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Abstract
Tb3Sc2Al3O12 crystals have been grown by optical floating zone methods for the first time. The XRD patterns of TSAG crystals match well with the standard cards and the full width half maximum of the analyzed peaks is 0.00832°. The unique emission properties of TSAG crystals have been demonstrated in detail. The Verdet constant of TSAG crystals in the whole region is nearly 20% higher than that of commercial TGG crystals. The terahertz time domain spectrum of TSAG crystals has been analyzed for the first time, and we find that the refractive index is nearly 2, and the absorption coefficient is 2/cm from 1THz to 5 THz.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Faraday devices are necessary optical elements of great importance in laser systems, which are used for laser radiation polarization controlling and elements protection. In different regions, various Faraday devices composed of different magnetooptical (MO) materials have been applied in the corresponding systems. Y3Fe5O12 (YIG) crystals have being applied in the region from 1200 nm to 5000nm, and Tb3Ga5O12 (TGG) crystals have being used in the region from 400 nm to 1100 nm as well as Tb3Al5O12 (TAG) crystals. Besides, the optical properties of TAG crystals are much better than that of commercial TGG crystals in the visible region.
However, due to the incongruent melting nature of TAG crystals, it is different to grow TAG single crystals by traditional methods. Sc ions have been chosen to stabilize the phase properties, and Sc co-doped Tb3Sc2Al3O12 (TSAG) crystals have been grown by micro-pulling down methods [1]. As reported, the Verdet constants of TSAG crystals is significantly higher than TGG crystals [2]. However, the crystalline properties are not perfect caused by outside layers composed of TbScO3 and TbAlO3 weakening the magnetooptical properties. Furthermore, the transmission of the previous reported crystals is far away of application [3]. Due to more advanced technology of crystal growing, the TSAG crystal has been grown by Czochralski methods [20,21], Edge-defined Film-fed Growth Method [22], and micro pulling down methods. As reported, the transmittance of the TSAG crystals is nearly 80% which emit green light with intense intensity excited by X-ray, proving that the TSAG crystals are promising for applying in the detecting area with considerable scintillation properties [21]. To make a research of the crystalline properties of Tb3Sc2Al3O12 crystals and promote the application research, Tb3Sc2Al3O12 crystals have been grown by Optical Floating Zone Method for the first time.
In this work, the growth of single crystalline Tb3Sc2Al3O12 by Floating Zone Method has been investigated in detail. The crystalline properties have been analyzed by X-ray diffractions, the luminescence properties have been studied systematically, magnetooptical properties have been tested by self-made Faraday Effect Tester and THz-TDS response have been analyzed detailly.
2. Experimental
Nominal Tb3Sc2Al3O12 polycrystal powders have been prepared by coprecipitation method [4]. Tb4O7, Sc2O3, Al2O3 powders of 4N purity were used as the starting materials which were weighted in corresponding nominal cationic ratios. The obtained Tb3Sc2Al3O12 polycrystal powders were prepared to polycrystalline material rod under isostatic pressing 30 MPa. Then the prepared rods would be sintered at 1450 °C to obtain the ceramic rods.
The Tb3Sc2Al3O12 crystal was grown with the prepared rods by Floating Zone Method in an infrared radiation furnace equipped with four 1.5kW halogen lamps positioned at the foci of four ellipsoidal mirrors. The crystal growth was then commenced when both rods were simultaneously lowered at the different rate, the speed of the upper rod was 3-4 mm/h and the speed of the lower rod was 2-3 mm/h. With reference to Ref [5], the growth condition was designed one by one. The crystal growth speed was 2-3 mm/h, the rotation rate was 5-8 rpm, the growth atmosphere was air, the molten zone temperature was nearly 1900 °C.
The cell parameters of the grown TSAG crystals were characterized by X-ray Diffraction (Ultima IV diffractometer, Rigaku, Japan) measurement using CuKα radiation at a scan width of 0.02° within 2θ = 10 – 80°. The spectral measurement was done in polished samples with thickness of 1 mm. The Faraday effect has been measured by the self-made Faraday Effect Tester. The time domain spectra have been measured in Shanghai University. All measurements were conducted at room temperature.
3. Results and discussions
3.1 X-ray diffraction and X-ray rocking curve
The TSAG samples was cut parallelly and polished optically. The X-ray Diffraction patterns and X-ray rocking curve were shown in Fig. 1. Compared to the standard card JCPDS-88-0575, the diffraction peaks match well with the corresponding peaks. None impurity peaks could be found in the picture, meaning that the quality of TSAG crystal is very good. The lattice parameters of TSAG crystals is 12.386 Å. Besides, X-ray rocking curve of (111) has been analyzed, and the peak was fitted by Vogit functions. The full width half maximum centered at 25.849° is 0.00832°, which is of great high quality。The size of grown TSAG crystals is , and the picture of the manufactured TSAG crystal has been attached in the X-ray rocking curves in Fig. 1.
3.2 XPS pattern analysis
X-ray Photoelectron Spectroscopy of TSAG crystals was detected at room temperature shown in Fig. 2. The various peaks at different binding energy has been separated to the corresponding valence state of elements [6], which has been labelled in the picture, and (A) represents auger. The crystal was consisted of Tb, Sc, Al, O elements. Particularly, the peak of Tb(4d) determines the valence state. The peak at nearly 150 eV was split into two peaks by Gauss function, Peak 1 and Peak 2. The calculated peak was centered at 150.73 eV, between the Tb (150 eV) and Tb2O3 (151.2 eV) [7] meaning that the valence state of Tb was + 3 without + 4.
3.3 Luminescence spectra analysis
The luminescence spectra of TSAG and TGG were excited by 318 nm at room temperature as shown in Fig. 3. When TSAG crystal was pumped by 318 nm, the Tb could be excited to higher energy levels 4f75d1, and then the energy would relax to 5D3. The emission band centered at 400 nm corresponds to 5D3-7FJ, which is different from the reported results [8]. Additionally, hardly could we find the emission peaks at 400 in TGG crystals which is mainly caused by different crystal structures. Tb3+ occupy the dodecahedron sites, and Ga3+ would occupy the octahedral and tetrahedral sites at the same time. However, TSAG crystals is different from TGG crystals. Tb3+ occupy the dodecahedron sites, Sc3+ occupy the octahedral sites and Al3+ occupy the tetrahedral sites. Compared to TGG crystals, TSAG crystals are relatively disordered resulting in the emission from 5D3 multiples.
Besides, the emission peaks in visible region are associated with 5D4-7F6 (492 nm), 5D4-7F5 (543 nm), 5D4-7F4 (586 nm), 5D4-7F3 (620 nm), 5D4-7F2 (660 nm) and 5D4-7F1 (690 nm) [8,9].
Typically, the emission band at 750 nm was analyzed in detail. The energy transfer and relaxation process would be shown with the following equations.
The lifetime of 5D4 level as reported was 3.92 ms [10]. It is of high possibility that the 5D4 could absorb the energy and pumped into higher levels [11]. Here, 486 nm was used as an example which is a typical absorption band 7F6-5D4 and emission band 5D4-7F6, and the calculated infrared emission wavenumber is 14.7103/cm which is located in the IR bands. In this way, the emission bands in the IR region was caused by the absorption of 5D4. Additionally, the lifetime of the 7F6-5D4 was calculated with a single exponential decay, and the value is 0.39 ms which is familiar with the reported results 0.38 ms [12].
3.4 Faraday effects analysis
The Faraday effect has been measured by the self-made Faraday Effect Tester [13], and the Verdet constants have been calculated with the following equation.
V is the Verdet constant, H is the applied magnetic field intensity, L is the thickness of the samples. The applied filed varies from zero to 1.1 T. The wavelength of the transmission beams are 532 nm, 632.8 nm and 1064 nm correspondingly, and the calculated results were shown in Fig. 4. The value of the calculated Verdet constant is 223.66 rad/Tm (532 nm), 179.57 rad/Tm (632.8 nm) and 51.41 rad/Tm (1064 nm), which is larger than those of TGG crystals [14]. To make a total understanding of the Faraday effect, these values have been fitted with Kamores Model [15] shown in Fig. 4. The function matches well with the mentioned models.The Verdet constants decrease rapidly with the increasing wavelength which could be explained by the following equations.The refractive index n is the function of wavelength λ, which decreases with the increasing wavelength.3.5 Terahertz energy spectrum analysis
Terahertz pulse has been applied to non-thermally excite and manipulate the spin precession through magnetic dipole coupling in materials [16], and Terahertz time domain spectroscopy (TDS) has been applied widely in researching spectral characteristic of various materials [17,18]. The sample was tested in the optical systems at room temperature in N2 atmosphere to improve the signal to noise ratio, and the results were shown in the Fig. 5. In the TDS spectra, the signals would be delayed nearly 10 ps after putting the samples in the detecting systems compared to the reference singles which were detected without samples. After Fourier transform, the frequency domain image of TSAG crystals would be obtained. The effective wavelength [19] is 1.2 THz, meaning that the singles are relatively unbelievable when the frequency is beyond 1.2 THz.
Besides, some other parameters have been calculated from the THz-TDS spectra [18].
is the incident singles, is the singles of samples, and is the transmission coefficient. In this way, the refractive index and absorption coefficient could be obtained with the following equations, and the calculated results were shown in Fig. 6.The index refraction of TSAG crystals increases to 3 at 0.4 THz, and then decreases to nearly 1.8 at 1 THz. From 1 THz to 5 THz, the average value of index is nearly 2. The absorption coefficient of TSAG crystals is nearly 2 cm−1 from 1 THz to 5 THz.
4. Conclusions
TSAG crystals proves great possibility to substitute the commercial TGG crystals due to the outstanding Magnetooptical properties. The Vedert constants decrease with the increasing wavelength, and the value of Verdet constant at 632.8 nm is 179.57 rad/Tm, much larger than TGG crystals. TSAG crystals grown by optical floating zone methods proves the unique emission properties, emitting the lights centered at 400 nm caused by the energy level transition from 5D3. The emission intensity is much higher than that of TGG crystals. Last but not least, the THz-TDs spectra have been analyzed for the first time and the refractive index is nearly 2 and absorption coefficient is 2 cm−1 from 1 T Hz to 5 THz. Besides some more work about the THz-TDs spectra is necessary in the future.
Funding
National Key Research and Development Program of China (2016YFB0402101); Strategic Priority Program of the Chinese Academy of Sciences (XDB16030000); National Natural Science Foundation of China (U1530152, 61635012, 51572275).
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