The temperature dependence of the Verdet constant of a 0.8 at % Ti-doped terbium aluminum garnet (TAG) ceramics was investigated using lasers with wavelengths of 632.8 and 1064 nm. A high value of the Verdet constant was obtained at 296 K – namely, 184 and 53 rad/Tm for 632.8 and 1064 nm, respectively. The Verdet constant of the Ti:TAG ceramics at 1064 nm is about 1.5 times higher than that of the terbium gallium garnet (TGG) ceramics. The transmittance of this sample was about 75% at the wavelength of 1 μm. This material represents a possible candidate for next-generation devices that utilize the magneto-optic effect.
© 2015 Optical Society of America
Magneto-optic elements that can be used for Faraday rotators and optical isolators are essential components in high-quality laser systems for polarization control, optical isolation, and birefringent compensation. However, slight absorption occurs inside conventional materials, and thermal effects such as thermal lensing and depolarization degrade the performance of high-average-power lasers [1–3]. In order to overcome these problems, it is necessary to combine a cooling system with an ideal magneto-optic material and a depolarization compensation technique, which was proposed in  and first demonstrated in [4,5]; details may be found in . In fact, 33 dB optical isolation was achieved for a laser power of 1500 W using terbium gallium garnet (TGG) single crystal . Additionally, the TGG ceramics have been reported as a new Faraday material , and an optical isolation of 35 dB was recently achieved for a laser power of 740 W . The most important properties of the material in question are a high Verdet constant with a low absorption coefficient for reducing the absorbed power, as well as a high thermal conductivity and thermo-optic properties for suppressing thermal effects.
Terbium aluminum garnet (TAG) is one of the candidate materials for the next-generation Faraday rotator owing to its high Verdet constant and high thermal conductivity [9–11]. However, it is difficult to grow a single crystal because the melting point of the TAG phase is incongruent in the Tb2O3–Al2O3 system. In order to overcome this obstacle, an earlier study  examined polycrystalline TAG that can be synthesized without melting processes. The obtained TAG ceramics was highly transparent and exhibited a high Verdet constant of 172.72 rad/Tm at 632.8 nm, which was larger than that of terbium gallium garnet (TGG), and a high thermal conductivity of 6.5 W/mK at room temperature. The TAG ceramics can thus be expected to function as an ideal Faraday material throughout the visible and near-infrared range. Additionally, doping with CeO2 was reported to enhance the Verdet constant [13,14], and an optical isolation ratio of higher than 30 dB was achieved under an average laser power greater than 300 W for TAG and Ce:TAG ceramics [15–17]. However, in order to manufacture the next-generation of Faraday isolators, it is important to investigate the characteristics of novel magneto-optic materials. According to the tests for finding more effective additives to improve the TAG optical quality, we found that doping with TiO2 is positive to the enhancement of the TAG properties. Additionally, the absorption due to Ti-doping around 1 μm could be negligible, and Ti:TAG has potential application in high-average power lasers operating at this wavelength range.
In this study, we investigate the temperature dependence of the Verdet constant for the wavelengths of 632.8 nm and 1064 nm in a Ti-doped TAG material that was synthesized for the first time to the best of our knowledge. The experimental results are compared to those of the undoped TAG and TGG ceramics. The crystal structure and optical properties of the sample are also studied.
2. Experimental method
2.1 Synthesis of Ti-doped TAG ceramics
The Ti-doped TAG ceramics used in this study was manufactured by the Shanghai Institute of Optics and Fine Mechanics (SIOM). High purity (99.999%) Tb4O7, Al2O3, and TiO2 powders were mixed according to the chemical composition of Tb3[Al(1-x)Tix]5O12 (x = 0.008) with ethanol by ball milling for 24 h. A mixture of 0.4 wt.% tetraethyl orthosilicate (TEOS) and 0.09 wt.% MgO and 1 wt.% Polyethylene glycol 400 (PEG-400) were added as sintering and dispersion aids, respectively. The slurry was dried at 80 °C. After sieving, the powders were uniaxially pressed under a pressure of 10 MPa. The pellets were subsequently cold-isostatically pressed (CIP) under a pressure of 200 MPa. The green body was pre-sintered in air at 750 °C for 4 h in order to remove the organic components. At this stage, vacuum sintering was carried out at ~1600 °C under a base pressure of 1.0 × 10−3 Pa.
Figure 1(a) shows a photo of the sample that was cut to dimensions of 5 mm × 5 mm × 3.9 mm. Both sides of the surface were polished. The crystal structure of the sample was studied by X-ray diffraction (XRD; Ultima IV, Rigaku), and the optical transmittance spectrum of the sample was measured with a UV/VIS/NIR spectrometer (UV-3100 PC, Shimadzu).
2.2 Temperature dependence of Verdet constant for Ti-doped TAG
Figure 1(b) shows the experimental setup used to measure the Verdet constant. The 3.9 mm long Ti:TAG ceramics was attached to a holder using Ag paste. The holder was conduction cooled by a temperature-controllable cryostat; the temperature of the sample was also monitored. A He-Ne laser (λ = 632.8 nm) and a Nd:YAG laser (λ = 1064 nm) were used as a probe beam. The polarization angle of the probe beam was determined by a polarizer, and passed though the Ti:TAG ceramics. The probe beam signal was detected by a power meter after passing through an analyzer. The angle of the analyzer was automatically rotated, and the signal intensity was measured at 1° intervals. This measurement was performed with and without a magnetic field of 1.17 T and in the range of 296–20 K for the He-Ne laser, as well as the range of 296–5.4 K for the Nd:YAG laser. The measurements were repeated three times, and the average values were used to determine the Verdet constant.
3. Results and discussion
3.1 Crystal structure and optical transmittance of Ti:TAG ceramics
Figure 2(a) shows the XRD results of the sample. In Fig. 2(a), the hkl indices of TAG diffraction peaks (JCPDF No. 76-0111) are also shown. The results show that the peaks match well to the diffraction of TAG, except for feeble traces of unidentified peaks. The results thus indicated that the obtained ceramics sample is an almost pure phase of TAG.
Figure 2(b) shows the optical transmittance of the sample. The signal noise around 800 nm is due to the detector switch of the instrument. From 800 to 1500 nm, the transmittance of the sample is over 70%, and a transmittance value of 75% is obtained at 1.0 μm wavelength range. An optical absorption peak can be found around 600 nm; we are currently investigating the explanation for this absorption. The absorption peak at 484 nm is due to the absorption of Tb3+ ions. For the Ce:TAG ceramics, an optical transmittance of 78% was achieved in the range of 550−1500 nm by optimizing the Ce-doping concentration . As with Ce:TAG, the transparency of the Ti:TAG sample might be further enhanced by optimizing the preparation and synthesis conditions.
3.2 Temperature dependence of Verdet constant
Figure 3 shows an example of the measured signal intensity as a function of the rotation angle of the analyzer at 296 K for the He-Ne laser. The red triangles and black circles show the results with and without the magnetic field, respectively. Fitting these results to sinusoidal curves gives the rotation angle of the linear polarization of the probe beam θ due to the magnetic field. In Fig. 3, the value of θ was 48.2°. The measurement errors evaluated were ± 0.23° and ± 0.27° for He-Ne and Nd:YAG lasers, respectively. The rotation angle θ can be expressed as follows:
The value of the Verdet constant for TAG, 0.3 at.%-doped Ce:TAG, and TGG ceramics (for the 632.8 nm laser) have been reported as 172.72, 199.55 , and 139.6 rad/Tm , respectively. Thus, the Verdet constant of the Ti-doped TAG ceramics is higher than that of the undoped TAG ceramics. These results indicate that doping with Ti ions influences the magneto-optic properties of TAG ceramics. Chen et al.  reported that the effect of doping on the magneto-optic properties of TAG ceramics may be related to the ionic radii and the magnetic properties of the dopant. Our results are consistent with their claim, because the Ti3+ ionic radius is larger than that of the Al3+ ion. Furthermore, it is possible that the crystal field of TAG would change as a result of the doped ion.
Figure 4 shows the experimental results of the temperature dependence of the Verdet constant for the 632.8 nm (open circles) and 1064 nm (closed circles) lasers. We obtained a high Verdet constant at room temperature – namely, 184 and 53 rad/Tm for the 632.8 and 1064 nm lasers, respectively. For comparison, the temperature dependence of the Verdet constant of the undoped TAG (squares)  and TGG ceramics (triangles)  are also shown. The Verdet constant of the TGG ceramics at room temperature is 36 rad/Tm for the 1054 nm laser. Thus, the value of the Verdet constant of Ti:TAG is about 1.5 times higher than that of the TGG ceramics.
The Verdet constant of the measured Ti:TAG ceramics, as well as that of the TGG ceramics, were inversely proportional to the temperature. The Verdet constant V is proportional to the magnetic susceptibility χ, which is in turn inversely proportional to temperature for paramagnetic materials . Thus, V can be described as follows:Fig. 4 represent the fitting curve obtained using Eq. (2) for the experimental data above 60 K. We obtained the following fitting parameters for the Ti:TAG ceramics: C = 55.7103 rad/TmK for the 632.8 nm laser; C = 16.2103 rad/TmK for the 1064 nm laser. In Fig. 4, there is a slight discrepancy between the fit and the experimental results below 30 K. We believe that this discrepancy is due to the unusual behavior of the magnetic susceptibility at extremely low temperatures for paramagnetic materials.
In comparison to the value at room temperature, the Verdet constant for the 1064 nm laser increased by factors of 1.5, 3.2, and 5.0 at 200, 100, and 60 K, respectively. TGG-based cryogenic Faraday isolators have been developed for lasers with high-average and high-peak power [21,22], and our results indicate that cryogenically cooled Ti:TAG can reduce the sample thickness of a Faraday rotator. As a result, we can reduce the excessive heat generation associated with high-power laser irradiation, as well as any thermal effects that degrade the laser performance. In addition, we can expect improvements in the thermal properties (thermal conductivity K, thermal expansion coefficient α, thermo-optic coefficient dn/dT, and photoelastic coefficients) of the material. In order to elucidate the laser power limitations for the Ti:TAG-based Faraday rotator, the measurement of the temperature dependence of these thermal properties is necessary.
The heat generated inside the materials increases linearly with not only the material length, but also with the absorption coefficient, which can be estimated from the depolarization ratio . Accordingly, the absorption coefficients were evaluated for TGG and TAG ceramics using a 1 μm laser source with a power of several hundred watts; these values were 1.310−3 cm−1 for the TGG ceramics  and 0.9–1.5 10−3 cm−1 for the TAG ceramics . Thus, the value of the absorption coefficient for the TAG ceramics is roughly equal to that of the TGG ceramics. The optical transmittance of the Ti-doped TAG ceramics used in this work is 75%, which is lower than that of the TAG ceramics. However, the optical transmittance might be further improved by optimizing the preparation process, and might possibly be made comparable to that of the TGG and TAG ceramics.
The temperature dependence of the Verdet constant of a 0.8 at.% Ti-doped TAG ceramics was measured for the first time to the best of our knowledge. The values of the Verdet constant at 296 K were 184 and 53 rad/Tm for the 632.8 nm and 1064 nm wavelength lasers, respectively; these values are higher than those of the TGG and TAG ceramics. The Verdet constant was measured between 296 K and 5.4 K for the 1064 nm laser, and its value was inversely proportional to the temperature down to 60 K. Additionally, an optical transmittance of 75% was achieved at the 1 μm wavelength. The present results suggest that, by improving the optical grade, it is possible that Ti:TAG may become an applicable magneto-optical material for high-average-power laser systems of the next generation.
This work was performed with the support and under the auspices of the NIFS Collaboration Research program (NIFS13KBAH006). This work was also supported by JSPS KAKENHI (Grant Nos. 15K18207 and 26709072), AMADA foundation, and NSFC (No. 61475172).
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