Magneto-optical Research Articles

Magneto-optical polarization texturing

Left and right circularly polarized transverse electromagnetic waves propagate at slightly different speeds in a magnetic material leading to a polarization rotation by an amount proportional to the projection of the magnetic field along the direction of the wave propagation. We show how this magneto-optical effect can serve as a vectorial polarization shaper if the input mode is either a radially-polarized or an azimuthally-polarized Laguerre-Gaussian (LG) mode. The specific polarization map of the output field can be achieved by choosing appropriately the magnetic material and/or its geometry. We show further that when the LG beam waist is comparable to the wavelength (i.e. w 0 ≈ λ ) the fields are no longer purely transverse but acquire an additional longitudinal (axial) component. We demonstrate how this modifies the polarization texturing.

Faraday rotation in a cavity integrated with layered van der collinear antiferromagnetic material

Abstract Two dimensional layered van der Waals (vdW) ferromagnetic (FM) and anti FM materials enable the alternative candidates for establishing the next-generation magnetooptical (MO) devices. However, the explorations of the MO effects are primarily focused on the layered FM materials and their nanostructures. How the MO responses behave in the structures with vdW layered anti FM materials remain largely unknown. Here, we identify that a layered collinear antiferromagnet supports the nontrivial MO Faraday rotation (FR) angle in the presence of the electric field instead of magnetic field due to its lifted spin degeneracy and asymmetric band structure at different valleys, whose value is comparable with those of vdW layered ferromagnets and closely correlates with the size of the electric field. Specifically, the FR angle of the layered collinear antiferromagnet can be substantially enhanced via integrating it into a cavity structure, which promises the chance of improving their MO performance. Our result highlights the fundamental opportunities of utilizing the collinear antiferromagnets for various MO applications ranging from magnetometry to high density holographic data storage.

Study on ion slicing of iron garnet magneto-optic materials for near and mid-infrared on-chip optical isolators

Achieving high-crystalline-quality, large-size iron garnet magneto-optic (MO) films on silicon substrates remains a critical challenge for CMOS-compatible on-chip non-reciprocal devices like isolators and circulators. In this study, we explored ion slicing on commercial yttrium iron garnet (YIG) crystals, bismuth-doped iron garnet (BIG), and newly developed YIG ceramics. After He+ ion implantation, wafer bonding and annealing, the BIG film on silicon was successfully fabricated, but its thickness and crystalline phase deviated from expectations. The underlying causes of these discrepancies were systematically investigated. In contrast, the YIG single crystals and ceramics showed blistering during annealing, which demonstrates their ion-slicing viability. Based on the magneto-optical constant dispersion relationships of the two materials, the nonreciprocal phase shift (NRPS) gap between BIG film on silicon and YIG film on silicon narrows significantly as the wavelength increases from 1.55 µm to 2.1 µm, dropping from 399% to 26%. As a proof of concept, we proposed a design for silicon-based TM-mode on-chip isolators at 1.5 µm and 2.1 µm using ion-sliced YIG ceramics, where the simulated insertion loss decreased from 2.78 dB to 0.35 dB due to the substantial reduction in material absorption with increasing wavelength. These results underscore the feasibility and promise of YIG ceramic ion slicing as a practical solution for CMOS-compatible on-chip isolators, particularly in the mid-infrared range.

Tunable directional radiation and reception based on magneto-optical photonic crystal waveguides

Abstract We have proposed a magneto-optical photonic crystal (MOPC) waveguide that can not only realize highly directional radiation but also work as an electromagnetic (EM) wave receiver. This waveguide consists of a square MOPC and a covering layer of Al2O3 PC tilted at 45° that allows for the penetration of EM waves. Due to the broken time-reversal symmetry caused by an external magnetic field, there exists a unidirectional leaky topological edge state within the photonic bandgap. Such the leaky edge state transmits unidirectional and radiates into the air simultaneously, leading to a broadside radiation beam with a low half-power beam width of less than 4°. More importantly, the radiation direction of the beam can be tuned over a wide angular range by adjusting the operation frequency or the magnetic field strength. Besides, this designed waveguide can also act as a tunable receiver for capturing incident EM waves effectively. These results have potential for various practical applications, such as antennas and sensors.

Modulation of Bessel-like vector vortex beam using the resonant magneto-optical effect in rubidium vapor

Abstract The Bessel-like vector vortex beam (BlVVB) has gained increasing significance across numerous applications. However, its practical application is restricted by manufacturing difficulties and polarization manipulation. Thus, the ability to manipulate its degrees of freedom is highly desirable. In this paper, the full-domain polarization modulation of BlVVB within a hot atomic ensemble has been investigated. We begin with the theoretical analysis of the resonant magneto-optical effect of atoms with a horizontal linear polarized beam and experimentally demonstrate precise manipulation of the polarization state across the entire domain of the BlVVB, achieving an error margin of less than 3° at various cross-sectional points. Our study provides a novel approach for the modulation of BlVVB based on atomic media, which holds potential applications in sensitive vector magnetometers, optical communications, and signal processing.

Magneto-Optics Features of Radiation Transitions of Non-Kramers Tm3+ Ion in Yttrium-Aluminum Garnet Crystals

The spectra of luminescence and magnetic circular polarization of a single crystal of thulium-yttrium garnet-aluminate Tm3+:YAG have been studied within the visible spectral range at a temperature of 90 and 300 K in a magnetic field of 10 kOe. Based on the analysis of optical and magneto-optical data, the presence of "quasi-degenerate" states of excited multiplets 1D2, 3F4,3G4 and the ground multiplet 3H6 of the Tm3+ RE ion in garnet-aluminate YAG at the radiative transitions 1G4→3H6, 1D2→3F4 and 1D2→3F3 has been determined. The effect of quantum mechanical “mixing” plays a significant role in the occurrence of magneto-optical effects on luminescence bands caused by “forbidden” 4f→4f transitions in the non-Kramers Tm3+ ion having a “quasi-doublet” structure in the energy spectra.

Magnetically tunable bound states in the continuum with arbitrary polarization and intrinsic chirality

Bound states in the continuum (BICs), which are exotic localized eigenstates embedded in the continuum spectrum and exhibit topological polarization singularities in momentum space, have recently attracted great attention in both fundamental and applied physics. Here, based on a magneto-optical (MO) photonic crystal (PhC) slab placed in external magnetic fields with time-reversal symmetry (TRS) breaking, we theoretically propose magnetically tunable BICs with arbitrary polarization covering the entire Poincaré sphere and efficient off-Γ chiral emission of circularly polarized states (C point). More interestingly, by further breaking the in-plane inversion symmetry of the MO PhC slab to generate a pair of C points spawning from the eliminated BICs and tuning the external magnetic field strength to move one C point to the Γ point, an at-Γ intrinsic chiral BIC exhibits chiral characteristics on both sides of the PhC slab with near-unity circular dichroism exceeding 0.99 and a high-quality factor of 46,000 owing to the preserved out-of-plane mirror symmetry. Moreover, the chirality of the chiral BICs can be inverted by flipping the magnetic bias. Our work opens an unprecedented avenue to explore the unique topological photonics of BICs with broken TRS and promises multiple applications in chiral-optical effects, structured light, and tunable optical devices.

Automatic Detection and Classification of Natural Weld Defects Using Alternating Magneto-Optical Imaging and ResNet50.

It is difficult to detect and identify natural defects in welded components. To solve this problem, according to the Faraday magneto-optical (MO) effect, a nondestructive testing system for MO imaging, excited by an alternating magnetic field, is established. For the acquired MO images of crack, pit, lack of penetration, gas pore, and no defect, Gaussian filtering, bilateral filtering, and median filtering are applied for image preprocessing. The effectiveness of these filtering methods is evaluated using metrics such as peak signal-noise ratio (PSNR) and mean squared error. Principal component analysis (PCA) is employed to extract column vector features from the downsampled defect MO images, which then serve as the input layer for the error backpropagation (BP) neural network model and the support vector machine (SVM) model. These two models can be used for the classification of partial defect MO images, but the recognition accuracy for cracks and gas pores is comparatively low. To further enhance the classification accuracy of natural weld defects, a convolutional neural network (CNN) classification model and a ResNet50 classification model for MO images of natural weld defects are established, and the model parameters are evaluated and optimized. The experimental results show that the overall classification accuracy of the ResNet50 model is 99%. Compared with the PCA-SVM model and CNN model, the overall classification accuracy was increased by 7.4% and 1.8%, and the classification accuracy of gas pore increased by 10% and 4%, respectively, indicating that the ResNet50 model can effectively and accurately classify natural weld defects.

Theoretical and experimental study on optical polarization states in overhead optical cables under complex weather conditions such as real lightning

Abstract By analyzing the changes in three Stokes vectors, the trajectory of polarization state on the Poincaré sphere, and the polarization change rate, both theoretical and practical aspects of polarization state variations in signal light within optical fiber composite overhead power ground wires (OPGWs) during real thunderstorm conditions, were examined. Our findings reveal that lightning and mechanical vibrations both impact the polarization state of the signal light in OPGW. Theoretical models demonstrate that lightning induces polarization changes through the Faraday magneto-optic effect and exhibits nonlinear electro-optic effects, leading to a maximum polarization change rate of Mrad s−1. In contrast, mechanical vibrations cause long-term, low-intensity polarization rate changes, with a maximum polarization rate of Krad s−1, highlighting a significant optical birefringence effect. Digital signal processing algorithms for coherent receivers with over 100G polarization multiplexing provide valuable guidance for recovering polarization states.

Synthesis and Characterization of Superparamagnetic CoPt Alloy Nanoparticles Exhibiting Magneto-Plasmonic Responses

Magnetic materials containing transition metals such as Fe, Co, and Ni have attracted considerable interest, in particular for their unique properties, which allow them to be applied to devices for magnetic control of optical communications and magneto-optical data storage. Magnetic control of catalysis and electrocatalysis is also a potential application of magnetic materials. Apparent catalytic activity can be controlled via a magnetohydrodynamic (MHD) effect, which affect mass transport rate in the vicinity of the magnetic material.[1] In addition, enantioselectivity can be given to the system by taking advantage of chiral-induced spin selectivity (CISS), which can be induced by magnetic materials.[2] If a magnetic material is broken down into nanoparticles, they could also obtain high catalytic activities. In addition, they would be superparamagnetic materials, which respond very rapidly to external magnetic fields without hysteresis. If the magnetic nanoparticles are metallic, they might exhibit magneto-optical responses such as magnetic circular dichroism (MCD), in association with localized surface plasmon resonance (LSPR).[3] LSPR would also allow the nanoparticles to be used for photocatalysis based on plasmon-induced charge separation (PICS).[4] These characteristics render magnetic nanomaterials suitable for catalysis, electrocatalysis, and photoelectrocatalysis with controllable activity and selectivity.With these points in mind, we synthesize superparamagnetic CoPt nanoparticles through a convenient wet chemical process[5] and examine their magneto-optical properties, more specifically, magnetic circular dichroism (MCD), in the UV-visible range. MCD characteristics would be important indices for magnetic materials to assess their magnetic properties including CISS. CoPt nanoparticles are well known for their high electrocatalytic activities as well as high magnetism due to strong spin-orbit interaction.[6] CoPt superlattice nanoparticles has been reported to show MCD in the near-infrared range and applied to magnetic switching of plasmonic laser.[7] However, MCD in the UV-visible range has not yet been reported for CoPt materials to the best of our knowledge.Co, Pt, and CoPt nanoparticles were synthesized via an oleylamine co-reduction method. Scanning electron microscopy (SEM) image shows that CoPt nanoparticles have a spherical shape with an average size of 6 nm (Fig. 1a). On the basis of X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and superconducting quantum interference device (SQUID) analyses, we have confirmed that the synthesized CoPt nanoparticles have face-centered cubic (fcc) structure with a small amount of cobalt oxides at the particle surface of CoPt, and show superparamagnetism at room temperature.The magneto-optical properties of the Co, Pt, and CoPt nanoparticles were evaluated by a dissymmetry factor, g MCD-factor which was obtained by normalizing MCD with extinction. As shown in Fig. 1b, the Pt nanoparticles showed almost no MCD responses, and the Co nanoparticles exhibited very weak MCD responses, although the latter are magnetic. In contrast, the CoPt nanoparticles showed sufficiently strong MCD signals over the UV-visible range examined. The value of their g MCD-factor was approximately 0.034, which was in the range of very high g MCD-factor values.The possible origin of the MCD properties is circular electron motion in the metallic nanoparticle induced by circularly polarized light. It could also be coupled with circular mode of LSPR. In an external magnetic field, a Lorentz force is applied to the rotating charges, and the resonant light energy depends on the direction of the rotation, resulting in lifted degeneracy (Fig. 1c). Although this type of MCD is also observed for non-magnetic Au nanoparticles, introduction of a magnetic component to the nanosystem enhances the local magnetic field and thereby MCD responses. Likewise, the magnetism of Co would be positive for the MCD responses in the present system. In addition, the spin-orbit coupling induced by Pt may enhance the MCD responses further.[1] S. Luo, K. Elouarzaki, and Z. J. Xu, Angew. Chem. Int. Ed. 61, e202203564 (2022).[2] B. Göhler, V. Hamelbeck, T. Z. Markus, M. Kettner, G. F. Hanne, Z. Vager, R. Naaman, and H. Zacharias, Science 331, 894 (2011).[3] F. Pineider, G. Campo, V. Bonanni, C. d. J. Fernández, G. Mattei, A. Caneschi, D. Gatteschi, and C. Sangregorio, Nano Lett. 13, 4785 (2013).[4] Y. Tian and T. Tatsuma, J. Am. Chem. Soc. 127, 7632 (2005).[5] Y. Yu, W. Yang, X. Sun, W. Zhu, X.-Z. Li, D. J. Sellmyer, and S. Sun, Nano Lett. 14, 5, 2778 (2014).[6] J. Okabayashi, Y. Miura, and H. Munekata, Sci. Rep. 8, 8303 (2018).[7] F. Freire-Fernández, J. Cuerda, K. S. Daskalakis, S. Perumbilavil, J.-P. Martikainen, K. Arjas, P. Törmä, and S. van Dijken, Nat. Photon. 16, 27 (2022). Figure 1

Magnetooptical gyrotropic effects in nanosized BiYIG films and diamagnetic YAG substrates

Magneto-optical gyrotropic Faraday and Kerr effects are studied in the BiY2Fe5O12 films with thicknesses ranging from 16 to 55 nm, in the wavelength range of 295 nm < λ < 830 nm (4.2–1.5 eV), and under magnetic fields of up to 1.2 T. It is shown that films produced by ac magnetron sputtering on the single-crystalline diamagnetic yttrium aluminum garnet substrates Y3Al5O12 (001) exhibit high structural and magneto-optical quality. The Verdet constant for diamagnetic Y3Al5O12 is determined, and the magnitude of the Kerr effect is estimated for the polished substrate. It is shown experimentally that for a substrate with diffusely reflective backside, the Kerr effect is about zero, except in the high-energy region. For all BiY2Fe5O12 films investigated, in the saturating magnetic fields of approximately 0.16–0.2 T, the value of specific Faraday rotation reaches 20 deg/μm (200 000 deg/cm), while the value of the Kerr effect reaches approximately 20 min (5.8 mrad). The critical thickness of the film–substrate interface was estimated, highlighting variations in the Kerr effect spectra associated with a decrease in the Bi content in thin films with the thicknesses of below 27 nm.

High-precision measurement of the magneto-optical Faraday effect via difference weak measurements

We propose a modified difference weak measurement scheme that permits precise measurements of the magneto-optical Faraday effect. By making normalized difference processing for a set of post-selected light intensity, a linear-response regime with a significant weak-value amplification effect is established. In the proof-of-principle experiment, we measure the magnetic intensity using the polarization system and achieve precision at the order of ∼10−7 T. Our scheme can be applied to measure other magneto-optical effects, providing a method for future ultra-sensitive sensing and metrology in magnetic physics.

Going deeper on magneto-optical Faraday effect analysis to detect fatigue crack with high-spatial resolution for non-destructive inspection

In this study, we introduce a high-resolution technique for defect detection that uses the magneto–optical (MO) Faraday effect. Our system combines a portable polarized microscope and an MO sensor for the high spatial observation of a magnetic domain structure. We accurately localized and characterized the fatigue defects through integrated image pre-processing and cross-power spectral density analysis. This approach enhances our analysis of the magnetic domain structure, increasing the spatial capabilities of the microscope to detect fatigue defects. We conducted experiments on fatigue cracks with defect depth angles of 0°, 30°, and 60°, analyzing their relationship through signal amplitude and the tendency of the signal shift with increasing defect angle. Our findings were validated using a tunnel magnetoresistance sensor. Future research will focus on the optimization of feature extraction complexity, considering the limited computational power available for portable non-destructive testing devices.

Demonstration of magnetically silent optically pumped magnetometers for the TUCAN electric dipole moment experiment

We report the performance of a magnetically silent optically pumped cesium magnetometer with a statistical sensitivity of 3.5 pT/Hz at 1 Hz and a stability of 90 fT over 150 s of measurement. Optical pumping with coherent, linearly-polarized, resonant light leads to a relatively long-lived polarized ground state of the cesium vapour contained in a measurement cell. The state precesses at its Larmor frequency in the magnetic field to be measured. Nonlinear magneto-optical rotation then leads to the rotation of the plane of polarization of a linearly polarized probe laser beam. The rotation angle is modulated at twice the Larmor frequency. A measurement of this frequency constitutes an absolute measurement of the magnetic field magnitude. Featuring purely optical operation, non-magnetic construction, low noise floor, and high stability, this sensor will be used for the upcoming TUCAN electric dipole moment experiment and other highly sensitive magnetic applications. Novel aspects of the system include commercial construction and the ability to operate up to 24 sensors on a single probe laser diode.

Nonlinear Faraday magneto-optic effects in a helically wound optical fiber

Nonlinear Faraday magneto-optic effects in a helically wound optical fiber

Time-resolved magneto-optical effects in the altermagnet candidate MnTe

α -MnTe is an antiferromagnetic semiconductor with above room temperature TN = 310 K, which is promising for spintronic applications. Recently, it was reported to be an altermagnet, containing bands with momentum-dependent spin splitting; time-resolved experimental probes of MnTe are, therefore, important both for understanding novel magnetic properties and potential device applications. We investigate ultrafast spin dynamics in epitaxial MnTe(001)/InP(111) thin films using pump-probe magneto-optical measurements in the Kerr configuration. At room temperature, we observe an oscillation mode at 55 GHz that does not appear at zero magnetic field. Combining field and polarization dependence, we identify this mode as a magnon, likely originating from inverse stimulated Raman scattering. Magnetic field-dependent oscillations persist up to at least 335 K, which could reflect coupling to known short-range magnetic order in MnTe above TN. Additionally, we observe two optical phonons at 3.6 and 4.2 THz, which broaden and redshift with increasing temperature.

Robust generation of intrinsic C points with magneto-optical bound states in the continuum.

C points, circular polarization in momentum space, play crucial roles in chiral wave manipulations. However, conventional approaches of achieving intrinsic C points using photonic crystals with broken symmetries suffer from a low Q factor and high sensitivity to structural geometry, rendering them fragile and susceptible to perturbations and disorders. We report magneto-optical (MO) bound states in the continuum (BICs) with a symmetry-preserved planar photonic crystal. We achieve intrinsic C points at Γ point that are robust against variation in both structural geometry and external magnetic field. MO coupling between two modes induces Zeeman splitting, leading to MO BICs and quasi-BICs with circular eigenstates for high-Q chiral responses. Furthermore, switchable C point handedness and circular dichroism are enabled by reversing the magnetic field. These findings unveil BICs and quasi-BICs with circular eigenstates and on-demand control of C points, paving the way for advanced chiral wave manipulation with enhanced light-matter interaction.

MODVORTEx: computer vision-driven automation for magnetic domain wall velocity analysis

Abstract This paper presents Magneto Optic Domain Velocity Observation and Real-Time Extraction (MODVORTEx), a comprehensive software solution for automating domain wall (DW) velocity measurements in magnetic materials using magneto-optic Kerr effect microscopy. Building upon our previous work on bubble domain structures, we introduce a versatile graphical user interface (GUI) that accommodates arbitrary domain shapes and employs advanced computer vision techniques. The software provides different methods for DW detection and velocity calculation, catering to various domain structures of arbitrary shape. Our approach significantly reduces the time and effort required for data extraction, transforming a process that previously took days of manual work into a task completable within minutes. We provide the details of the algorithmic implementation which is organized into pre-processing, DW detection, displacement measurement, and velocity extraction. This tool is applicable to a wide range of scenarios, including bubble domain dynamics, Dzyaloshinskii–Moriya interaction studies in perpendicular magnetic anisotropy systems, current-driven DW motion in patterned strips etc. By providing both GUI and Application Programing Interface, our software offers flexibility for integration into existing measurement systems and adaptability for specific research needs. This automation promises to accelerate research in spintronics and magnetic materials, enabling more comprehensive and accurate studies of DW dynamics.

Tb3+-doped fluoro-borosilicate magneto-optical glass with a large Verdet constant for current sensing

Rare-earth-ion-doped magneto-optical (MO) glasses, characterized by their high transparency, ease of processing, and large Verdet constant, exhibit substantial potential in sensing and modulation systems. In this study, a series of transparent, fluoro-borosilicate MO glasses doped with a high Tb3+ content and composed of SiO2-B2O3-Tb2O3-TbF3 were prepared. The Tb3+ ion concentration in these MO glasses was approximately 71 wt%, with an exceptionally high Verdet constant of −146.91 rad/(T∙m) at 638 nm, exceeding that of the widely used terbium gallium garnet crystal. By exploiting these exceptional characteristics, an MO glass-based current sensor was fabricated, which exhibited a high current sensitivity of 2.794°/A. These findings are expected to guide the development of high-performance MO glasses and advance their application in current sensing and related technologies.

Magneto-optical Goos–Hänchen shift in uniaxial anisotropic epsilon-near-zero metamaterials

Abstract We theoretically investigate the enhancement of the magneto-optical Goos-Hänchen (MOGH) shift in multilayer structure combining uniaxially anisotropic epsilon-near-zero (ENZ) metamaterials with magneto-optical (MO) materials. The structure enables the excitation of magneto-optical surface plasmon resonance (MOSPR) without the need for a prism or grating coupler, thereby achieving a significant enhancement of the MOGH shift. The effect of the thickness of the ENZ layer and the type of magnetic metal in this structure on the MOGH shift is also discussed. Based on the above structure, we design a high-sensitivity miniature biosensor that is used for the detection of hemoglobin concentration in human blood. The maximum sensitivity achievable with this sensor is 1.4×106 λ/RIU. The results of this investigation contribute to the development of biosensors towards miniaturization and integration.