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Transparent Ho2O3 ceramics are fabricated and their magneto-optical characteristics are reported for the first time, to the best of our knowledge. The value of the Verdet constant was measured in the 560–1064 nm wavelength range, and the value at 1064 nm is 46.3 rad/Tm. This corresponds to the Verdet constant for terbium aluminum garnet (TAG), which is ~1.3 times higher than that of terbium gallium garnet (TGG). The in-line transmittance at 1 μm wavelength is only ~60%, but the optical properties can be further improved by optimizing sintering conditions. This is a new potential magneto-optic material that can be applicable for high-average-power lasers.
© 2017 Optical Society of America
1. Introduction
Magneto-optic elements (MOE) are one of the more important components in high-average power lasers for optical polarization control and as an optical isolator. Recent progress in the high-power solid state laser field based on Yb or Nd-doped materials has revealed that MOEs, which can be applicable in kilowatt class lasers are extremely desirable. To satisfy these demands, new functional magneto-optic materials with high Verdet constants, low absorption coefficients, and large scalable sizes are being developed.
Currently, terbium gallium garnet (TGG) ceramics, which have a moderate Verdet constant of 36.4 rad/Tm at 1064 nm [1,2], a low optical absorption of 1.3 × 10−3 cm−1 [3], and a large size scalability of 100 cm2 [4], is the most common magneto-optic material used in high power lasers. On the other hand, researchers are constantly investigating materials for new MOEs with higher Verdet constants than TGG, such as terbium aluminum garnet (TAG) [5]. The ceramics properties can also be improved by adding dopants. For example, it has been reported that doping TAG with Ce or Ti improved their magneto-optic characteristics [6,7].
Researchers also have shown that a 0.2 at.% Ho-doped TGG single crystal has a Verdet constant two times higher than that of TGG [8]. These studies show that it is important not to only search for a new host material but also effective dopants and dopant concentrations to obtain ideal MOEs.
Besides the Tb3+ ion, Dy3+ or Ho3+ ions are among the most magnetically active candidates. In fact, the Verdet constants of rare-earth aluminum garnets are measured in the visible wavelength range, and DyAlG and HoAlG represent the second- and third-largest Verdet constants, respectively [9]. Furthermore, it is predicted that rare earth sesquioxides (Re2O3) have larger magneto-optic characteristics than complex oxide structures because they have a higher volume percentage of Re3+ ion. However, Re2O3 has a high melting point of ~2400 °C; therefore, not every composition can be grown from a melt.
A low temperature ceramic process is often employed for making transparent polycrystalline ceramic materials. Most recently, Snetkov et. al., fabricated transparent Tb2O3 ceramics and reported on their magneto-optic characteristics [10]. The Verdet constant of Tb2O3 at 1064 nm was 128.4 rad/Tm, which is 3.5 times higher than that of TGG, and it is the highest Verdet constant among the transparent materials in this wavelength range, to our knowledge. However, it may be challenging to improve optical properties that approach the theoretical transmittance because of the difficulty in controlling the valence of Tb3+ ions.
In 2011, transparent Dy2O3 ceramics were manufactured for designing novel Faraday rotators at visible wavelengths [11]. In that study, the optical transmittance was ~50% at 633 nm, and the Verdet constant at the same wavelength was 300 rad/Tm, which is 2.25 times higher than that of TGG. Unfortunately, the Dy3+ ion has strong absorption characteristics at 1 μm wavelength due to the 6H15/2 to 6H7/2, 6F9/2 energy transition; it cannot be used for high average power laser systems based on Yb or Nd-doped materials.
The next candidates are Ho2O3 ceramics, which have no dominant absorption between 1000 nm and 1100 nm, making them a prime candidate for further investigation of magneto-optic properties. In this paper, we will discuss the fabricated transparent Ho2O3 ceramics and its magneto-optic characteristics in the 560–1064 nm wavelength range. The optical transmittance and crystal structure are also presented.
2. Experimental method
In this study, a transparent Ho2O3 ceramics was fabricated by a pulse electric current sintering technique, known as spark plasma sintering (SPS). The advantages of this sintering technique compared with other conventional techniques such as hot isostatic pressing (HIP) or pressure-less vacuum sintering are shorter sintering times of several hours due to higher heating rates (greater than 100 °C/min) and lower sintering temperatures. Thus, this sintering method is very useful for evaluating dopant effects or developing a database for various materials.
Commercially obtained Ho2O3 powder (Shin-Etsu Chemical Co., Ltd., Japan; 99.9% purity) with an average particle size of 350 nm was used. The powder was put into a graphite die and sintered under vacuum. The sintering temperature was 1100 °C, and it was pressed to 80 MPa during the entire sintering process. After annealing in air to compensate for oxygen defects and to remove carbon contamination, both surfaces of the sample were polished. We prepared two specimens with 0.60 and 1.01 mm thickness for the following experiments. The sample thickness was measured by using a micrometer screw gauge.
The optical transmittance spectrum and the crystal structure were measured for the 0.60-mm-thick specimen using a UV/VIS/NIR spectrometer (UV-3100 PC, Shimadzu, Japan), X-ray diffraction (XRD; Ultima IV, Rigaku, Japan). The wavelength dependence of the Verdet constant was evaluated using the 1.01-mm-thick specimen. For the measurements, the Faraday rotating angle was measured using a supercontinuum source (SuperK COMPACT, NKT Photonics) in the 560–1020 nm wavelength range, spectrometers (USB2000, Ocean optics), two Glan prism polarizers, and a permanent magnet with 1.17 T. We also measured the Verdet constant at 1064 nm using a Nd:YAG laser.
3. Results and discussion
3.1 Crystal structure and optical transmittance of Ho2O3 ceramic
Figure 1 shows the XRD results and a photo of the Ho2O3 ceramic sample. In Fig. 1, the hkl indices of Ho2O3 diffraction peaks (JCPDF No. 44-1268) are also shown. The results show that the peaks match well to the diffraction pattern of Ho2O3, and thus the obtained sample can be considered an almost pure phase of Ho2O3.
Figure 2 shows an optical transmitted spectrum of the specimen. The thickness of the Ho2O3 ceramics was 0.60 mm. There are some absorption peaks around 1800–2000 nm (5I8 to 5I7), 1100–1200 nm (5I8 to 5I6), 900–920 nm (5I8 to 5I5), 630–670 nm (5I8 to 5F5), and 520–560 nm (5I4 + 5S2). For the 1000–1100 nm wavelength range, the absorption is relatively small, and in our first result, we obtained 58% in-line transmittance at 1064 nm.
The dashed line in Fig. 2 shows the transmittance of Ho2O3 calculated using the dispersion formula:
where n is the refractive index, λ is the wavelength of light, and A = 0.0124 (μm2) and B = 0.3875 are the mean dispersion parameters [12]. Using this equation, the refractive index and the ideal transmittance at λ = 1064 nm are calculated to be n = 1.912 and T = 81.3%, respectively. Although the optical property of our sample can be further improved by optimizing the sintering conditions, it is satisfactory for measuring the magneto-optic coefficient. For use in high-average-power lasers, ideal magneto-optic materials require not only high Verdet constants but also low absorption coefficients at the incident laser wavelengths. We would like to discuss the absorption coefficient of Ho2O3 ceramics at 1-μm wavelength range after further improving the optical properties.3.2 Verdet constant of Ho2O3 ceramics
Figure 3 shows the measured signal intensity as a function of the rotation angle of the analyzer for the Nd:YAG laser. The red and black marks represent the results without and with the magnetic field of 1.17 T, respectively. The results were fitted by a trigonometric function, and the Faraday rotating angle θ was obtained based on the phase difference. The value of θ with standard deviation was 0.05467 ± 0.000449 rad, which corresponds to 3.13° ± 0.026° as shown in the inset of Fig. 3, and the value of the Verdet constant V at 1064 nm is estimated to be 46.26 ± 0.38 rad/Tm by θ = VBL. Here, B = 1.17 T and L = 1.01 mm were used.
Figure 4 shows the wavelength dependence of the Verdet constant of Ho2O3. The experimental data at the absorption band were eliminated in the figure. In the figure, the error bar for each experiment is shown, and the measurement errors for a supercontinuum source were less than ± 1.0 rad/Tm. In Fig. 4, the Verdet constant at 1064 nm measured with a Nd:YAG laser is also shown (blue square). In this experiment, the values of the Verdet constant are 178.1 rad/Tm and 46.3 rad/Tm at the 600 nm and 1064 nm wavelengths, respectively. These values are similar to those of TAG ceramics reported in Refs [5,13]. The Verdet constant at 1064 nm is ~1.3 times higher than that of TGG.
The dashed line in Fig. 4 shows the wavelength dependence of the Verdet constant fitted by a formula
where A and λ0 are the approximation parameters. A is proportional to the concentration of the Re3+ ion and the transition probability, and λ0 represents the transition wavelength. A detailed discussion about these parameters can be found in Ref [14]. From our experimental data, the fitting parameters were A = 5.81 × 107 rad nm2/(Tm), and λ0 = 173 nm, respectively. However, the theoretical curve poorly matched the experimental data because Eq. (2) is simplified for a single electronic transition, whereby a Ho3+ ion has many electronic transitions, as shown in Fig. 2. To obtain a more precise fitting curve and to discuss the wavelength dependence of the Verdet constant in more detail, the experimental data should be analyzed using a formula describing several transitions between electronic states in the material, as shown in Ref [14]. In future, we will increase the value of the Verdet constant with respect to doping concentration, temperature, and wavelength over a wide range to advance this analysis.Table 1 lists the Verdet constants at a 1 μm wavelength for different candidate materials for next generation high-average power lasers. For comparison, the ratio of the Verdet constant of the candidate material to the Verdet constant of TGG (V/VTGG) is also listed. Table 1 shows that the highest Verdet constant is for Tb2O3, followed by Ho:TGG [8]. In their study of Ho:TGG, Chen et al. explain that a Ho-Tb interaction dramatically improves the magneto-optic property of Ho-doped TGG single crystals, although the doping concentration of the Ho3+ ion was only 0.2 at.%. It can be expected that Tb-doped Ho2O3 will also have a higher Verdet constant. Of course, another dopant such as Ce would be effective as well TAG for increasing the Verdet constant [15]. The effect of dopants is under investigation, and this will be reported in the future.
Table 1. Comparison of Verdet constant for different materials at 1 μm wavelength range.
The Verdet constant of Ho2O3 is close to that of undoped TAG ceramics; however, the optical quality is not as high for the current Ho2O3 sample. Nevertheless, some sesquioxide laser ceramics such as Yb:Y2O3 [16], Nd:Lu2O3 [17], and Ho:Y2O3 [18] have already been developed by manufacturing companies, and we have a plan for the synthesis of high-optical-grade Ho2O3 after confirming the optimal additive conditions.
Additionally, one of the advantages of Ho2O3 over TAG and TGG is its simpler synthesis process. For the Tb-based materials, the valence of ion should be controlled from Tb4+ to Tb3+ because Tb4+ reduces both magneto-optic and optical properties, although this issue would be less for Ho2O3. Furthermore, reasonably accurate mixing process is required for the complex oxide materials; however, this process is unnecessary for sesquioxide materials. Finally, we believe that preparation of several candidates for next-generation magneto-optic materials is important for advancement in the solid-state laser field.
4. Conclusion
In conclusion, we have fabricated transparent Ho2O3 ceramics and analyzed their magneto-optic characteristics in the 560–1064 nm wavelength range. The Verdet constant of Ho2O3 ceramics at 1064 nm corresponds to TAG ceramics, and is ~1.3 times higher than that of TGG. The XRD results show a pure Ho2O3 phase, and an in-line optical transmittance of over 58% at 1064 nm was achieved. To our knowledge, this is the first investigation into the magneto-optics of Ho2O3, which has the potential to be used for a Faraday rotator in high-average power lasers. It lays the groundwork for further investigations that may provide significant improvements for the properties of MOEs. Since this manuscript presents the first reported data for Ho2O3, the detailed analysis of the wavelength dependence of the Verdet constant has not yet been performed. We will present the detailed discussion on the temperature- and wavelength-dependencies of the Verdet constant in future studies.
The next step of this study is to explore dopant effects on magneto-optic properties of Ho2O3. The Verdet constant of several dopants at various concentrations are under investigation in order to determine the optimal dopant combination. In addition, the optical transmittance will be further improved by adjusting the sintering conditions such as temperature, heating rate, holding time, and pressure. Fabrication of transparent Ho2O3 ceramics by other sintering techniques such as pressure-less vacuum sintering or HIP may also be explored. After improving the optical properties, thermally induced depolarization and thermal lens effects of Ho2O3 will be examined for the ceramic’s suitability as a high-average power laser MOE. For a more detailed analysis, thermal conductivity and thermo-optic effects will be also investigated.
Funding
This work was performed with the support and under the auspices of the NIFS Collaboration Research program (NIFS15KBAH010). This work was also supported by JSPS KAKENHI (Grant Nos. 15K18207, 15K13386 and 26709072).
Acknowledgments
The authors appreciate the assistance of David Vojna in the magneto-optic experiment.
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