Preparation and Faraday rotation of Bi-YIG/PMMA nanocomposite

Bismuth-substituted yttrium iron garnet (Bi-YIG) nanoparticles (NPs) were prepared by coprecipitation and subsequent heating treatment. Thermal gravity-differential thermal analysis was performed to investigate the thermal behavior of the Bi-YIG precursors and to decide the best annealing temperature. Phase formation of garnet NPs was investigated by X-ray powder diffraction. The size of Bi-YIG NPs was investigated by transmission electron microscopy, and the magnetic properties of Bi-YIG NPs were measured using a vibrating sample magnetometer. The results show that the temperature needed for the transformation of Bi-YIG from the amorphous phase to the garnet phase decreases with increasing Bi content, and Bi-YIG NPs with sizes of 28–78 nm are obtained after heating treatment at 650–1000 °C. The saturation magnetization of Bi-YIG NPs increases as the Bi content increases. Moreover, the Faraday rotation of polymethyl methacrylate (PMMA) slices doped with Bi-YIG NPs was investigated. The results indicate that the angle of Faraday rotation increases with increasing Bi content in PMMA composites, and the maximum value of the figure of merit is 1.46°, which is comparable to the value of a sputtered film. The Bi-YIG NPs-doped PMMA slices are new promising materials for magneto-optical devices.     

用于光纤电流传感器的掺Bi稀土铁石榴石单晶生长与磁光性能

用熔盐法生长了两种Bi替代的高法拉第旋转、温度稳定的稀土铁石榴石磁光单晶Bi-HoYbIG和Bi-GdYIG,测试分析了其在近红外波段的磁光性能及其温度特性。Ho3-z-y,YbyiBixFe512(x＝1.03,y=1.20)单晶的比法拉第旋转角为-891°/cm(λ=1.31μm)和-767°/cm(λ＝1.55μm),Y3-z-yGdyBixFe5O12(x=0.46,y=0.24)单晶的值为－1067°／cm(λ=1.31μm)和-882°／cm(λ=1.55μm),两者都比纯YIG晶体高出许多。而且这两种晶体的法拉第旋转角随温度变化较小,在250～400K范围内,其温度灵敏度S分别为4. 60×10-4／K和4. 20×10-4/K。分析表明,Bi的掺入能大大提高晶体的法拉第旋转角,而yb3+、Gd3+等离子的掺入可以有效降低Bi3+替代磁光单晶的法拉第旋转的温度灵敏性,Bi-HoYbiG和Bi-GdYIG等磁光晶体非常适合作为高灵敏度、温度稳定的光纤电流传感器中的法拉第转子材料。     

Wavelength and temperature characteristics of BiYbIG film/YIG crystal composite structure for magneto-optical applications

We report the wavelength and temperature characteristics of novel Bi-substituted rare-earth iron garnet films grown on a YIG substrate by a modified liquid phase epitaxy (LPE) technique. The Faraday-rotation spectrum was measured by the magneto-optically modulated dual-frequency technique with the wavelength varied from 800 nm to 1700 nm. The resultant Bi_0.37Yb_2.63Fe₅O₁₂ (BiYbIG) LPE film/YIG crystal structure showed an increased Faraday-rotation coefficient due to Bi³⁺-ion doping on the dodecahedral sites of the iron garnet without increasing absorption loss; therefore, a good magneto-optical figure of merit, defined by the ratio of Faraday rotation and optical absorption loss, has been achieved (21.5 deg/dB and 30.2 deg/dB at 1300-nm and 1550-nm wavelengths, respectively, at room temperature). In addition, since the Yb³⁺ and Y³⁺ ions provide opposite contributions to the wavelength and temperature characteristics of the Faraday rotation, the resultant BiYbIG LPE film/YIG crystal structure showed a flat Faraday-rotation curve versus wavelength and temperature. The Faraday-rotation wavelength coefficient was reduced to 0.06 %/nm at 1550-nm wavelength. The Faraday-rotation temperature derivative was reduced to 0.006 deg/°C at 1300-nm wavelength and 0.007 deg/°C at 1550-nm wavelength, respectively. PACS 78.20.Ls; 81.15.Lm; 75.50.Gg     

掺Bi钇铁石榴石薄膜光波导的线性双折射及消除方法

分析了光的线性双折射对磁光波导ＴＥ－ＴＭ模式转换，法拉第旋转，波导光隔离器隔离比及插入损耗的影响。研究了掺Ｂｉ钇铁石榴石薄膜光波导中的形状线性双折射，应力感生线性双折射及生长感生线性双折射的来源，特性和消除方法。     

Growth and characterization of cerium-substituted yttrium iron garnet single crystals for magneto-optical applications

This paper is concerned with the preparation and characterization of cerium-substituted yttrium iron garnets, which are known to be oxides having a large Faraday rotation effect. Using the improved flux method we successfully grew bulk single crystals of iron garnet doped with Ce³⁺ ions with maximum substitutions up to 0.349. Here we investigate different solution compositions for maximum Ce³⁺ substitution. The Faraday rotation and optical absorption spectra were measured in the near infrared region for different Ce³⁺ ion substituted iron garnets. The specific Faraday rotation of Ce_0.349Eu_0.195Y_2.456Fe₅O₁₂ was found to be 1430 deg/cm at a wavelength of 780 nm and –1280 deg/cm at 1150 nm. The Ce substitution prominently enhances the Faraday rotation effect, and Yb³⁺ and Eu³⁺ ions substituted for Y³⁺ in the dodecahedral sites of YIG can increase the concentration of Ce³⁺ ions, depressing the formation of nonmagnetic Ce⁴⁺ ions by charge compensation. Received: 24 January 2001 / Accepted: 2 March 2001 / Published online: 27 June 2001     

Tunable optical properties of nano-Au on vanadium dioxide

The optical properties of Au nanoparticles deposited on thermochromic thin films of VO₂ are investigated using spectroscopy. A localized modification on the transmittance spectrum of VO₂ film is formed due to the presence of Au nanoparticles which exhibit localized surface plasmon resonance (LSPR) in the visible-near IR region. The position of the modification wavelength region shows a strong dependence on the Au mass thickness and shifts toward the red as it increases. On the other hand, it was found that the LSPR of Au nanoparticles can be thermally tunable because of the thermochromism of the supporting material of VO₂. The LSPR wavelength, λ_SPR, shifts to the blue with increasing temperature, and shifts back to the red as temperature decreases. A fine tuning is achieved when the temperature is increased in a stepwise manner.     

Spatially selective modification of optical and magneto-optical properties in Fe- and Au-doped glasses irradiated with femtosecond-laser

The optical property and the magneto-optical response were space-selectively modified in transparent Fe³⁺- and Au³⁺-doped glasses by using infrared femtosecond- (fs-) laser irradiation and subsequent annealing. This irradiation process induces the precipitation of not only magnetic spinel-type Fe-oxide nanoparticles but also Au nanoparticles inside the glasses, which shows localized surface plasmon resonance absorption at the wavelengths larger than 500 nm. As the annealing time and the temperature increases, the position of the LSPR peaks exhibits red shifts, which is due to the growth of Au nanoparticles. Faraday rotation angles as a function of wavelength were measured, and the difference spectra exhibit distinct positive peaks, indicating that the coupling between the LSPR due to the Au nanoparticles and the diamagnetism of the matrix glass is effective. To decrease the coupling with the diamagnetic glass, a two-step annealing process (at 450 °C for 90 min and at 550 °C for 30 min) was carried out after irradiation with fs-laser. The preliminary annealing at the lower temperature contributes to the precipitation of ferrimagnetic magnetite nanoparticles. Au nanoparticles were subsequently grown by annealing at 550 °C. In this case, effective coupling between the LSPR and ferrimagnetic nanoparticles has significantly suppressed the intensity of the positive peak in the Faraday spectra compared with the single annealing process.     

Compact 1-D magnetophotonic crystals with simultaneous large magnetooptical Kerr rotation and high reflectance

We have investigated the case of reflection-type one-dimensional magnetophotonic crystals (MPCs) in order to achieve high reflectance and high magnetooptical Kerr effect, simultaneously, utilizing a low number of magnetic layers. In a MPC structure including only one Bi:YIG magnetic layer, we have achieved a Kerr rotation as huge as θ_k ≈ 90° with a reflectance of R > 97%. In addition, we have introduced a thinner structure having three Bi:YIG layers and with approximately the same Kerr rotation and reflectance. Both the structures are very compact with lengths of less than 9 μm.     

Magneto-optical Kerr effects of yttrium-iron garnet thin films incorporating gold nanoparticles

We report an experimental study on magneto-optical (MO) Kerr effects of yttrium-iron garnet (YIG) thin films incorporating Au nanoparticles. The polar MO Kerr spectra in the wavelength between 400 and 800 nm show that, by incorporating the Au nanoparticles, Kerr rotation angles become negative values in the region, where the localized surface plasmon polariton (SPP) resonance of the Au nanoparticles is located. The anomalous Kerr rotation indicates a possible coupling between the MO Kerr effect of YIG and the SPP. A mechanism for the coupling is discussed.     

Polarization mode conversion in YIG rib waveguide

This paper describes the experimental results on the orthogonally polarized mode conversion in a YIG rib channel waveguide, in which a small magnetic field induced around the electrode deposited on the waveguide was used. The dependence of mode conversion efficiency on the coupling length and the effect of a d.c. bias field applied to the waveguide were investigated. The Faraday rotation angle of the fabricated YIG rib channel waveguide was found to be 160° cm^–1, which was a reasonable value compared with that in the YIG planar waveguide reported previously. Furthermore, an increase in conversion efficiency and a decrease in switching time were found in the presence of a d.c. magnetic bias field.     

A compounded rare-earth iron garnet single crystal exhibiting stable Faraday rotation against wavelength and temperature variation in the 1.55 μm band

The Bi, Tb and Yb partially substituted iron garnet bulk single crystals of Tb_3−x−yYb_yBi_xFe₅O₁₂ were grown by using Bi₂O₃/B₂O₃ as flux and accelerated crucible rotation technique for single-crystal growth. Faraday rotation (FR) spectra showed that the specific FR of the (Tb_0.91Yb_1.38Bi_0.71)Fe₅O₁₂ crystal under magnetic field at saturation was measured to be about −1617°/cm at λ=1.55 μm, Faraday rotation wavelength coefficient (FWC, 0.009%/nm) in the wavelength range of 1.50–1.62 μm and Faraday rotation temperature coefficient (FTC, 3.92×10⁻⁵/K) at λ=1.55 μm were even smaller than that of YIG. It is proven that through combining two types of Bi-substituted rare-earth iron garnets with opposite FWC and FTC signs, the compound rare-earth iron garnets with low FWC and FTC may be obtained due to the compensation effect. The saturation magnetization of (Tb_0.91Yb_1.38Bi_0.71) Fe₅O₁₂ crystal is 0.48×10⁶ A/M and is also much smaller than that of YIG. We have found empirically that there is a simple relationship between the FR θ_f(x) and Bi content x for Tb_3−x−yYb_yBi_xFe₅O₁₂, which is given by θ_f(x)=(−2759x+400)°/cm.     

Growth-induced magnetic anisotropy behavior in thin garnet films fabricated by pulsed laser deposition

The effect of target rotation rate on the magneto-optical properties and induced magnetic anisotropy of pulse laser deposition grown Bi:YIG garnets has been investigated. The rotation rate dependence of magnetic properties of thin films has been studied using magneto-optical Faraday rotation combined with magneto-optical Kerr measurement setup. Our results show that decreasing the target rotation rate can also increase the polar coercive field and hence magnetic anisotropy. Also the spectral answer of samples show the capability of use of them in one off optical ranges such as visible, near infrared and far infrared regions.     

The effect of target rotation rate on structural and morphological properties of thin garnet films fabricated by pulsed laser deposition

The effect of target rotation rate on the structural and morphological properties of pulsed laser deposition grown Bi:YIG garnets is investigated. The rotation rate dependence of the surface morphology and magnetic properties of the thin films were studied using atomic force microscopy combined with a magneto-optical measurement setup. The results show that decrease in the target rotation rate can also increase the roughness, the index of refraction, and the surface skewness and can decrease Faraday rotation by an order of magnitude.     

Effect of post-annealing temperature on the microstructure and magnetic properties of Ce:YIG thin films deposited on Si substrates

Amorphous Ce₁Y₂Fe₅O₁₂ (Ce:YIG) thin films deposited on single crystal Si(1 0 0) and thermally oxidized Si(1 0 0) substrates by pulsed laser deposition were annealed in the temperature range of 700-1000 °C in air. The annealing temperature dependence of microstructure and magnetic properties of Ce:YIG films was studied using X-ray diffraction combined with vibrating sample magnetometer. The results show that single phase of polycrystalline Ce:YIG thin films can be obtained by the post-annealing of as-deposited films at the temperature of 700 °C. However, two steps of phase segregation of Ce:YIG occur as the post-annealing temperature increases: at first, Ce:YIG is decomposed into YIG and non-magnetic CeO₂ when annealed at 800 °C; then YIG continues to be decomposed forming Fe₂O₃ when the temperature is increased up to 900 °C. Consequently, the saturation magnetization of Ce:YIG films decreases first and then increases with the post-annealing temperature going up, which indicates that the saturation magnetization of Ce:YIG films is mainly related to the phase composition of the films. Meanwhile, the presence of SiO₂ buffer layer can significantly enhance the saturation magnetization of Ce:YIG films.     

Spectral dependence of Faraday rotation in magnetite-polymer nanocomposites

We report the size-dependent magneto-optical properties of nanometer-sized magnetite particles embedded in poly(methyl methacrylate). The nanocomposite material contains Fe₃O₄ particles with diameters ranging from 8 to 200 nm. Faraday rotation spectra are measured in the wavelength range of 400-900 nm. A broad spectral band centered at 650 nm (1.91 eV) is observed in the for the larger (200 nm) particles. Decreasing of nanoparticle size leads to a significant narrowing of this band and appearance of an additional peak in the 2.5-3.2 eV range. We propose that the changes to the spectrum are caused by structural changes in the small particles, which affect the magneto-optically active charge transfer and orbital promotion electronic transitions. In addition, the Faraday rotation spectrum of the composite containing 8 nm particles is sensitive to nanoparticle concentration. Increasing the concentration of nanoparticles in the composite results in a red shift of the spectral feature at approximately 450 nm. We propose that the shift of the peak in Faraday rotation spectrum is due to particle-particle interactions in the concentrated sample. Ferromagnetic Resonance measurements confirm a magneto-static interaction in the concentrated sample not present in the diluted sample.     

热稳定法拉第旋转TbYbBiIG磁光单晶及性能

采用高温溶液法，以Ｂｉ２Ｏ３／Ｂ２Ｏ３为助熔剂成功地生长出掺铋复合稀土铁石榴石（ＴｂＹｂＢｉ）３Ｆｅ５Ｏ１２（简称ＴｂＹｂＢｉＩＧ）晶体。晶体外形规则，最大尺寸为７×６×４ｍｍ３，Ｘ射线衍射分析证实，生长的晶体为ＴｂＹｂＢｉＩＧ单相单晶体，扫描电镜能谱分析其组成为Ｔｂ２．０６Ｙｂ０．４６Ｂｉ０．４８Ｆｅ５Ｏ１２。在１．０μｍ～１．７μｍ波段测量出晶体法拉第旋转谱和光吸收谱。当λ＝１．５５μｍ时，在１０°Ｃ～８０°Ｃ温度范围内测得法拉第旋转θＦ的温度系数为ｄθＦ／ｄＴ＝－２．３×１０－２ｄｅｇ·ｍｍ－１Ｋ－１。研究结果表明，ＴｂＹｂＢｉＩＧ单晶体在近红外波段θＦ约为ＹＩＧ单晶的３倍，温度系数小，是制作高性能光隔离器的一种好材料     

Geometrical confinement effects on the magnetization and polarization response in resonant magneto-optic rotator waveguides

Control of magnetization is central to the performance of magneto-optical switches and isolators. Photonic crystal technology on these devices can yield significant improvements in polarization rotation efficiency and an overall reduction in device dimensions. The optical response and field reversal characteristics of resonant magneto-optic polarization rotators fabricated on chip are presented herein and analyzed by micromagnetic simulation. By introducing domain-strip structures into the resonant cavity of Bragg gratings formed on magnetic garnet films, a bi-stable magnetic state is demonstrated and the enhancement of characteristic saturation field is studied. Domain closure loops between the strips affect the hysteresis response in the resonant cavity. Large magneto-optic rotations exceeding 45° are produced near resonance between 1500 and 1580 nm in the stop-bands, although the presence of linear birefringence in these gyrotropic waveguides strongly suppresses the Faraday rotation outside the stop-bands and degrades the linearity of the output polarization.     

Bi induced step-flow growth in the homoepitaxial growth of Au(1 1 1)

Homoepitaxial growth of Au on Bi-covered Au(1 1 1) was studied at room temperature using reflection high-energy electron diffraction (RHEED) and Auger electron spectroscopy (AES). From observations of RHEED it is found that the Au(1 1 1) (23 × 1) reconstruction structure changes to a (1 × 1) by about 0.16-0.5 ML deposition of Bi and to a (2√3 × 2√3)R30° by about 1.0 ML deposition of Bi, respectively. The surface morphology evolution by Bi deposition leads to a change of Au homoepitaxial growth behavior from layer-by-layer to step flow. This indicates that the surface diffusion distance of Au atoms on the Bi-precovered (1 × 1) and (2√3 × 2√3)R30° surfaces is longer than that on the Au(1 1 1) (23 × 1) clean surfaces. A strong surface segregation of Bi was found at top of surface. It is concluded that Bi atoms acted as an effective surfactant in the Au homoepitaxial growth by promoting Au intralayer mass transport.     

Analysis of an anisotropic plate Faraday rotator coated with multilayer dielectric plates

The analysis of an anisotropic plate Faraday rotator coated with multilayer dielectric plates is presented by means of the matrix methods in conjunction with Fourier transformation technigues. It provides a closed-form representation of the electromagnetic field over the whole multilayer regions. The treatment does not limit the direction of the biased magnetic field and the numbers of the coated dielectric plates. Numerical results are presented and compared with that calculated with the method in ref.[1], and full-waveguide-band bandwidth for 47.5°±1.5° rotation angle is obtained for transmission type rotator in W band. Calculation for reflection type rotator is also presented and compared with the experiments.     

Composition-Controlled Laser-Induced Alloying of Colloidal Au–Cu Hetero Nanoparticles

Due to their optical properties (localized surface plasmon resonance, LSPR), colloidally dispersed metal nanoparticles are well suited for selective heating by high-energy laser radiation above their melting point without being limited by the boiling point of the solvent, which represents an excellent complement to wet-chemical nanoparticle synthesis. By combining wet-chemical synthesis and postsynthesis laser treatment, the advantages of both methods can be used to specifically control the properties of nanoparticles. Especially in the colloidal synthesis of nanoalloys consisting of two or more metals with different redox potentials, wet-chemical synthesis quickly reaches its limits in terms of composition control and homogeneity. For this reason, the direct synthesis path is divided into two parts to take the strengths of both methods. After preparing Au–Cu hetero nanoparticles by wet-chemical synthesis, nanoalloys with previous adjusted composition can be formed by postsynthesis laser treatment. The formation of these nanoalloys can be followed by different characterization methods, such as transmission electron microscopy (TEM), where the fusion of both metal domains and the formation of spherical and homogeneous Au–Cu nanoparticles can be observed. Moreover, the alloy formation can be followed by different shifts of X-ray diffraction (XRD) reflections and LSPR maxima depending on the composition.