its better internal optical quality and lower internal scattering losses (see the Section 2.2). The low roundtrip losses found for the Lu0.25 also explain why with low output coupling the lasing occurs at 1048 nm, due to the gain tuning effect.
The difference in the laser emission in CW pumping conditions can also be explained by the different internal losses; moreover, the Lu0.50 sample is probably more prone to thermal effects due to its lower thermal conductivity in comparison with the other two samples (see Kuwano ). The beam quality factor M2 resulted quite similar for all the samples, and slightly higher than for TEM00 mode. This is due to the fact that the cavity fundamental mode cross section is smaller than the pump beam size (as it was exposed in Section 3.1) so that the laser emission is inherently multimode. It must be considered that the set up was optimized for maximum power extraction, and no specific attempt was done to optimize the beam quality. Regarding the tuning range, from the analysis of the emission and absorption cross section spectra one should expect as similar performance from all the samples. Indeed, the Lu0.25 sample show a somewhat broader tuning range than the other two sample. This can be explained again considering that this sample has a lower level of internal losses, which enhances the output power on the wings of the tuning range, where the emission cross section is small (on the longer wavelength side) or where the ground level absorption is more significant (on the shorter wavelength side).
The comparison of the optical and laser characteristics of the various samples suggests that the fabrication technique is nearly optimal for the Lu0.25 composition, while it has still some margin for improvement for the Lu0.50 and Lu0.75 compositions.
In conclusion, the mixed LuYAG samples investigated here have shown overall a high laser efficiency. We have demonstrated that high quality ceramics can be obtained also from the Yb-doped mixed LuAG-YAG, across the whole range of compositions spanning from LuAG to YAG, with all the advantages for application in high power solid state lasers. For the Lu0.75 samples this is also the first demonstration of laser emission for a mixed garnet with this composition. The differences in laser efficiency in QCW pumping conditions and in the tuning range seems to be mainly due to the different optical quality of the samples. On the other hand, the mixed composition, which should result in a locally fluctuating chemical composition, has only a limited impact on the spectroscopic properties of the Yb3+.
Activity supported by CNR-AVCR Joint Project 2013-2015 “Influence of composition and defects on the properties of transparent ceramics and crystals for laser and scintillator applications”. This work was also partially supported by the National Natural Science Foundation of China (Grant No. 61575212) and Chinese Academy of Sciences Visiting Professor for Senior International Scientists (Grant No. 2013T2G0004).
Portions of this work were presented at the SPIE Conference “Solid State Lasers XXV: Technology and Devices” in 2016 (see  for details).
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