Magneto-optical Elements Research Articles

Beam commissioning of the SARAF Injector and MEBT (protons)

The SARAF Phase II cryogenic linear accelerator (linac) is currently under construction and will be commissioned through collaborative efforts between SNRC and CEA. The linac will accelerate a 5 mA pulsed/continuous wave (CW) proton and deuteron beam, reaching energies of up to 35 MeV and 40 MeV, respectively. The linac injector consists of Phase I components, including an ECR ion source, a Low-Energy Beam Transport (LEBT) line, and a 4-rod Radio Frequency Quadruple (RFQ). Additionally, a new Medium Energy Beam Transport (MEBT) line has been installed and integrated into the injector infrastructure. The MEBT includes three rebuncher resonators, magneto-optical elements, and beam diagnostics. The MEBT beam diagnostics, as well as the Phase I D-plate installed downstream, were used for beam characterization. The downstream-installed Temporary Beam Line (TBL) and a prototype of the Gallium Indium Liquid Target (GALIT) were used for commissioning, operating at high beam duty cycles, including CW mode. The report describes the proton beam commissioning process of the injector prior to the delivery of the cryomodules, scheduled for the second half of 2023.

Selection of Magneto-Optical Material for a Faraday Isolator Operating in High-Power Laser Radiation

Faraday isolators are the inherent components of complex laser systems. The isolation degree is essentially determined by the effects that occur in its magneto-optical element, so the choice of material from which it is made is very important. The principal approaches to choosing a magneto-optical material for Faraday isolators are addressed. Characteristic features of materials for Faraday devices operating in laser radiation with high average and high peak power are considered. Some trends in magneto-optical ceramics and the advantages and shortcomings of a number of ceramic samples are analyzed. Using the proposed approaches and recommendations will allow to create devices with unique characteristics for any wavelength range for different practical applications.

A miniaturized Faraday isolator magnet structure design for near-infrared wavelength

Faraday isolators are widely used in laser systems. The magneto-optical element in the Faraday isolator has thermally induced depolarization characteristics, which affects the overall performance. The solution is to use a stronger magnetic field and shorter magneto-optical elements. In this paper, a new small-scale Faraday isolator magnet system is designed for using in laser systems with near-infrared wavelengths. The magnet system consists of five magnetic rings that are alternately magnetized in the radial and axial directions. The influence of different magnetic ring configurations on the intensity of the magnetic field was analyzed, and a magnet structure was designed and fabricated. Using this structure, the magneto-optical element is shortened to 7.7 mm under the 780 nm laser. This research is helpful to the development and optimization of the Faraday isolator in the laser system of the near-infrared wavelength.

Faraday Isolator With Composite Magneto-Optical TGG-Sapphire Elements

Faraday isolator with composite terbium gallium garnet TGG/sapphire magneto-optical elements that were produced by the surface-activated direct bonding method was created. Thermally induced depolarization and thermal lens for composite elements were investigated experimentally and numerically. The resulting structures operated successfully at a high radiation power, providing an isolation ratio exceeding the classical single element made of TGG by 5 dB and amounted to 34 dB at a laser power of 700 W. The maximum operating power at which the isolation ratio was higher than 30 dB was estimated to be over 2 kW, which is almost 3 times more than in the case of a single TGG.

Compensation for Thermally Induced Depolarization in Magneto-Optical Media Made of Materials With a Negative Optical Anisotropy Parameter

Thermally induced depolarization of radiation is the principal limiting factor of using Faraday devices in laser radiation with a high average power. In this work thermally induced depolarization is analyzed in a system of two optical elements made of cubic magneto-optical materials with a negative optical anisotropy parameter separated by a quartz rotator. The parameters of the optical scheme of a Faraday isolator (FI) based on single crystals cut in the [C] orientation, at which effective compensation of thermally induced depolarization is realized, are found. These parameters are universal and do not depend on the material of the magneto-optical element. Analytical expressions for the integral thermally induced depolarization are derived and the contributions of thermally induced linear birefringence and of the temperature dependence of the Verdet constant are analyzed. The parameter that allows assessing the contributions to depolarization of each of the thermal effects and predicting the efficiency of using the compensation scheme for a specific magneto-optical material is introduced. The proposed FI scheme with compensation is investigated for a number of known magneto-optical materials possessing a negative optical anisotropy parameter. The results obtained will significantly reduce thermally induced polarization distortion of radiation, and will allow developing FIs providing a high isolation ratio in powerful laser radiation.

Quantum process tomography of a magneto-optic transformation

We characterize a magneto-optic transformation in the Faraday modality via quantum process tomography (QPT) with quantum light comprising of antibunched single photons generated through heralded spontaneous parametric downconversion. We derive a general process matrix for a unitary transformation which effectively describes the action of the magneto-optic element. This matrix quantifies the polarization attributes of the single photon as it traverses through the magneto-optic element. The magnitude of rotation and ellipticity are extracted through a maximum likelihood based optimization of the process matrix. Measurements are performed for a wide range of temperatures (8–100 K) and applied magnetic fields. The estimated rotation angles are curve fit to determine the Curie-Weiss constant for a terbium gallium garnet crystal. The extracted angles are also utilized to map the rotations on the surface of Bloch sphere.

ZnSe-based Faraday isolator for high-power mid-IR lasers

High-power Faraday isolators (FIs) for mid-IR radiation are of great practical importance, since they expand the capabilities of constructing optical schemes of solid state mid-IR lasers, which have progressed significantly in recent years. Isolators for high-power radiation with a wavelength of 1.5–2 µm are especially important for the solid-state lasers in order to cover the entire mid-IR range. We developed the FI for high-power mid-IR radiation based on CVD-ZnSe polycrystals and studied its characteristics. The isolation of radiation with a wavelength of 1940 nm was demonstrated. No thermally induced depolarization effects were observed for radiation with a wavelength of 1940 nm up to 20 W. For studying the limitations of the isolator in terms of operating power, experiments with more powerful radiation at a wavelength of 1075 nm were performed. Estimates show the possibility of isolating radiation with a power of up to 200 W with the isolation ratio of the device of ∼25 dB. Ways to improve the device are discussed. The level of cold depolarization can be reduced, by annealing the magneto-optical elements. The device can be compactified by using a different optical isolation scheme and a different organization of the magnetic system. This path will also allow compensating thermally induced depolarization and, according to the estimates, will ensure the isolation of mid-IR radiation of a kilowatt power level.

Reduction of thermal-stress-induced birefringence in a Faraday rotator based on a crystal with a negative optical anisotropy parameter

Thermally induced polarization distortions in a Faraday rotator based on a crystal with a negative value of the optical anisotropy parameter was investigated in experiments. A terbium-containing cubic zirconia crystal was taken as a magneto-active medium. A crystal was cut in the orientation in which the axes of thermal-stress-induced birefringence are oriented in one direction. It was demonstrated that the distortions in such a Faraday rotator were reduced by an order of magnitude compared to the minimum possible depolarization arising with the use of a magneto-optical element with the standard [001] orientation, but were still higher than the calculated values. It was shown that for successful development of Faraday isolators based on crystals with a negative optical anisotropy parameter, the theoretical model describing thermal-stress-induced birefringence needs improvement.

Прогнозирование халькошпинелей состава ABCX4 (X — S или Se)

New chalcospinels of the most common compositions were predicted: AIBIIICIVX4 (X — S or Se) and AIIBIIICIIIS4 (A, B, and C are various chemical elements). They are promising for the search for new materials for magneto-optical memory elements, sensors and anodes in sodium-ion batteries. The parameter “a” values of their crystal lattice are estimated. When predicting only the values of chemical elements properties were used. The calculations were carried out using machine learning programs that are part of the information-analytical system developed by the authors (various ensembles of algorithms of: the binary decision trees, the linear machine, the search for logical regularities of classes, the support vector machine, Fisher linear discriminant, the k-nearest neighbors, the learning a multilayer perceptron and a neural network), — for predicting chalcospinels not yet obtained, as well as an extensive family of regression methods, presented in the scikit-learn package for the Python language, and multilevel machine learning methods that were proposed by the authors — for estimation of the new chalcospinels lattice parameter value). The prediction accuracy of new chalcospinels according to the results of the cross-validation is not lower than 80%, and the prediction accuracy of the parameter of their crystal lattice (according to the results of calculating the mean absolute error (when cross-validation in the leave-one-out mode)) is ± 0.1 Å. The effectiveness of using multilevel machine learning methods to predict the physical properties of substances was shown.

Nanostructures with magnetooptical and plasmonic response for optical sensors and nanophotonic devices

It was proposed photonic nanostructures with magnetooptical and plasmonic properties that can be effectively used for optical sensors and nanophotonic devices. Optical, magnetooptical and functional properties of photonic crystals with magnetooptical structural elements in the form of films of bismuth-substituted iron garnets and gold plasmonic layer were considered. Simultaneous excitation of optical Tamm states (OTS) and Fabry-Perot resonance or OTS, Fabry-Perot resonance and surface plasmon-polaritons in proposed nanostructures was considered.

Permanent-magnet Faraday isolator with the field intensity of more than 3 tesla

A permanent-magnet system with a maximum magnetic field exceeding 3 T, which is a record value for Faraday isolators, has been created. The magnitude of the field in the system is close to the theoretical limit that can be achieved at an optimal magnetization distribution. This is possible thanks to the use in the central area of the system of magnetic conductors that possess a saturation induction higher than the residual induction of the strongest magnets and provide more optimal magnetization distribution than magnets. But the main feature of the system is control of demagnetizing fields in the center of the system by optimizing the position and shape of magnetic conductors, thus retaining the maximum volume of the magnets in the core of the system without the risk of their magnetization reversal. The technology of controlling demagnetizing fields by means of magnetic conductors is proven for the first time. The created system is promising for the development of Faraday isolators for high-power lasers as it allows decreasing the length of the magneto-optical element and reducing parasitic heat generation. Record results on the maximum permissible operating power of Faraday isolators based on CeF3 and TSAG crystals were obtained with the use of this system. It is also promising for the development of Faraday isolators for ‘eye-safe’ and mid-infrared radiation since the Verdet constant of magneto-active crystals in these wavelength ranges reduces substantially compared to the near-infrared. The creation of strong magnetic field becomes of principal importance for providing a needed angle of Faraday rotation.

Research on beam bunching and deflection characteristics of low-power laser under the electromagnetic effect

The bunching and deflection characteristics of low-power laser beam were investigated under electromagnetic field. On the basis of the Faraday effect, the cylindrical electromagnetic cavity was designed and implemented in the experiments. Several types of the magneto-optical elements were placed in the electromagnetic cavity individually. In the test of the deflection characteristics of low-power laser, the rotating angle, the polarization plane of linearly polarized light which passed through electromagnetic cavity, was measured by polarization extinction. We focus on the relation between the coil current and the rotating angle. The experimental data show that when the coil current varies in the range of 0–5 A, the rotating angles changed from 0° to 24.1°. Then, a fitting formula about the coil current and the rotating angle was obtained from the experimental data using the least square algorithm. The analysis shows that the rotating angle is proportional to the excitation current and the correlation coefficient is more than 0.9995. In order to study the beam bunching characteristics of low-power laser, the area of the laser facula was measured after the low-power laser passed through the electromagnetic cavity. The experiment data show that the laser facula area changes in a small range and the experimental data meet 3σ criteria.

High-power Faraday isolator on a uniaxial CeF3 crystal.

We have created the first high-power Faraday isolator on an anisotropic magneto-optical element (MOE). The isolator is based on one MOE of a uniaxial CeF3 crystal and ensures an isolation degree of 30dB at a high average laser radiation power of 700W. The limitations due to the anisotropic nature of the crystal do not impose significantly more stringent requirements, either for the beam or the MOE.

Thermo-optical properties of EuF2-based crystals

High-power solid state lasers of the mid-IR spectral range are extremely interesting for both basic research and technological applications and they are actively developing at present. However, the implementation of projects with the use of such lasers is hampered by the absence of Faraday isolators (FIs) for the radiation with the corresponding characteristics. The need for such devices is growing with laser power enhancement. They reduce the risk of self-excitation of the damage of amplifiers and optical elements and are a handy tool for organizing multipass schemes. Magneto-active crystals based on Eu2+ ions are promising media for the development of FIs for high-power lasers of the mid-IR spectral range because of the unique combination of their spectral and magneto-optical properties. Thermo-optical characteristics are also of principal importance when operating with high-power laser radiation. This letter presents the results of investigation of thermo-optical constants Q and P of the EuF2.11 solid solution crystal which determines the values of polarization and phase distortions of passing laser radiation. The measured values can be considered as the corresponding values for the EuF2 crystal, which is of greatest interest for the development of FIs for high-power mid-IR lasers. The obtained results can be used to determine the characteristics of FIs based on EuF2 crystals in various implementations of their optical schemes for various parameters of laser radiation. According to the estimates, for the radiation wavelength of 2 μm, the maximum permissible laser power of the Faraday isolator based on one magneto-optical element with [001] orientation will be more than 200 W.

Improvement of the polar magneto-optical Kerr effect in the spatial tunable one-dimensional magnetophotonic crystals

We have studied the magnetophotonic crystals (MPCs) with excellent capabilities to enhance reflection and Kerr rotation simultaneously. In order to simultaneously investigate the symmetric and nonsymmetric spatial structures, we have considered the different repetition numbers for dielectric photonic crystals around the magnetic defect layer. To construction of dielectric Bragg mirrors, we have used of Al2O3 due to low refractive index and its unique optical features. The Ce:YIG with high magneto-optical features is utilized as magnetic material to increasing the magneto-optical responses of structures. By adjusting spatial features such as repetition numbers of PCs and thickness of magnetic defect layers, we have achieved the magneto-optical Kerr rotation more than 70 degrees and reflection close to 1. We have introduced the structures with single and several cavities. For investigation of light localization at defect layer, the electric field distribution along the MPCs is simulated using finite element method (FEM). With light localization inside the cavity and multiple reflections in it, the magneto-optical (MO) responses of these MPCs are significantly increased. Our proposed structures having high MO responses, are suitable cases to design MO elements incorporated in integrated photonic devices and optical telecommunication tools.

Voltage-driven magneto-optical Kerr effect in a glass/Au/NiFe/dielectric/WS_2magneto-plasmonic structure

The current study was undertaken to design a voltage-driven magneto-plasmonic device analog to a field effect transistor (FET) but alternatively proposed for a transverse magneto-optical Kerr effect (TMOKE) response. The suggested structure is a glass/Au/NiFe/dielectric/WS2 multilayer with very similar constituted layers to the substrate/metal-gate/dielectric/conductive-channel FET design. Here, a Au/NiFe bilayer is of metal that Au is individually working as a plasmonic excitation layer and NiFe is a ferromagnetic layer for a TMOKE response. The WS2 monolayer is a conductive-channel layer separated from the metal-gate layer via a dielectric layer. The WS2 refractive index and absorption/reflection can be controlled upon applying voltage to the Au/NiFe metal-gate bilayer. Light reflection and TMOKE responses from this structure in a Kretschmann configuration were determined based on a transfer matrix method. Significant change in TMOKE was predicted as the result of large changes in the refractive index of the WS2 monolayer versus voltage as recently reported by Yu et al. [Nano Lett.17, 3613 (2017)NALEFD1530-698410.1021/acs.nanolett.7b00768]. Dielectric layers having different thicknesses were examined to increase the TMOKE response and reduce the reflectance. The results offer design options for achieving miniaturized electrically driven magneto-optical response elements.

Magneto-optical responses of microcavity-integrated graphene photonic crystals in the infrared spectral region

The magneto-optical responses and photonic band gap properties of the microcavity-integrated graphene photonic crystals were numerically studied. The structure consists of a graphene sheet embedded between two mirror symmetric Bragg reflectors, under the influence of an external static magnetic field. The properties of the microcavity resonance mode were investigated, considering the right- and left-handed circular polarization transmission coefficients and their phases, together with the Faraday rotation angle and ellipticity of the output light. The effects of the repetition number of the Bragg reflectors, thickness of the microcavity central layer and refractive indices of the graphene adjacent layers were considered. The obtained results revealed that a pure linear polarized output light with no ellipticity and high transmittance enhanced Faraday rotation can be achieved. These results can be utilized in designing a variety of graphene based photonic devices and magneto-optical integrated elements, such as miniaturized isolators or circulators.

Wavelength dependence of Verdet constant of Tb3+:Y2O3 ceramics

Samples of the magneto-active material—Tb3+:Y2O3 ceramics with Tb3+ ion concentrations of 10%, 20%, 30%, and 100% (Tb2O3)—were prepared and studied. The wavelength dependence of Verdet constant in the 380 nm–1750 nm range was approximated for all investigated ceramic samples and was predicted for a pure Tb2O3 material. Tb2O3 ceramics demonstrates a more than three times higher Verdet constant in comparison with terbium gallium garnet crystal or ceramics. The linear dependence of the Verdet constant on Tb3+ ion concentration in the Tb3+:Y2O3 ceramics was demonstrated. The obtained data will be useful for fabricating magneto-optical elements of Faraday devices based on Tb3+:Y2O3 with arbitrary Tb3+ ion concentration operating at room temperature in the wavelength range of 380 nm–1750 nm.

Faraday isolator stably operating in a wide temperature range

A method of stabilizing Faraday isolator characteristics at varying temperatures is proposed in this letter. The method is based on the use of a magnetic system with an inhomogeneous magnetic field and a magneto-optical element holder made of a material with a high value of thermal expansion coefficient. Changing the rotation angle of the polarization plane of radiation caused by the temperature variation of the magneto-optical element and the temperature dependence of its Verdet constant is compensated by its shifting in the magnetic field. The developed device demonstrates an isolation ratio of more than 40 dB at a temperature range of 25 °C. Estimates show the possibility of providing an isolation ratio of more than 45 dB in a temperature range of 60 °C.

The effect of optical dispersion on magneto-optical responses of single-cavity and dual-cavity magnetophotonic crystals at near infrared wavelengths

In this paper the magneto-optical responses of single-cavity and dual-cavity type one-dimensional magnetophotonic crystals have been investigated considering the constituent layers optical dispersion in near infrared region. The 4 × 4 transfer matrix method has been utilized for calculations and analysis. The obtained results revealed that the resonance transmittance mode is split into two separated modes for specific repetition numbers. These modes are shifted toward the center of photonic band gap regarding the stronger dispersion relations, in both single- and dual-cavity structures. But the amounts of the shifts and their fine behavior are strongly dependent on the type of the structure. We believe that the results of the present work would be practically helpful to precisely design the miniaturized magneto-optical elements in micro- and nano-scale.