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Abstract
We fabricated an advanced transparent Yb-doped hexagonal fluorapatite ceramic with an average grain size of only 90 nm; we also demonstrated its laser oscillation. To the best of our knowledge, this is the smallest grain size for laser ceramics. The in-line transmittance was 86.1% at 1 µm for 0.79-mm-thick ceramics (corresponding to a loss coefficient of 0.45 cm-1). Although the ceramic crystal grains were randomly oriented, scattering at the grain boundary was suppressed because the grains were considerably smaller than the wavelength. This novel ceramic possesses both a fine microstructure and densification and is expected to be the foundation of many non-cubic laser ceramics.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Polycrystalline transparent ceramics composed of multiple crystal grains are used in many optical applications such as lasers [1,2], Faraday materials [3,4], scintillators [5], and phosphors [6] owing to their excellent structural and functional properties. The average grain size of typical transparent ceramics is greater than 1 µm [7]—equivalent to or greater than the wavelength of light in the near-infrared region. Therefore, the in-line transmittance is attenuated by grain boundary scattering due to the birefringence of optically anisotropic materials (i.e., non-cubic materials), such that the only transparent polycrystalline ceramic material with laser grade is the optically isotropic cubic material.
The development of transparent non-cubic ceramics is a breakthrough for next-generation optical materials and technologies because the advantages of ceramic technology, such as enlargement, ease of compositing, optical uniformity, and high concentration of soluble active elements, can be applied to non-cubic crystal materials as well. Furthermore, such ceramic technology enables production of transparent bulk materials at temperatures below the melting point, even for high-melting-point materials or decomposition melting materials, for which single crystals are difficult to obtain. This indicates the possibility for developing new optical materials that have never been realized.
Many researchers have been working on experimental and theoretical studies to overcome this challenging issue, particularly with alumina (Al2O3) [8–10]. The scattering coefficient at the birefringent grain boundary based on Rayleigh-Gans-Debye theory γ can be represented as [11]:
where d, Δn, and λ are the grain size, average difference in the refractive indices of grains, and wavelength of light, respectively. From Eq. (1), it is evident that one of the effective ways to decrease γ is reducing Δn. In 2011, Akiyama et al. reported the laser oscillation of Nd-doped hexagonal fluorapatite (Ca10(PO4)6F2: FAP) by controlling the crystal orientation using a magnetic field and reducing Δn [12]. This was the first laser demonstration of non-cubic ceramic materials. Further, Sato et al. developed transparent crystal-oriented Yb:FAP ceramics and demonstrated its Q-switch laser operation [13].By contrast, the other approach to decrease γ is reducing d. In 2019, we firstly demonstrated the fabrication and laser oscillation of transparent Nd:FAP ceramics with randomly oriented crystal grains by reducing the average grain size d [14]. To achieve both fine microstructure and full densification, low-temperature sintering by pulsed electric current and uniaxial pressing were adopted. The realized average grain size, d, was approximately 140 nm—approximately one order of magnitude lower than that of conventional transparent ceramics; the volume of one crystal grain is as small as 1/1000. We consider this non-cubic laser ceramic with random crystal orientation to be a novel laser ceramic material.
The fabrication and evaluation of rare-earth ion doping materials, except Nd3+, are effective for laser applications. Particularly, Yb-doped materials are known to have higher Stokes efficiency than Nd3+-doped materials and are expected to be promising high-power laser materials. Thus, the purpose of this study is to demonstrate the laser oscillation of fabricated high-optical-quality Yb:FAP ceramics having a fine microstructure.
2. Experimental method
As the fabrication of transparent FAP ceramics is detailed in an existing paper [14], we have briefly explained it for the sake of completeness. First, 1 at.% Yb:FAP powder was synthesized by a wet chemical route. YbCl3·6H2O, calcium hydroxide, and phosphoric acid were used as the starting materials, which were mixed to prepare the Yb-doped hydroxyapatite (Ca10(PO4)6(OH)2: HAP) precursor. Thereafter, this Yb:HAP precursor was mixed with a moderate amount of CF3CONH2 for fluorine substitution. The mixture was dried and heated at 600°C for 2 h in air to synthesize the Yb:FAP powder. The size of the powder was approximately 50 nm. Second, the powder was sintered via spark plasma sintering (SPS) to produce a fine microstructure [15]. A graphite mold with a 10 mm inner diameter and a corresponding punch were used. The die and punch were heated at a rate of 5 °C/min under a uniaxial pressure of 80 MPa. The sintering temperature was 900 °C. We prepared several Yb:FAP ceramics under the same conditions for material characterization and lasing tests. After sintering, both surfaces of the ceramics were mirror-polished. The final samples used to measure the optical properties and conduct lasing tests were 10 mm in diameter and 0.79 mm in thickness.
3. Results and discussion
3.1 Characterization of crystal structure and microstructure
Figure 1 shows the X-ray diffraction spectrum of the Yb:FAP powder and ceramics. The diffraction angles of the ceramics coincide well with those of the standard FAP powder (JCPDS card No. 71-881), indicating that the ceramics consist of randomly oriented grains as well as the initial Yb:FAP powder. In addition, no secondary phase can be observed.
Figure 2(a) shows a photograph of the sample, which demonstrates sufficiently high optical transmittance in the visible range. The optical transmittance at λ = 500 nm was over 80% despite the fact that optical scattering increases at shorter wavelengths. To observe the microstructure, the polished surface of the Yb:FAP ceramics was thermally etched in air at 800 °C for 1 h. The microstructure of the ceramics examined by field-emission scanning electron microscopy (FE-SEM) is shown in Fig. 2(b). This micrograph shows a dense microstructure and no secondary phases, which agree well with the results of XRD as shown in Fig. 1. The average grain size was evaluated from FE-SEM micrographs of over 500 grains by assuming that the crystal grains are spheres. From the average cross-sectional area per grain Sg, the average grain size d was evaluated as Sg = (1/6)πd2 [11,15]. The realized average grain size d was approximately 90 nm that was significantly smaller than that of our previously reported Nd:FAP (d of approximately 140 nm). Because the Nd:FAP was sintered at 950 °C while Yb:FAP was sintered at 900 °C, we attributed the difference in the average grain size to the sintering temperature. To the best of our knowledge, no other laser ceramics with a microcrystalline structure having a grain size less than 100 nm exist. This special ceramic has been realized using advanced powder synthesis and sintering technology.
Fig. 2. (a) Photo of Yb:FAP ceramics with a thickness of 0.79 mm, and (b) FE-SEM image of a polished and thermally etched Yb:FAP ceramics.
3.2 Optical and fluorescence properties
The optical in-line transmittance of Yb:FAP was measured using a spectrometer. For the measurement, an optical aperture with an inner diameter of 5 mm was used. Figure 3 shows the measured in-line transmittance for a 0.79-mm-thick sample and the evaluated loss coefficient, including both absorption and scattering, of the Yb:FAP ceramics. The dashed line represents the theoretical transmittance of FAP calculated from the average refractive index dispersion [14,16]. Typically, grain-boundary reflections may be expected to increase with this type of ultrafine microstructure. In this case, however, the maximum discontinuity of the refractive index (Δnmax) at the grain boundaries for randomly oriented FAP is quite low, i.e., Δnmax ∼ 0.003. Thus, the total reflection for 1-mm-thick ceramics with an average grain size of 90 nm (that is, ∼ 1.1 × 104 grain boundaries) can be evaluated to be below 1%, even assuming that the light passes through all grain boundaries at a normal angle. Therefore, the reflection of light at grain boundaries can be neglected in the present FAP ceramics in this study, similar to the findings in [11].
In Fig. 3, Yb:FAP is seen to have strong absorption peaks at 905 and 983 nm. A small absorption peak can be observed at 1043 nm due to the reabsorption triggered by Yb3+ ions. In addition, we attribute a medium absorption peak at 910 nm to Yb:HAP that could not be substituted to Yb:FAP owing to the lack of fluorine compound during the powder synthesis process.
The loss coefficient of the Yb:FAP ceramics at a wavelength of 1 µm was 0.45 cm-1, although the grain boundary scattering coefficient was calculated using Eq. (1) to be 0.06 cm-1. This discrepancy may be due to few residual pores that were unconfirmed by FE-SEM because the sintering temperature of Yb:FAP was lower, and any secondary phases were not observed by XRD and FE-SEM. In this case, residual pores may be removed by performing low-temperature HIP treatment, and the total loss coefficient may also be reduced further.
The fluorescence spectrum was measured using an optical spectrum analyzer and laser diode operating at approximately 905 nm. Figure 4(a) shows the fluorescence spectrum of the Yb:FAP ceramics; strong fluorescence peaks can be observed at wavelengths of 982.8 and 1044 nm, which reasonably agree with those of Yb:FAP single crystal [17]. Figure 4(b) shows the fluorescence decay curve of the ceramics at room temperature. From the experimental data, we determined the lifetime of the Yb:FAP to be 309 µs, which is considerably smaller than that of Yb:FAP single crystal, which was reported to be 1100 µs [18]. One of the reasons for this discrepancy may be fluorescence quenching owing to non-radiative energy transfer between Yb3+ and residual hydroxy oxide ions [19]. We believe that the amount of the hydroxy oxide can be reduced by optimizing the powder synthesis procedure, resulting in a longer lifetime.
3.3 Laser properties
Figure 5(a) shows the experimental setup for the lasing test. The cavity length was about 1 mm with a flat dichroic mirror (DM; anti-reflection (AR) coated at 800–1000 nm and high-reflection coated at 1020–1200 nm) and a flat output coupler (OC; reflectivity of 95% at 1000–1100 nm). The Yb:FAP sample without AR coating was attached to the DM. A continuous wave (CW) of 30 W fiber-coupled laser diode (LD) was used as the pump source. The core diameter of the fiber was 100 µm, and the fiber end face image was relayed to the sample using lens groups. The center wavelength of the LD was 910.8 nm at the maximum output power under CW operation. Because the sample was not cooled in the test, the pump source was driven under a quasi-CW mode with a 5 ms pump duration and 10 Hz repetition rate to prevent any thermal issues.
Fig. 5. (a) Experimental setup for lasing test, and (b) laser output power as a function of absorbed pump power. Inset shows the typical lasing spectrum with emission spectrum (gray dashed line) as reference.
Figure 5(b) shows the output average power as a function of the absorbed pump power. A slope efficiency of 4.6% was obtained, which is comparable to that of Nd:FAP ceramics [14]. The laser performance can be further increased by improving the fluorescence lifetime and optical quality, reducing the Fresnel loss by depositing an AR-coating, optimizing the cavity design, and introducing a cooling system. The inset shows a typical lasing spectrum, confirming lasing at 1043 nm. The gray dashed line represents the emission spectrum as shown in Fig. 4(b) as reference. Yb:S-FAP single crystal can oscillate at both 985 and 1047 nm [20,21]; similarly, Yb:FAP ceramics can oscillate at both 983 and 1044 nm if the coating of the mirror used for the cavity is optimized.
Apart from laser characteristics, we will also investigate the difference in fluorescence properties between randomly oriented crystal ceramics and single crystals in future; the grain size dependence of basic physical properties such as mechanical strength and thermal conductivity are also important parameters for thermal analyses.
Additionally, powder synthesis and densification of Yb:S-FAP, which has already been attempted for fabrication without lasing [22,23], will be more relevant for high power laser applications. Furthermore, it may be beneficial to dope other rare-earth elements that radiate at longer wavelengths (e.g., Er, Ho, and Tm) because the scattering coefficient at the grain boundary is inversely proportional to the square of the wavelength and the scattering becomes low at longer wavelengths, as shown in Eq. (1). Moreover, we believe that regulating the magnetic field orientation of this kind of fine microstructural ceramic material can be effectively reduce the boundary scattering, and improve the optical quality further, particularly for highly anisotropic non-cubic host materials besides apatite.
4. Conclusion
Transparent ceramic technology for non-cubic materials is promising for next-generation laser and optical technologies. In this study, we fabricated an advanced transparent hexagonal Yb:FAP ceramics having a randomly oriented ultrafine crystal microstructure and demonstrated its laser characteristics at 1043 nm. The average grain size was only 90 nm, and the scattering at the grain boundary due to birefringence was highly reduced. High optical transparency was obtained, and the in-line transmittance in the 1 µm wavelength range was 86.1%—close to the theoretical transmittance. Although the slight scattering may be attributed to the small number of residual pores, a low-temperature HIP treatment may further improve the optical transmittance. We believe that many laser ceramics with non-cubic crystal structures, which have never been realized, will be actively developed in the future.
Funding
Japan Society for the Promotion of Science (18K04687); Matsuo Foundation.
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 may be obtained from the authors upon reasonable request.
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