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[Hiroaki Furuse](https://orcid.org/0000-0002-9008-1697), Kazuya Takimoto, [Sanae Koizumi](https://orcid.org/0000-0003-4680-2482), Hiroyasu Sone

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[Fluorescence and laser properties of Nd-doped (Ca                    <sub>0.5</sub>                    Sr                    <sub>0.5</sub>                    )                    <sub>10</sub>                    (PO                    <sub>4</sub>                    )                    <sub>6</sub>                    F                    <sub>2</sub>                    ceramics](https://mdr.nims.go.jp/datasets/fe2151a0-3df0-40bc-9946-1b476ae8ed5d)

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Fluorescence and laser properties of Nd-doped (Ca0.5Sr0.5)10(PO4)6F2 ceramicsResearch Article Vol. 16, No. 4 / 1 Apr 2026 / Optical Materials Express 992Fluorescence and laser properties of Nd-doped(Ca0.5Sr0.5)10(PO4)6F2 ceramicsHIROAKI FURUSE,1,* KAZUYA TAKIMOTO,1,2 SANAE KOIZUMI,1AND HIROYASU SONE21National Institute for Materials Science, 1-2-1 Sengen, Tsukuba, Ibaraki, Japan2Kitami Institute of Technology, 165 Koencho, Kitami, Hokkaido, Japan*FURUSE.Hiroaki@nims.go.jpAbstract: Transparent ceramics of Nd3+-doped (Ca0.5Sr0.5)10(PO4)6F2 (CS-FAP), a solidsolution of hexagonal fluorapatite Ca10(PO4)6F2 (C-FAP) and Sr10(PO4)6F2 (S-FAP), wassuccessfully fabricated. Laser oscillation from Nd:CS-FAP was demonstrated for the first time.The ceramics consisted of fine grains with an average grain size of approximately 100 nm.Despite a non-cubic crystal structure, light scattering due to birefringence was small, and thetotal loss coefficient near 1 µm was approximately 0.5 cm−1. The full width at half maximum(FWHM) of the fluorescence spectra was about 3.5 nm (31 cm−1) at 1 µm and 6.4 nm (36 cm−1)at 1.3 µm, respectively. They were substantially broader than those of C-FAP and S-FAP (bothapproximately 1 nm), resulting from lattice distortion due to solid solution formation. Thefluorescence peak wavelengths and laser oscillation wavelengths (1061 nm and 1333 nm) wereintermediate between those of C-FAP and S-FAP. The Ca to Sr ratio can be controlled, providingcompositional design flexibility and the potential of CS-FAP ceramics as novel ultrashort-pulselaser materials.© 2026 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement1. IntroductionCeramic laser media are regarded as promising candidates for high pulse energy and highrepetition rate laser applications, owing to their thermal conductivity being comparable to thatof single crystals, the feasibility of fabricating large-scale and bonding materials, and superioroptical homogeneity [1–4]. However, ceramic sintered bodies are more likely to exhibit residualpores, secondary phases, and grain boundary scattering, all of which can act as optical scatteringsources and limit the realization of optical properties required for laser applications [5].It is crucial to minimize optical loss sources in ceramics for laser applications, as they induce anexponential decrease in the in-line transmittance of the ceramics, which is described as follows:T = (1 − R)2 exp[−(α + δp + δs.p + δg · · ·)L], (1)where T is the in-line transmittance, R is the Fresnel reflection, α, δp, δs.p. and δg are theloss coefficients due to absorption, residual porosity, secondary phases, and grain boundaries,respectively, and L is the thickness of the material. In general, a total loss coefficient of< 0.01 cm−1is required to achieve laser-grade quality. Consequently, attaining high optical quality in ceramiclaser media requires rigorous control of microstructural features that contribute to scatteringand absorption losses. While various crystal structures, including non-cubic structures, can beselected for single-crystal laser materials, polycrystalline ceramics are subject to grain boundaryscattering δg induced by birefringence. As a result, laser-grade optical quality has typically beenlimited to materials with cubic crystal structures, such as YAG and cubic sesquioxides, includingY2O3, Lu2O3, and Sc2O3.Hexagonal fluorapatite compounds, such as Ca10(PO4)6F2 (FAP or C-FAP) and Sr10(PO4)6F2(S-FAP), have been used as single-crystal laser media [6–8]. In particular, Yb:S-FAP single crystalsexhibit a higher stimulated emission cross-section than that of YAG, making them promising#591337 https://doi.org/10.1364/OME.591337Journal © 2026 Received 26 Jan 2026; revised 11 Mar 2026; accepted 11 Mar 2026; published 23 Mar 2026https://orcid.org/0000-0002-9008-1697https://doi.org/10.1364/OA_License_v2#VOR-OAhttps://crossmark.crossref.org/dialog/?doi=10.1364/OME.591337&amp;domain=pdf&amp;date_stamp=2026-03-23Research Article Vol. 16, No. 4 / 1 Apr 2026 / Optical Materials Express 993candidates for high-power laser applications [9,10]. Recently, there has been considerable interestin fabricating these materials using ceramic techniques.To achieve laser-grade optical quality in non-cubic polycrystalline ceramics, all loss sources inEq. (1) must be minimized as much as possible. The magnitude of grain boundary scattering δg,caused by birefringence can be expressed based on Mie scattering theory as follows [11]:δg =3π2d∆n22λ2 . (2)where ∆n is the refractive index difference, d is the average grain size composing the ceramics,and λ is the wavelength of light. Previous approaches to reducing the grain boundary scatteringcoefficient δg in fluorapatite ceramic laser media include the use of a magnetic field to alignthe crystal orientation of grains during the forming process prior to sintering. Rare-earthdoping enhances the magnetic anisotropy of otherwise diamagnetic apatite powders, therebyenabling crystal orientation in a magnetic field [12]. This alignment reduces refractive indexdifferences, ∆n, as demonstrated in previous studies reporting the first laser oscillation [13] andQ-switched operation [14] in non-cubic ceramics. Another approach is to control the grainsize d to approximately 100 nm, which is significantly smaller than the wavelength of light[15–17]. This is achieved by highly controlled powder preparation and sintering techniques.Laser oscillation has been demonstrated using both methods, indicating the potential for furthermaterial development in this field.Recently, the development of transparent Nd-doped CS-FAP: (CaxSr1−x)10(PO4)6F2) ceramics,which are solid solutions composed of Ca and Sr, has been reported [18]. In such mixed-crystalmaterials, compositional disorder and differences in the ionic radii of the constituent ions reducesite symmetry, resulting in a more inhomogeneous crystal field and significantly broadenedabsorption and emission bands [19]. Such compositional tuning enables the developmentof materials with broad emission bands suitable for ultrashort-pulse generation and has beensystematically investigated in various rare-earth-doped mixed materials [20–22]. Accordingly,Nd3+-doped CS-FAP ceramics have potential applications as novel materials for ultrashort-pulselasers; however, laser oscillation has not yet been achieved in Nd:CS-FAP. To the best of ourknowledge, the only reported laser oscillation in mixed fluorapatite materials was for Yb:CS-FAPsingle crystal [23,24].In this study, we fabricated transparent Nd:CS-FAP ceramics via ceramic processing techniquesto obtain broadened emission spectra, with the aim of developing new broadband laser materialsfor ultrashort pulse generation. We evaluated their fluorescence and laser properties and comparedthe fluorescence properties of CS-FAP ceramics with those of C-FAP and S-FAP to assess thepotential of CS-FAP as a broadband laser material.2. Experimental method2.1. Material preparationC-FAP and S-FAP powders with a nominal Nd3+ concentration of 1 at.% were initially synthesizedfollowing the procedures described in our previous studies, where detailed information on theircrystal structure and microstructure can be found [15,17]. Trivalent Nd3+ ions can substitute forthe divalent Ca2+ or Sr2+ sites in the apatite lattice, preferentially occupying the Ca(II) or Sr(II)sites (coordination number = 7). However, owing to the difference in valence, this substitutionrequires charge compensation, which commonly occurs via the incorporation of O2− ions or theformation of fluorine vacancies, thereby maintaining overall charge neutrality [9].These powders were then mixed at a 1:1 molar ratio of Ca to Sr using a ball mill andsubsequently dried. This equimolar ratio was selected because the effects of compositionaldisorder on lattice distortion and spectral broadening are generally most pronounced near equalproportions of the two cations in mixed-crystal systems. The mixed powders were sintered toResearch Article Vol. 16, No. 4 / 1 Apr 2026 / Optical Materials Express 994produce Nd-doped CS-FAP ((Ca0.5Sr0.5)10(PO4)6F2) transparent ceramics using a spark plasmasintering system (LABOX-315, Sinter Land), which enables sintering under a vacuum withuniaxial pressure. The sintering temperature was 950°C, the holding time was 20 min, and theapplied pressure was 80 MPa. After sintering, both surfaces of the ceramics were mirror-polished.The resulting sample had a diameter of 10 mm and a thickness of approximately 0.8 mm.For material characterization, the in-line transmittance, crystal structure, and microstructurewere evaluated using a UV-VIS-NIR spectrometer (UV-3600i Plus, Shimadzu), X-ray diffraction(XRD, RINT-TTR III, Rigaku), and field emission scanning electron microscope (FE-SEM,S-4800, Hitachi).2.2. Emission and laser propertiesFluorescence spectrum measurements and laser oscillation tests at both 1 µm and 1.3 µm wereconducted for Nd:CS-FAP ceramics. A fiber-coupled laser diode (LD) with a wavelength of808 nm was used as the pump source. The fluorescence and lasing spectra were measured usingan optical spectrum analyzer (Q8383, Advantest). Figure 1 presents a schematic of the laser test.The configuration for laser oscillation tests was the same as that used in our previous study [25].The core diameter and maximum output power of the fiber-coupled LD were 105 µm and 60 W,respectively. The fiber output was focused to approximately 105 µm on the Nd:CS-FAP ceramicsusing a pair of lenses with identical focal lengths. The LD was operated in quasi-continuouswave (QCW) mode with a pulse width of 1 ms and a repetition rate of 10 Hz, to avoid thermalissues in the gain medium. The laser tests were performed using a cavity length of approximately1 mm formed by a flat dichroic mirror and a flat output coupler, and the average output powerwas measured using a power meter.Fig. 1. Experimental setup of the laser test for Nd:CS-FAP ceramics.For the laser tests at 1 µm, the output coupler reflectivity was varied from 70–99% to determinethe optimal output coupler reflectivity. For the 1.3 µm wavelength range, a reflectivity of 90%was used.3. Results and discussion3.1. Material characteristicsFigure 2(a) shows a photograph of the sample and in-line transmittance spectrum of Nd:CS-FAP ceramics. The dashed line indicates the theoretical transmittance calculated from theaverage refractive indices of C-FAP and S-FAP [9,26]. The in-line transmittance at the laserwavelength of 1060 nm was 85.7%, which corresponds to a total loss coefficient of approximately0.5 cm−1. The reduction in transmittance at shorter wavelengths was mainly attributed to asmall number of residual pores and grain boundary scattering, which cannot be eliminated inResearch Article Vol. 16, No. 4 / 1 Apr 2026 / Optical Materials Express 995fine polycrystalline ceramics. Laser-grade optical quality (total loss coefficient < 0.01 cm−1)requires minimizing these loss sources. Achieving this will require further refinement of ceramicprocessing conditions, including optimization of powder synthesis, high-density compaction,and post-sintering treatments such as hot isostatic pressing (HIP).Fig. 2. (a) In-line transmittance and (b) absorption intensity of Nd:CS-FAP ceramics.Figure 2(b) shows the normalized absorption spectra around the excitation wavelength. Theabsorption values were first calculated using the total loss coefficient derived from the in-linetransmittance and sample thickness based on Eq. (1). The baseline scattering coefficient wasthen subtracted to isolate the absorption attributable only to Nd3+ ions. Each spectrum wassubsequently normalized to its maximum value. The absorption peak wavelengths for C-FAP,S-FAP, and CS-FAP were 808.7 nm, 806.2 nm, and 807.4 nm, respectively. Among thesecompositions, CS-FAP exhibited a much broader absorption band.Figure 3 shows the XRD patterns of Nd:CS-FAP ceramics, along with those of Nd:C-FAPand Nd:S-FAP for comparison. XRD analysis confirmed that Nd:CS-FAP formed a single-phaseapatite structure. Furthermore, the XRD patterns indicate that the ceramics were composed ofgrains with nearly random orientations. Figure 3 also presents an enlarged view of the peaksbetween 30° and 32.5°, revealing that the main diffraction peak of Nd:CS-FAP was locatedbetween those of C-FAP and S-FAP. These results suggest that Ca and Sr were homogeneouslyincorporated into the apatite lattice to form a solid solution. The lattice parameters estimatedfrom these XRD peak positions are as follows (in Å): Nd:C-FAP (a= 9.38, c= 6.89), Nd:CS-FAP(a= 9.56, c= 7.09), and Nd:S-FAP (a= 9.73, c= 7.28). The gradual increase in both latticeparameters with increasing Sr content provides further evidence of the formation of a continuoussolid solution.Figure 4(a) shows the microstructure of the polished ceramic surface after thermal etchingat 800°C. The ceramics were dense, with no significant residual pores, secondary phases, orother scattering sources observed. Figure 4(b) shows the grain size distribution evaluated fromFE-SEM images. The average grain size was calculated from at least 300 individual grains bydetermining the equivalent circular diameter under assumption of spherical grains [11]. Thegrain sizes ranged from approximately 50 to 200 nm, with an average grain size of about 100 nm.This grain size and morphology are comparable to those previously reported for Nd:C-FAP andNd:S-FAP ceramics [15,17].Research Article Vol. 16, No. 4 / 1 Apr 2026 / Optical Materials Express 996Fig. 3. XRD peak pattern for Nd:C-FAP, Nd:CS-FAP and Nd:S-FAP ceramics.　Fig. 4. (a) FE-SEM image of the polished surface of Nd:CS-FAP ceramics. (b) Grain sizedistribution of the ceramics, showing an average grain size of approximately 100 nm.Research Article Vol. 16, No. 4 / 1 Apr 2026 / Optical Materials Express 997In addition to improving the ceramic processing to enhance optical quality, evaluating opticalhomogeneity is also important. In future work, we aim to further improve material quality byassessing the transmitted wavefront after polishing and conducting elemental analysis of themicrostructure.3.2. Fluorescence and laser propertiesFigures 5(a) and (b) show the fluorescence spectra of Nd:CS-FAP in the 1 µm and 1.3 µmwavelength ranges, which correspond to the 4F3/2 → 4I11/2 and 4F3/2 → 4I13/2 transitions ofNd3+ ions, respectively. For comparison, the spectra of the Nd:C-FAP and Nd:S-FAP were alsoincluded. In the spectra of Nd:CS-FAP, a peak shift and spectral broadening were observed atboth wavelengths, attributable to the solid solution effects of Ca and Sr. The FWHM values forC-FAP and S-FAP are approximately 1 nm (corresponding to 9 cm−1 at 1060 nm and 6 cm−1 at1330 nm), while Nd:CS-FAP exhibited FWHM values of 3.5 nm (31 cm−1) at 1061 nm and 6.4 nm(36 cm−1) at 1330 nm, respectively. These spectral widths correspond to potential pulse durationsof 340 fs and 290 fs at 1061 nm and 1330 nm, achievable through mode-locked operation. Theseresults highlight the potential of Nd:CS-FAP as a novel ultrashort-pulse laser material.Fig. 5. Comparison of the emission properties of Nd:C-FAP, Nd:CS-FAP and Nd:S-FAPfor (a) 1 µm and (b) 1.3 µm wavelengths.Figure 6 shows the temporal decay curves of Nd:CS-FAP ceramics together with those ofNd:C-FAP and Nd:S-FAP. The fluorescence was detected at around 1 µm (corresponding tothe 4F3/2 → 4I11/2 transition) using a Si photodetector and a 1000 nm long-pass filter. Nosignificant quenching processes were observed for CS-FAP ceramics, and the fluorescencelifetime, determined by single-exponential fitting, was 186 µs. This value lies between thelifetimes of C-FAP (156 µs) and S-FAP (218 µs). The trend of an increasing fluorescencelifetime with a higher Sr concentration was also observed in a previous study [18], possibly dueto differences in the phonon energies of the C-FAP and S-FAP hosts, which affect the rate ofnon-radiative transitions.Figures 7(a) and (b) show the laser properties at wavelengths of 1 µm and 1.3 µm, respectively.Typical lasing spectra are also shown, with oscillation observed at 1061 nm and 1333 nm. Themaximum output and slope efficiency of 6.3 mW and 6.1%, respectively, at 1 µm was obtainedwith an output coupler reflectivity of 90%. The slope efficiency was somewhat lower than thevalues exceeding 10% reported for Nd:C-FAP and Nd:S-FAP. This reduction may be attributedResearch Article Vol. 16, No. 4 / 1 Apr 2026 / Optical Materials Express 998Fig. 6. Lifetimes of Nd-doped fluorapatite ceramics.to the broader emission spectrum, which likely resulted in a lower stimulated emission crosssection. At around 1333 nm, laser oscillation was also observed with a slope efficiency of 1.2%.To the best of our knowledge, this is the first demonstration of laser oscillation using Nd:CS-FAP.Fig. 7. Average output power as a function of the absorbed pump power for Nd:CS-FAPceramics at (a) 1 µm and (b) 1.3 µm wavelengths. The insets show typical lasing spectra.Fitting the data for 1 µm yields lasing thresholds of 66.4 mW (R= 70%), 46.7 mW (80%),41.9 mW (85%), 34.5 mW (90%), 31.4 mW (95%), and 33.3 mW (99%). Although the thresholddecreases monotonically up to R= 95%, it increases slightly at R= 99%. The precise reason forthis anomaly remains unclear; however, one possible explanation is that the finite internal loss ofthe ceramics (∼ 0.5 cm−1) becomes the dominant loss when the output coupling is extremely low.From the relationship between the output coupler reflectivity (70–95%) and threshold, the cavityloss was estimated to be 19.2%.Research Article Vol. 16, No. 4 / 1 Apr 2026 / Optical Materials Express 999Future work will focus on further improving the optical quality of the ceramics throughmaterial development. In addition, the introduction of optical coatings and the optimizationof crystal thickness and cavity design are planned to enhance laser performance. Additionally,future research will focus on realizing ultrashort pulse operation, thereby advancing this materialas a promising candidate for next-generation laser applications.4. ConclusionIn this study, transparent ceramics of hexagonal mixed fluorapatite Nd:CS-FAP were successfullyfabricated, and laser oscillation was demonstrated for the first time. The ceramics exhibited a highoptical transmittance of 85.7% for the 0.8 mm thick sample, corresponding to a loss coefficientof 0.5 cm−1. The ceramics exhibited a hexagonal single-phase with an average grain size ofapproximately 100 nm. Compared with C-FAP and S-FAP, CS-FAP showed broader absorptionand fluorescence spectra, indicating its potential as an ultrashort-pulse laser material. Laseroscillation was observed at both 1061 nm and 1333 nm. Further enhancements in performancecan be expected through improvements in the optical quality of the material and optimization ofthe cavity design.Funding. Japan Science and Technology Agency (JPMJFR203S).Acknowledgment. Portions of this work were presented at Advanced Solid-State Lasers 2025 (ASSL2025), paperJTu2A.16, and has been revised for this manuscript.Disclosures. The authors declare no conflicts of interest.Data availability. Data underlying the results presented in this paper are not publicly available at this time but maybe obtained from the authors upon reasonable request.References1. P. Mason, M. Divoký, K. Ertel, et al., “Kilowatt average power 100 J-level diode pumped solid state laser,” Optica4(4), 438–439 (2017).2. M. Divoký, J. Pilař, M. Hanuš, et al., “150 J DPSSL operating at 1.5 kW level,” Opt. Lett. 46(22), 5771–5773 (2021).3. 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).4. S. Tokita, M. Divoký, H. Furuse, et al., “Generation of 500-mJ nanosecond pulses from a diode-pumped Yb:YAGTRAM laser amplifier,” Opt. Mater. Express 4(10), 2122–2126 (2014).5. A. Ikesue and Y. L. Aung, “Ceramic laser materials,” Nat. Photonics 2(12), 721–727 (2008).6. R. C. Ohlmann, K. B. Steinbruegge, and R. 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