Magneto-optical Media Research Articles

Magnetically induced Stark-like splitting and asymmetric refractive index effect in plasmonic nanostructures

We develop an analytical method to investigate the eigenmodes of the surface plasmonic polaritons propagating along the interface of metal and magneto-optical (MO) medium by using perturbation theory. The solution shows that the MO activity can eliminate the eigenfrequency degeneracy of a pair of time-reversed waves. Analogously to the electron's magneto-Stark effect, we show this splitting can be understood as a result of “Lorentz force” acting on photons and it exhibits a Stark-like shape, which causes a difference between the forward and backward refractive index. It can be further enhanced by the surface plasmonic polaritons. Then, a hybrid structure composed with MO medium and metallic array is proposed to demonstrate this effect. In such a hybrid structure, the one-way property of surface wave presents a much higher efficiency than that of conventional magnetic photonic crystal. This Stark-like splitting at the interface of metal and magneto-optical (MO) medium can be used in the design of nanophotonic circuits, such as surface wave diode and asymmetric prism.

Coherent multiple light scattering in Faraday active materials

Wave propagation in multiple scattering media shows various kinds of coherent phenomena such as coherent backscattering [1, 2] or Anderson localization [3], both of which are intimately connected to the concept of reciprocity. Manipulating reciprocity in such media is a powerful tool to study these phenomena in experiments [4]. Here we discuss the manipulation of reciprocity in reflection and transmission geometry for the case of light propagation in magneto-optical media. We show new experiments on coherent backscattering and speckle correlations in strongly scattering samples containing Faraday active materials (CeF3) with transport mean free path in the μm range, at low temperatures (T < 10 K) and high fields (B = 18 T). Under such conditions we observe the effect of a Faraday rotation saturation in multiple scattering measurements.

High magnetic field measurement utilizing Faraday rotation in SF11 glass in simplified diagnostics.

With the commercialization of powerful solid-state lasers as pointer lasers, it is becoming simpler nowadays for the launch and free-space reception of polarized light for polarimetric applications. Additionally, because of the high power of such laser diodes, the alignment of the received light on the small sensor area of a photo-diode with a high bandwidth response is also greatly simplified. A plastic sheet polarizer taken from spectacles of 3D television (commercially available) is simply implemented as an analyzer before the photo-receiver. SF11 glass is used as a magneto-optic modulating medium for the measurement of the magnetic field. A magnetic field of magnitude more than 8 Tesla, generated by a solenoid has been measured using this simple assembly. The measured Verdet constant of 12.46 rad/T-m is obtained at the wavelength of 672 nm for the SF11 glass. The complete measurement system is a cost-effective solution.

Nonreciprocal Transverse Photonic Spin and Magnetization-Induced Electromagnetic Spin-Orbit Coupling

We present a formulation of electromagnetic spin-orbit coupling in magneto-optic media, and propose an alternative source of spin-orbit coupling to non-paraxial optics vortices. Our treatment puts forth a formulation of nonreciprocal transverse-spin angular-momentum-density shifts for evanescent waves in magneto-optic waveguide media. It shows that magnetization-induced electromagnetic spin-orbit coupling is possible, and that it leads to unequal spin to orbital angular momentum conversion in magneto-optic media evanescent waves in opposite propagation-directions. Generation of free-space helicoidal beams based on this conversion is shown to be spin-helicity- and magnetization-dependent. We show that transverse-spin to orbital angular momentum coupling into magneto-optic waveguide media engenders spin-helicity-dependent unidirectional propagation. This unidirectional effect produces different orbital angular momenta in opposite directions upon excitation-spin-helicity reversals.

Contributions to Verdet constant of magneto-optical materials

Verdet constant is one of the key parameters to characterize the material magneto-optical properties, and dependent on wavelength and temperature. In order to thoroughly analyze the influence mechanisms of the incident wavelength and temperature on the Verdet constant and then uncover its essence, both the advantages and disadvantages of the classical electronic dynamics theory and quantum theory are discussed on account of basic theories and test data. However, neither of the two theories can be separately used to fully explain the Verdet constant and the correlative test data. Therefore, based on the essential property of the magneto-optical effect, the interactions between the incident light and magnetic matter in a magnetic field are studied, and then a hypothesis which suggests that the Faraday effect result from the combination of various factors is proposed. Furthermore, a theory of wave-transition contribution to the Verdet constant is deduced by adopting the theory of wave-particle duality. That is, the Faraday effect is caused by two different contributions simultaneously. One is the wave contribution, which is the interaction between the wave aspect of light and the magneto-optical medium, and the other refers to the transition contribution, which comes from the electronic transition. When the light enters into a deflection angle, the wave contribution is positive while the transition contribution is negative. In a diamagnetic material, since the wave contribution is greater than the transition contribution, the diamagnetic Verdet constant is positive while in a paramagnetic material, on the contrary, the transition contribution is much larger than the wave contribution, so the paramagnetic Verdet constant is negative. According to the above-mentioned theory, the diamagnetic Verdet constant model and the paramagnetic Verdet constant model are proposed by combining the two parts together. Taking the typical diamagnetic material ZF1 and the typical paramagnetic terbium gallium garnet for example, the influences of the incident wavelength and the temperature on the Verdet constant are analyzed, and the deduced theory together with the corresponding models is tested and verified by analyzing the relevant parameters and the test data. Accordingly, the research turns out that the theoretical results correspond to the real values, which proves the rationality of the hypothesis and the authenticity of the deduced theory. Compared with the traditional theories, the wave-transition contribution theory and its model are superior in the sense of precisely describing the material Verdet constant.

Periodic soliton trains in nonlinear magneto-optical media

The combination of the vector nature (i.e. polarization) of light in materials with nonlinearity can give rise to outstanding phenomena in the transmission of nonlinear optical fields. When the optical medium has been initially altered by a magnetic field, magneto-optical effects introduce additional interesting phenomena such as the possible control of optical signals by a magnetic field. The present work addresses the problem of optical-soliton generation from pulsed radiations interacting with a quasi-resonant magneto-optical medium, under normal Zeeman effect. It is shown that multi-soliton structures can be generated in the magneto-optical medium under specific conditions, these include periodic trains of bright and dark optical solitons having finite temporal periods.

Magneto-optical imaging deviation model of micro-gap weld joint

Seam tracking is important for butt joint laser welding. A magneto-optical imaging approach is proposed to detect the micro-gap weld whose width is less than 0.2mm. The symmetry of the magnetic field above the weld joint is an important precondition to ensure the detection accuracy of the magneto-optical imaging method. However, in actual complex industrial scene, it is difficult to guarantee complete symmetry of the magnetic field. This paper proposes an effective method for solving the problem of magneto-optical imaging deviation under an asymmetric magnetic field. Two possible factors causing the asymmetry of magnetic field above weld joint are firstly investigated using finite element analysis. By analyzing the characteristics of the magneto-optic medium used in the sensor and measuring the magnetic field distribution of weld joint at different lift-off height and different excitation voltage, the prediction model of deviation between the weld position detected by magneto-optical imaging and the actual weld position is built by back propagation (BP) neural network. The experimental result of weld seam tracking based on magneto-optical imaging shows that the change of the lift-off height will affect the detection accuracy of the weld position, and sufficient accuracy can be ensured after correcting the deviation according to the prediction model of magneto-optical imaging deviation.

Self-Trapped Band and Semi-Opening Movable Cavity

Localization of light is usually achieved by external mechanisms, such as metallic reflection, photonic bandgap, and total internal reflections. Here, we propose a new concept about localization of electromagnetic waves, named self-trapped state. We design an ordinary photonic crystal waveguide with two array rods made of magneto-optical media on its two sides. The waveguide modes form a self-trapped band in which electromagnetic waves are trapped. The trapping does not rely on the external mechanisms; instead, it comes from band's own properties. Based on the self-trapped band, the virtual reflection walls and semi-opening cavity can be achieved. The semi-opening cavity has some unique advantages compared with ordinary cavities, e.g., electromagnetic field in it can be either trapped or released; its position and the field amplification in it are tunable. Our study opens a new research door for the combining of magneto-optical materials and photonic crystals.

Optimization of Magneto-Optical Kerr Effect in Cu/Fe/Cu Nano-structure

We use an analytical approach to describe the optical response of a magnetoplasmonic structure upon surface plasmon polariton (SPP) excitation in glass/Cu/Fe/Cu multilayer. The proposed structure is based on Kretschmann prism couplers, Fe layer as magneto-optical (MO) medium, and Cu layer as plasmonic metal layer, to enhance the MO effects thanks to the resonant excitation of surface plasmons. The influence of constituent layer thicknesses and layer order is investigated to obtain the maximum MO Kerr signal and figure of merit (FOM) in polar geometry. In addition, we show a range of parameters to determine the maximum Kerr rotation accompanied by minimum Kerr ellipticity which is desirable for data storage applications. Results demonstrate the important role of film thicknesses and incidence angle distribution on resonant excitation of surface plasmons.

Nonreciprocal propagating electromagnetic modes without phase gradients

We introduce a concept of a $k$-zero mode, an electromagnetic wave with a nonzero group velocity $(\ensuremath{\partial}\ensuremath{\omega}/\ensuremath{\partial}k\ensuremath{\ne}0)$ but a null phase gradient $(k=0)$. Existence conditions of $k$-zero modes are discussed. We emphasize that the existence of $k$-zero modes is closely related to the the unique nonzero lateral displacement when reflected from a magneto-optical (MO) medium at normal incidence. Consequently, the waveguide should be suitably designed to avoid a destructive interference between the lateral displacements at different MO-dielectric interfaces. Numerical full-field simulation proves our analysis. The importance of this study is discussed.

Magneto-optics enhancement with gain-assisted plasmonic subdiffraction chains

We study the enhancement of the magneto-optical effect in subdiffraction plasmonic chains. We show that in a periodic chain of the plasmonic nanoparticles embedded in a magneto-optical medium, propagation of a guided mode is accompanied by rotation of electrical dipoles (the Faraday effect). The angle of rotation per 1 {\mu}m is two orders of magnitude greater than that in the same bulk magneto-optical medium. We also demonstrate that the effect of Ohmic losses can be significantly reduced by using a gain-assisted chain composed of active core-shell nanoparticles (spasers). The dipole mode in such an array of magneto-optical spasers exhibits high values of the Faraday rotation and propagation length.

Analyzing combinations of circular birefringence, linear birefringence, and elliptical dichroism in magneto-optical rotators

We derive analytic expressions for the polarization characteristics of light emerging from a magneto-optical medium possessing arbitrary contributions from linear and circular birefringence as well as magnetic circular dichroism. The medium is placed inside a static magnetic field. The rotation of the plane of polarization and the ellipticity of the resultant light exhibit interesting characteristics that can be a useful guide in the design and analysis of new photonic devices. Furthermore, the Jones matrices are derived in all cases, including for elliptical dichroism, indicating the role of hyperbolic trigonometric functions in modeling the effects of dichroism. Finally, implications for experimental detection of the polarization state and the limits on the performance of optical isolators are discussed.

Study on photonic angular momentum states in coaxial magneto-optical waveguides

By rigorously solving Maxwell's equations, we develop a full-wave electromagnetic theory for the study of photonic angular momentum states (PAMSs) in coaxial magneto-optical (MO) waveguides. Paying attention to a metal-MO-metal coaxial configuration, we show that the dispersion curves of the originally degenerated PAMSs experience a splitting, which are determined by the off-diagonal permittivity tensor element of the MO medium. We emphasize that this broken degeneracy in dispersion relation is accompanied by modified distributions of field component and transverse energy flux. A qualitative analysis about the connection between the split dispersion behavior and the field distribution is provided. Potential applications are discussed.

Faraday isolator based on TSAG crystal for high power lasers.

A Faraday isolator based on a new magneto-optical medium, TSAG (terbium scandium aluminum garnet) crystal, has been constructed and investigated experimentally. The device provides an isolation ratio of more than 30 dB at 500 W laser power. It is shown that this medium can be used in Faraday isolators for kilowatt-level laser powers.

The optical, magnetic, and electrical characteristics of Fe2Si thin films

In this study, Fe2Si thin films were sputtered onto a glass substrate at thicknesses of 100, 200, 300, 400, 500, and 600nm. X-ray diffraction (XRD) was applied to identify film structures. The XRD patterns displayed a Fe2Si (100) significant diffraction peak at 2θ of approximately 25° at room temperature. The surface morphology and grain-size distribution of Fe2Si thin films were observed using a scanning electron microscope (SEM). The authors observed that the crystallization and grain size of thicker Fe2Si thin films were stronger and larger than those of thinner Fe2Si thin films. The Fe2Si (100) texture induced magneto-crystalline anisotropy, thus reducing electrical resistivity and yielding the highest low-frequency alternative-current magnetic susceptibility (χac). The value of χac increased as the thickness was increased because of magneto-crystalline anisotropy. The maximal value of χac was obtained at a thickness of 600nm at the optimal resonance frequency (fres) of 10Hz, which produced maximal spin sensitivity. The resistivity (ρ) decreased as the Fe2Si thickness increased, because grain boundaries and the thin-film surface scattered the electrons; therefore, the thinner films were more resistant. The 600nm-thick Fe2Si thin film exhibited a minimal ρ of approximately 9300Ωcm. The transmission of Fe2Si thin films indicated that the thin Fe2Si film exhibited a high transmittance of approximately 60%. The high transmittance decreased from 60% to 25% as the thickness was increased. Therefore, the optimal Fe2Si thickness was 600nm, yielding the highest χac value of approximately 0.84 with an fres of 10Hz, a minimal ρ of approximately 9300Ωcm, and a transmittance of approximately 25%, because of a strong Fe2Si (100) texture which could be applied in components for a magneto-optical recording medium.

Improving characteristics of Faraday isolators based on TAG ceramics by cerium doping

A Faraday isolator (FI) based on a new magneto-optical medium-Ce:TAG ceramics-has been fabricated and studied in experiments. Compensation of thermally induced depolarization in the FI increases the isolation ratio from 31 to 39 dB at the laser power of 300 W. Estimates predict stable operation of the device with kilowatt laser power.

Magnetic, optical, and electrical properties of Fe40Pd40B20 thin-films

In this study, FePdB thin-films were sputtered onto a glass substrate and their magnetic, electrical, and optical characteristics were investigated. X-ray diffraction (XRD) indicated that each Fe40Pd40B20 thin-film had a FePd (001) significant crystalline peak within 2θ of approximately 25° at room temperature (RT). XRD demonstrated that thicker FePdB was more crystalline than thinner FePdB. The FePd (001) texture induced a magneto crystalline anisotropy, yielding the highest low-frequency alternative-current magnetic susceptibility (χac) and reducing electrical resistivity. The value of χac increased as the thickness increased because of magneto crystalline anisotropy. The maximal value of χac was obtained at a thickness of 300Å at the optimal resonance frequency (fres) of 10Hz, which produced maximal spin sensitivity. Moreover, the resistivity (ρ) and sheet resistance (Rs) reduced as the FePdB thickness increased, because grain boundaries and the thin-film surface scattered the electrons, therefore thinner films were more resistant. The 300Å of thick FePdB exhibited a minimal ρ of approximately 0.009Ωcm and a minimal Rs of approximately 0.102Ω/□. The transmission of FePdB thin-films showed that the thin FePdB exhibited a high transmittance of approximately 80%. Based on these results, the optimal FePdB thickness was 300Å, yielding the highest χac value of approximately 0.54 with an fres of 10Hz, a minimal ρ of approximately 0.009Ωcm, and a transmittance of approximately 55% because of a strong FePd (001) texture, which could be used in components for a magneto-optical recording medium.

An optical unidirectional device tunable by a magnetic field

A tunable optical unidirectional device or optical isolator composed of a metal subwavelength grating covered with magneto-optical rotation medium is proposed. The results of a simulation show that the difference of transmissivity in opposite directions varies from 0 to 83% with an external magnetic field at room temperature. The largest field needed to control transmissivity is only 1.1T, so the working conditions can be easily achieved. A theoretical method to calculate transmissivity is presented and the unique transmission characteristics of this new device are explained satisfactorily.

Effect of grain size on optical and electrical properties of Ni80Fe20 thin films

In this investigation, the transmittance and electrical properties of NiFe thin films were measured under three conditions. NiFe thin films were sputtered on a glass substrate with a thickness (tf) from 300Å to 1500Å under the following conditions: (a) substrate temperature (Ts) maintained at room temperature (RT), (b) post-annealing at TA=150°C for 1h, and (c) post-annealing at TA=250°C for 1h. Transmission electron microscopy (TEM) demonstrated that the NiFe film yielded a strong face-centered cubic (FCC) (111) selected-area-diffraction (SAD) pattern. A spectral analyzer was utilized to measure transmittance through various thicknesses. Post-annealing treatment promoted the growth of grains, yielding a large average grain size, and therefore a small transmittance. However, electrical measurements revealed that increasing the electron mobility reduces the resistivity (ρ) and sheet resistance (Rs). At a thickness of 300Å with post-annealing 250°C, the optimal maximum transmittance is 50%; the optimal ρ is 110μΩcm, and the optimal Rs is 13Ω/□. Accordingly, a 300Å-thick NiFe thin film with favorable electrical and optical properties can be utilized in the components of a magneto-optical recording medium.

High-power Faraday isolators based on TAG ceramics

The Faraday isolator based on a new magneto-optical medium--TAG (terbium aluminum garnet) ceramics was implemented and investigated experimentally. The magneto-optical element was temperature-stabilized using water cooling. The device provides a stable isolation ratio of 38 dB at 300 W laser power. Estimates show high performance of the device at a kilowatt laser power.