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[Hiroaki Furuse](https://orcid.org/0000-0002-9008-1697), Hiroyuki Tanaka, Yuki Kagami, Hiyori Uehara, [Ryo Yasuhara](https://orcid.org/0000-0001-5377-637X)

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[Flatness-insensitive pulsed electric current bonding of sapphire/Nd:YAG ceramics](https://mdr.nims.go.jp/datasets/b1306275-8921-4fc9-82a0-23f0fdc224d9)

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Flatness-insensitive pulsed electric current bonding of sapphire/Nd:YAG ceramicsResearch Article Vol. 15, No. 10 / 1 Oct 2025 / Optical Materials Express 2633Flatness-insensitive pulsed electric currentbonding of sapphire/Nd:YAG ceramicsHIROAKI FURUSE,1,* HIROYUKI TANAKA,2 YUKI KAGAMI,2 HIYORIUEHARA,3 AND RYO YASUHARA31National Institute for Materials Science, 1-2-1 Sengen, Tsukuba, Ibaraki, Japan2Kitami Institute of Technology, 165 Koen-cho, Kitami, Hokkaido, Japan3National Institute for Fusion Science, 322-6 Oroshi-cho, Toki, Gifu, Japan*FURUSE.Hiroaki@nims.go.jpAbstract: Initial surface-flatness-insensitive bonding of sapphire and YAG ceramics for high-average-power laser materials was achieved via pulsed electric current bonding (PECB), atype of thermal diffusion bonding. Nd:YAG ceramics with surface flatness of λ/10, λ/5, and λ(λ= 632.8 nm) were bonded to sapphire (flatness< λ/10). All samples exhibited excellent opticalperformance irrespective of initial flatness. In-line transmittance approached theoretical limits,transmitted wavefront distortion was below 0.264λ, and polarization exceeded 35 dB. Minorunbonded regions appeared near edges for λ-flat samples, but overall performance remainedunaffected. TEM analysis revealed interfacial microstructures, providing deeper insights. Thesefindings demonstrate that PECB enables high-quality bonding with minimal dependence oninitial flatness, making it advantageous for manufacturing large-aperture, high-average-power,and high-pulse-energy laser materials.© 2025 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement1. IntroductionThe development of high-average-power lasers requires effective management of thermal effectssuch as thermal lensing, thermal birefringence, and thermal stress fracture, as these issuessignificantly degrade laser performance. These effects primarily arise from heat accumulation inthe laser gain medium and the resulting temperature gradients caused by cooling. Therefore, itis essential to consider both material geometry and cooling techniques in order to effectivelymitigate these effects.One effective approach for thermal management involves bonding the laser medium to highlythermally conductive optical materials, such as diamond [1] or sapphire [2–8], which serveas heatsinks. These composite structures effectively dissipate heat along the optical path andsuppress radial temperature gradients.Surface-activated bonding (SAB) is among the most effective methods for bonding dissimilaroptical materials, such as YAG and heatsinks. SAB can be performed at room temperature,making it insensitive to thermal expansion mismatches between different materials and thereforehighly suitable for bonding materials with significantly different coefficients of thermal expansion.Prior studies have reported that the damage threshold at the sapphire/YAG interface bonded bySAB is as high as that of the bulk [9], and that the thermal resistance at the interface is negligiblysmall [10]. Consequently, nanosecond pulses with joule-class pulse energies have been achieved[11]. However, SAB requires stringent conditions, including high-quality initial surfaces (surfaceflatness less than λ/10) and ultrahigh vacuum (∼ 10–5 Pa) [1,6]. These stringent requirementscould limit the scalability of laser media, particularly for larger apertures exceeding 10 cm, whichare essential for 100 J-class high-pulse-energy lasers to prevent damage issues [12–14].Alternative bonding methods for dissimilar optical materials have also been reported, includingglass bonding [15,16] and thermal diffusion bonding [3,5]. In glass bonding, significant effortshave been made to minimize degradation of mechanical and optical properties and reducing#575741 https://doi.org/10.1364/OME.575741Journal © 2025 Received 9 Aug 2025; revised 24 Sep 2025; accepted 24 Sep 2025; published 1 Oct 2025https://orcid.org/0000-0002-9008-1697https://orcid.org/0000-0001-5377-637Xhttps://doi.org/10.1364/OA_License_v2#VOR-OAhttps://crossmark.crossref.org/dialog/?doi=10.1364/OME.575741&amp;domain=pdf&amp;date_stamp=2025-10-01Research Article Vol. 15, No. 10 / 1 Oct 2025 / Optical Materials Express 2634refractive index mismatch at the interface [17–19]. However, improvements in laser performanceusing this method have remained limited.With respect to thermal diffusion bonding, to the best of our knowledge, the first successfulbonding of Nd:YAG and sapphire, which have different crystal structures, was achieved usingpulsed electric current bonding (PECB), also referred to as spark plasma sintering [3]. Theresulting enhancement in laser performance has been attributed to the conductive cooling effectof sapphire. Although thermal diffusion bonding of dissimilar materials is generally challenging,the relatively small mismatch in thermal expansion coefficients between sapphire and YAG,combined with the low-temperature (1100°C) and short-time (1 hour) bonding process enabledby uniaxial pressure (64 MPa) in the PECB method, are considered key factors for successfulbonding. Compared with SAB, PECB can be performed under moderate vacuum conditions(several Pa) and enables the simultaneous bonding of multiple layers in a single process, making ita simpler and more practical approach. Therefore, PECB is a promising technique for fabricatingsapphire/YAG composites.To extend the applicability of PECB to high-average-power laser materials, a detailedunderstanding of the required bonding conditions is necessary. In particular, the influence ofthe initial surface flatness on achieving sufficient optical performance in these composites hasnot been comprehensively investigated. Clarifying these requirements is crucial for fabricatinglarge-diameter composite laser media for high-pulse-energy lasers. Furthermore, although theprevious study has primarily focused on optical transmittance, other critical characteristics havenot been systematically examined [3]. Specifically, evaluating transmitted wavefront distortionand stress-induced depolarization is vital for laser applications. Additionally, high-resolutionelectron microscopy analysis of the bonded interface is essential for optimizing the bondingprocess.In this study, we fabricated sapphire/Nd:YAG composite materials with varying initialsurface flatness using PECB to investigate the conditions required for successful bonding. Theoptical characteristics, including transmitted spectra, transmitted wavefront, stress-induceddepolarization, and laser performance, were systematically compared. Moreover, the bondedinterface microstructure was examined using transmission electron microscopy (TEM) for bothsapphire/Nd:YAG ceramic and single-crystal composites to clarify the differences between them.2. Experimental method2.1. Pulsed electric current bonding (PECB) of sapphire/ Nd:YAG ceramic compositeThe starting materials were commercially available a-cut sapphire (Orbe Pioneer Co. Ltd.,Japan) and Nd:YAG ceramics (Konoshima Chemical Co. Ltd., Japan), each with dimensions of10 mm in diameter and 2 mm in thickness. Figure 1 shows a schematic of the sapphire/Nd:YAGcomposite material.The bonded surfaces of the Nd:YAG ceramics were polished to approximately λ/10 (twopieces), λ/5 (three pieces), and λ (two pieces), whereas the sapphire surfaces were polishedto less than λ/10 (seven pieces), where λ= 632.8 nm. Surface flatness was evaluated based onpeak-to-valley (PV) values measured using a laser interferometer (Verifire, ZYGO, USA) andprovided by the polishing company. Detailed information on the initial surface flatness of allsamples is summarized in Table 1.After assembling each pair of surfaces, the samples were bonded using a PECB system(Labox-315, Sinter Land Co. Ltd., Japan). The bonding temperature and applied pressure were1100°C and 64 MPa, respectively, as described in our previous study [3]. After bonding, allsurfaces of the composites were polished again for subsequent characterization.Research Article Vol. 15, No. 10 / 1 Oct 2025 / Optical Materials Express 2635Fig. 1. Schematic of sapphire and Nd:YAG ceramic composite material.Table 1. Initial surface flatness at the polished surfaces for the compositesample. λ=632.8 nm.Peak-to-valley values of initial surface flatness (λ)Sample Nd:YAG ceramics sapphireComposite-1 0.096 (∼ λ/10) 0.033 (< λ/10)Composite-2 0.096 (∼ λ/10) 0.041 (< λ/10)Composite-3 0.194 (∼ λ/5) 0.063 (< λ/10)Composite-4 0.196 (∼ λ/5) 0.071 (< λ/10)Composite-5 0.198(∼ λ/5) 0.041 (< λ/10)Composite-6 0.973(∼ λ) 0.096 (< λ/10)Composite-7 0.977 (∼ λ) 0.036 (< λ/10)2.2. Characterization of optical propertiesThe transmitted wavefronts (PV values) of all polished sapphire/Nd:YAG composites were alsomeasured by the polishing company using the laser interferometer. Transmission spectra weremeasured using a UV/VIS/NIR spectrometer (UV-3600 Plus, Shimadzu Co. Ltd., Japan).The experimental setup for measuring the degree of polarization is shown in Fig. 2. A 632.8 nmlinearly polarized He-Ne laser was used as the probe beam. The laser beam was expanded andcollimated using two lenses, and the sample was irradiated at normal incidence. The polarizerwas adjusted so that the laser polarization aligned with the sapphire c-axis, and an analyzer wasplaced behind the sample. The analyzer was rotated, and the transmitted power was measuredusing a power meter (PD300-3W, Ophir Optronics Solutions Ltd., Israel).2.3. TEM observation of bonding interfaceThe microstructure of the bonded interfaces was observed using high-resolution transmissionelectron microscopy (TEM, JEM-2800, JEOL Ltd., Japan). Elemental mapping was performedby energy-dispersive X-ray spectroscopy (EDS) to identify constituent elements. To furtherunderstand the bonding mechanism, additional observations were conducted on sapphire/Nd:YAGsingle-crystal composite bonded under the same conditions as the ceramic composites. Thespecimens used for these observations were prepared from optically polished sapphire andNd:YAG materials, obtained from their respective manufacturers and used as received.Research Article Vol. 15, No. 10 / 1 Oct 2025 / Optical Materials Express 2636Fig. 2. Experimental setup for degree of polarization.3. Results and discussion3.1. Optical characteristicsFigure 3 shows photographs of Composite-1, Composite-3, and Composite-6. For samples withan initial surface flatness up to λ/5, all composites exhibited high-quality bonding; no interfacefringes or cloudiness were observed. By contrast, the sides of Composite-6 (and Composite-7;λ-flatness samples) appeared to be poorly bonded, as indicated by the red arrow, which is likelydue to the lower initial surface flatness of the Nd:YAG ceramics. However, within an aperturesize of approximately 8 mm in Composite-6, bonding quality was sufficient, and the opticalproperties were comparable to those of the other composites.Fig. 3. photographs of each bonded composite.Figure 4 shows the transmission spectra for all composites. For improved readability, Fig. 4(a)and 4(b) present enlarged views around the excitation wavelength (808 nm) and lasing wavelength(1064 nm), respectively. At the excitation wavelength, no significant differences were observedamong the composites. At the lasing wavelength, all composites except Composite-5 exhibitednearly identical transmittance values, ranging from 84.4% to 84.6%. These values were almostequal to the theoretical transmittance of the sapphire/YAG composite (84.7%), regardless of theinitial surface flatness.Table 2 summarizes the transmittance at 1064 nm and the transmitted wavefront (PV value)for all composites. The PV values ranged from 0.128λ to 0.264λ, and, similar to transmittance,Research Article Vol. 15, No. 10 / 1 Oct 2025 / Optical Materials Express 2637Fig. 4. Transmission spectra of all composites around the (a) excitation wavelength (808 nm)and (b) lasing wavelength (1064 nm).showed no correlation with initial surface flatness. Typically, a transmitted wavefront ofapproximately λ/5 (i.e., 0.2λ) was obtained, indicating that the quality is sufficient for use as alaser material.Table 2. Transmittance at 1064 nm and transmitted wavefront (PV value) for eachcomposite. λ=632.8 nm.Sample Transmittance at 1064 nm (%) Transmitted wavefront (λ)Composite-1 (λ/10) 84.6 0.128Composite-2 (λ/10) 84.5 0.259Composite-3 (λ/5) 84.5 0.171Composite-4 (λ/5) 84.6 0.264Composite-5 (λ/5) 84.1 0.178Composite-6 (λ) 84.4 0.213Composite-7 (λ) 84.4 0.220Figure 5 shows the transmitted power and extinction ratio of the composites. The degreesof polarization for the He-Ne laser (without composite), Composite-1, Composite-3, andComposite-6 were 41.9 dB, 34.3 dB, 32.0 dB, and 35.0 dB, respectively. These results indicatethat depolarization in the composites was negligible, implying minimal residual stress inside thecomposites. Therefore, appropriate bonding can be achieved even when the initial surface flatnessof the YAG ceramics is around λ, which is highly advantageous for large-aperture applications.3.2. Laser characteristicsTo compare the laser performance of composites with different initial surface flatness, weconducted laser oscillation tests. Figure 6 shows the experimental setup for the lasing test. Thecavity length was approximately 25 mm, comprising a flat dichroic mirror and a concave laseroutput coupler (R= 50 mm). The reflectivity of the output coupler was 95%. A continuous-wave(CW) 60 W fiber-coupled laser diode (LD) with a core diameter of 105 µm was used as thepump source. The LD output was focused onto the Nd:YAG ceramics with a spot diameter ofapproximately 105 µm, and the laser output power was measured using a power meter (30A,Research Article Vol. 15, No. 10 / 1 Oct 2025 / Optical Materials Express 2638Fig. 5. Degree of polarization of the probe beam (w/o composite) and for each bondedcomposite.Ophir Optronics Solutions Ltd., Israel). For this measurement, composite samples 1, 3, and6 (listed in Table 1) were tested. For comparison, a single-piece Nd:YAG ceramic was alsomeasured. The cavity configuration, including the LD focusing point, was kept fixed and onlythe sample was replaced to ensure a fair comparison. Consequently, the sample position wasapproximately fixed within the laser cavity.Fig. 6. Experimental setup for laser oscillation.Figure 7 shows the laser output characteristics of the composite materials. As the excitationpower increases up to 10 W, the output power of all samples initially increases. Beyond thispoint, the output power of the unbonded Nd:YAG ceramics (without sapphire) decreases morerapidly than that of the bonded composites, most likely owing to thermal effects. In the bondedsamples, the decrease occurs at higher excitation powers, resulting in an output improvement ofapproximately 13% compared to the unbonded ceramics at high power. This enhancement canbe attributed to the conductive cooling effect provided by sapphire, as reported in our previousstudy [3].Furthermore, no significant differences were observed in the laser outputs among the compositematerials. The slope efficiency was evaluated using data points for excitation powers up to 12 W,as shown in Fig. 7. Both the lasing threshold and slope efficiency were nearly identical for allsamples, indicating that total cavity losses, including those from the laser medium, were similar.Research Article Vol. 15, No. 10 / 1 Oct 2025 / Optical Materials Express 2639Fig. 7. Laser output power as a function of pump power for single Nd:YAG ceramics andbonded composites.Therefore, the optical loss at the bonded interfaces can be considered negligibly small, which isconsistent with the transmittance results shown in Fig. 4.In the experiments, the decrease in laser output under high pump power, which is attributed tothermal effects, occurred at nearly the same pump power for all bonded composites, regardless ofthe initial surface flatness. This suggests that thermal management is not significantly influencedby surface flatness within the range investigated. Although additional characterization (forexample, far-field beam profiles and detailed wavefront measurements) would be required forfurther clarification, which will be addressed in the future studies, the results obtained in thisstudy indicate that pulsed electric current bonding of sapphire/Nd:YAG ceramics is insensitive toinitial flatness with respect to laser output characteristics.3.3. TEM observationsFigure 8 shows TEM images of the sapphire/Nd:YAG ceramic composites interfaces. For com-parison, similar observations were conducted on the sapphire/Nd:YAG single-crystal composite.As shown in Fig. 8(a), in the case of Nd:YAG ceramics, an interlayer approximately 20 nm thickwas clearly observed at the interface. This interlayer was not detected in our previous study usingFE-SEM due to its limited resolution [3]. TEM analysis revealed that the interlayer was crystalline.By contrast, as shown in Fig. 8(b), no such interlayer was observed at the single-crystal compositeinterface, indicating that interlayer formation is characteristic of bonding with ceramics.Figure 9 presents the elemental mapping results of the sapphire/Nd:YAG ceramic interfaceobtained by EDS. The interlayer consists of Y, Si, O, and Nd, with almost no Al detected. The Siis likely derived from sintering additives in the YAG ceramics. Previous studies have shownthat Si can diffuse during heat treatment via grain boundary diffusion [20–22], leading to itsaccumulation at the sapphire interface, where it may form a Y–Si complex oxide, such as yttriumsilicate.Although the interlayer at the sapphire/Nd:YAG ceramic interface could potentially act as alight-scattering source, its small thickness (approximately 20 nm) did not cause significant opticalloss. Furthermore, high-quality bonding was also achieved at the sapphire/Nd:YAG single-crystalinterfaces, suggesting that successful direct bonding of dissimilar materials does not depend onResearch Article Vol. 15, No. 10 / 1 Oct 2025 / Optical Materials Express 2640Fig. 8. TEM images of the bonded interfaces for (a) sapphire/Nd:YAG polycrystallineceramics and (b) sapphire/Nd:YAG single crystal.Fig. 9. EDS elemental mapping of sapphire/Nd:YAG ceramics composite interface.interlayer formation. This finding provides valuable insights for bonding not only different typesof substrates but also similar ceramic materials using thermal diffusion bonding by PECB.4. ConclusionIn this study, we investigated flatness-insensitive pulsed electric current bonding (PECB) ofsapphire and Nd:YAG ceramics for high-average-power laser applications. Nd:YAG ceramicswith varying initial surface flatness levels (λ/10, λ/5, and λ) were bonded to sapphire, and nosignificant differences were observed in in-line transmittance, transmitted wavefront, degree ofResearch Article Vol. 15, No. 10 / 1 Oct 2025 / Optical Materials Express 2641polarization, or laser performance over most of the bonded area. These results demonstrate thathigh-quality bonding can be achieved even when the initial surface flatness is approximatelyλ, highlighting the feasibility of using PECB for large-aperture optical elements—a significantadvantage over conventional bonding methods.Notably, the present experiments were conducted using relatively small-diameter samples(ϕ10 mm). Further studies with large-diameter samples are necessary to confirm these findings.Additionally, TEM observations revealed the accumulation of silicon from the sintering additivesand the formation of an interlayer approximately 20 nm thick, which was not observed in samplesbonded to Nd:YAG single crystal.Future work will focus on bonding with c-cut sapphire (which exhibits no polarizationdependence), developing sandwich structures, and scaling up to large-aperture samples. Theseinvestigations are expected to enhance understanding and promote the practical application offlatness-insensitive PECB in high-average-power laser development.Funding. National Institute for Fusion Science (NIFS25KIII021, NIFS23KIIH019).Disclosures. The authors declare no conflicts of interest.Data availability. The data underlying the results presented in this paper are not publicly available at this time butmay be obtained from the authors upon reasonable request.References1. H. Ichikawa, K. Yamaguchi, T. Katsumata, et al., “High-power and highly efficient composite laser with an anti-reflection coated layer between a laser crystal and a diamond heat spreader fabricated by room-temperature bonding,”Opt. Express 25(19), 22797–22804 (2017).2. L. Zheng, A. Kausas, and T. Taira, “Drastic thermal effects reduction through distributed face cooling in a high powergiant-pulse tiny laser,” Opt. Mater. Express 7(9), 3214–3221 (2017).3. H. Furuse, Y. Koike, and R. Yasuhara, “Sapphire/Nd:YAG composite by pulsed electric current bonding forhigh-average-power lasers,” Opt. Lett. 43(13), 3065–3068 (2018).4. Y. Chen, Y. Lin, J. Huang, et al., “Efficient continuous-wave and passively Q-switched pulse laser operations ina diffusion-bonded sapphire/Er:Yb:YAl3(BO3)4/sapphire composite crystal around 1.55 µm,” Opt. Express 26(1),419–427 (2018).5. Z. Feng, C. Ma, S. Hu, et al., “Transparent YAG ceramic/sapphire composite fabricated by pressureless direct thermaldiffusion bonding,” J. Eur. Ceram. Soc. 41(15), 7845–7851 (2021).6. K. Fujioka, X. Guo, M. Maruyama, et al., “Room-temperature bonding with post-heat treatment for compositeYb:YAG ceramic lasers,” Opt. Mater. 91, 344–348 (2019).7. L. Zheng, A. Kausas, and T. Taira, “>30 MW peak power from distributed face cooling tiny integrated laser,” Opt.Express 27(21), 30217–30224 (2019).8. I. Kuznetsov, A. Pestov, I. Mukhin, et al., “Composite Yb:YAG/sapphire thin-disk active elements for high-energyhigh-average power lasers,” Opt. Lett. 45(2), 387–390 (2020).9. A. Kausas and T. Taira, “Laser-induced damage study of bonded material for a high-brightness laser system,” Opt.Lett. 47(12), 3067–3070 (2022).10. Y. Sato, A. Kausas, and T. Taira, “Enhanced thermal conductivity of distributed face-cooled composite laser mediumincluded thermal resistance at the bonding interface,” Opt. Express 33(11), 24039–24049 (2025).11. V. Yahia, A. Kausas, A. Tsuji, et al., “Joule-class sub-nanosecond pulses produced by end-pumped direct bondedYAG/sapphire modular amplifier,” Opt. Express 32(8), 14377–14393 (2024).12. P. Mason, M. Divoký, K. Ertel, et al., “Kilowatt average power 100 J-level diode pumped solid state laser,” Optica4(4), 438–439 (2017).13. M. Divoký, J. Pilař, M. Hanuš, et al., “150 J DPSSL operating at 1.5 kW level,” Opt. Lett. 46(22), 5771–5773 (2021).14. T. Sekine, T. Kurita, Y. Hatano, et al., “253 J at 0.2 Hz, LD pumped cryogenic helium gas cooled Yb:YAG ceramicslaser,” Opt. Express 30(25), 44385–44394 (2022).15. Y. Yu, W. Zhu, Y. Ma, et al., “Novel design of sapphire/spinel transparent ceramic joints with double glass interlayersby coating and bonding,” Ceram. Int. 50(1), 1591–1600 (2024).16. M. Sakajio, N. Pears, M. Dov, et al., “Glass bonding of YAG/Nd:YAG and Sapphire/Nd:YAG windows,” Opt. Mater.151, 115363 (2024).17. J. Lin, P. Lin, R. Ao, et al., “Microstructure evolution and mechanical properties of YAG/YAG joint usingbismuth-borate glass,” J. Eur. Ceram. Soc. 41(4), 2847–2854 (2021).18. W. Zhu, Y. Chen, H. Zou, et al., “Microstructure, mechanical and optical properties of MgAl2O4/MgAl2O4 jointsbonded using CaO-Al2O3-SiO2 glass filler,” J. Eur. Ceram. Soc. 42(6), 2994–3003 (2022).19. X. Liu, D. Han, X. Mao, et al., “Joining transparent spinel ceramics using refractive index–matched glass,” J. Eur.Ceram. Soc. 42(8), 3579–3585 (2022).https://doi.org/10.1364/OE.25.022797https://doi.org/10.1364/OME.7.003214https://doi.org/10.1364/OL.43.003065https://doi.org/10.1364/OE.26.000419https://doi.org/10.1016/j.jeurceramsoc.2021.08.020https://doi.org/10.1016/j.optmat.2019.03.032https://doi.org/10.1364/OE.27.030217https://doi.org/10.1364/OE.27.030217https://doi.org/10.1364/OL.384898https://doi.org/10.1364/OL.456760https://doi.org/10.1364/OL.456760https://doi.org/10.1364/OE.554536https://doi.org/10.1364/OE.518251https://doi.org/10.1364/OPTICA.4.000438https://doi.org/10.1364/OL.444902https://doi.org/10.1364/OE.470815https://doi.org/10.1016/j.ceramint.2023.10.251https://doi.org/10.1016/j.optmat.2024.115363https://doi.org/10.1016/j.jeurceramsoc.2020.12.002https://doi.org/10.1016/j.jeurceramsoc.2022.01.042https://doi.org/10.1016/j.jeurceramsoc.2022.02.041https://doi.org/10.1016/j.jeurceramsoc.2022.02.041Research Article Vol. 15, No. 10 / 1 Oct 2025 / Optical Materials Express 264220. R. Boulesteix, A. Maître, J. F. Baumard, et al., “The effect of silica doping on neodymium diffusion in yttriumaluminum garnet ceramics: implications for sintering mechanisms,” J. Eur. Ceram. Soc. 29(12), 2517–2526 (2009).21. K. Fujioka, A. Sugiyama, Y. Fujimoto, et al., “Ion diffusion at the bonding interface of undoped YAG/Yb:YAGcomposite ceramics,” Opt. Mater. 46, 542–547 (2015).22. K. Fujioka, T. Mochida, Y. Fujimoto, et al., “Heat treatment of transparent Yb:YAG and YAG ceramics and itsinfluence on laser performance,” Opt. Mater. 79, 353–357 (2018).https://doi.org/10.1016/j.jeurceramsoc.2009.03.003https://doi.org/10.1016/j.optmat.2015.05.023https://doi.org/10.1016/j.optmat.2018.03.047