Magneto-optic Gratings Research Articles

Strong enhancement of near-field radiative heat transfer between anisotropic magneto-optical gratings by a weak magnetic field

The highly tunable surface magneto-plasmon polaritons supported by magneto-optical materials provide reliable means for active adjusting of near-field radiative heat transfer (NFRHT). In our study, we proposed an anisotropic system consisting of two graphene-covered magneto-optical gratings, and theoretically investigate the NFRHT between them controlled by a weak magnetic field. We find that an external magnetic field open a broad-spectrum photo tunneling channel and modify the energy distribution of surface electromagnetic modes in the wave-vector space. The near-field heat flux between two magneto-optical gratings can achieve a remarkable maximum enhancement of 6 times at the magnetic field H=0.9T compared to that without a magnetic field. Owing to the coupling between highly tunable surface magneto-plasmon polaritons of graphene and surface modes of InSb, the regulation ratio of the heat flux exceeds 10 from 0.5 T to 0.9 T. In addition, the effects of the separation distance, chemical potential of graphene and structural configuration of two gratings are also investigated. We look forward to the proposed system being used as a thermal switch, providing convenience for tuning thermal radiation and improving the efficiency of energy-utilization devices.

Magneto-plasmonic grating-based structure for biosensing application

Gold with satisfactory plasmonic properties can be combined with magneto-optical material to be used as biosensors. In this theoretical work, our suggestion is to use optimized Au strips mounted on a periodic magneto-optical grating, with measurements of the transverse magneto-optical Kerr effect (TMOKE) as the basis for the sensing method. The thickness of the gold strips used is 20 nm after optimizing the structure. Considering the high cost of using gold, since the thickness of the obtained gold layer is small and only placed on the grating, this structure is expected to be economical. Also, in this research, its sensing application has been investigated, and significant results have been obtained.

Steering light with magnetic textures

We study the steering of visible light using a combination of magneto-optical effects and the reconfigurability of magnetic domains in yttrium-iron garnet films. The spontaneously formed stripe domains are used as a field-controlled optical grating, allowing for active spatiotemporal control of light. We discuss the basic ideas behind the approach and provide a quantitative description of the field dependence of the obtained light patterns. Finally, we calculate and experimentally verify the efficiency of our magneto-optical grating.

Transient magnetic gratings on the nanometer scale.

Laser-driven non-local electron dynamics in ultrathin magnetic samples on a sub-10 nm length scale is a key process in ultrafast magnetism. However, the experimental access has been challenging due to the nanoscopic and femtosecond nature of such transport processes. Here, we present a scattering-based experiment relying on a laser-induced electro- and magneto-optical grating in a Co/Pd ferromagnetic multilayer as a new technique to investigate non-local magnetization dynamics on nanometer length and femtosecond timescales. We induce a spatially modulated excitation pattern using tailored Al near-field masks with varying periodicities on a nanometer length scale and measure the first four diffraction orders in an x-ray scattering experiment with magnetic circular dichroism contrast at the free-electron laser facility FERMI, Trieste. The design of the periodic excitation mask leads to a strongly enhanced and characteristic transient scattering response allowing for sub-wavelength in-plane sensitivity for magnetic structures. In conjunction with scattering simulations, the experiment allows us to infer that a potential ultrafast lateral expansion of the initially excited regions of the magnetic film mediated by hot-electron transport and spin transport remains confined to below three nanometers.

Deep‐UV Lithography of Nanocomposite Thin Films into Magnetooptical Gratings with Submicron Periodicity

AbstractIncompatibility of conventional magneto‐optical (MO) materials with photonic‐system fabrication processes constitutes a major technological roadblock limiting their integration into photonic devices, especially for micro‐ or nano‐patterned films. This paper proposes a photolithography approach toward MO micro‐ and nano‐patterns, based on hybrid photosensitive matrices doped with magnetic nanoparticles to concentrations as high as 20 % vol. The as‐prepared and characterized nanocomposite thin films exhibit excellent MO properties: a high Faraday rotation (3300°/cm), comparable with the best existing materials that nevertheless require much more involved processes. The photosensitive matrices allow patterned gratings with a wide range of periods (100 μm to 500 nm), using a simple procedure at room temperature. The fabricated gratings exhibit well‐defined regular structures and resolutions suitable for optical applications, including those at telecommunications wavelengths. Finally, direct applications of the proposed material and its associated process are demonstrated on two examples: 1) a magneto‐optical grating in free‐space and 2) in combination with a waveguide. The obtained optical and MO results are consistent with simulations.

Plasmonic Magnetic Sensor Based on Graphene Mounted on a Magneto-Optic Grating

In this paper, we propose a magnetic sensor utilizing the non-reciprocal propagation of surface plasmons (SPs) on a graphene layer mounted on a magneto-optic grating. An incident electromagnetic wave upon the graphene layer can be absorbed and, under certain conditions, coupled to the SPs along the layer. The sensor structure is assumed to be parallel to the applied magnetic field, the parameter which should be sensed and measured. Contrary to the case with no magnetic field bias and due to magneto-optic characteristics of the grating substrate, wave absorbance for each incident mode can be realized in two different frequencies with non-reciprocal behavior. The frequency difference between these distinct modes depends on the applied magnetic field. This idea can provide a proper approach to sense and measure the magnitude of the applied magnetic field. The performance of the proposed structure is studied analytically and confirmed by numerical simulations. The sensor can be utilized extensively in various systems employing the magnetic field capabilities such as medical diagnostic devices and physicists' experimental setups.

Diffraction-modified Kerr rotation from patterned garnet films

Bismuth-substituted yttrium iron garnet (Bi:YIG)-based two-dimensional (2D) magnetophotonic structures were fabricated on top of opal thin films. Autocloning approach was used to transpose 2D hexagonal symmetry of the opal (111) plane into sputtered Bi:YIG. Resultant 2D patterned Bi:YIG films had two realizations. As-sputtered films had a hemispherical profile of structural elements. These films were additionally etched to obtain films with a cone profile of scatterers. Optical and magneto-optical spectra of the films demonstrated resonances addressed to diffraction from the patterned Bi:YIG films. Structural parameters of these magneto-optical gratings were found to govern the magnitude of Kerr rotation.

Polarization conversion in resonant magneto-optic gratings

Strong polarization conversion effects, including large Kerr rotation and ellipticity, are predicted in zero-order reflection of light from a Bi : YIG grating embedded in a dielectric material. The field localization that gives rise to the enhanced magneto-optic effects is due to the excitation of leaky guided waves, which propagate in (and around) the high-index Bi : YIG layer in directions perpendicular to the external static magnetic field as well as the incident, transmitted and reflected optical fields. The resonant interaction with the incident field then results in narrow-band reflection spectra. Large magneto-optic polarization conversion effects result from the splitting of the degeneracy of right- and left-circularly polarized eigenmode resonances with highly dispersive phase spectra near the resonance peak.

Nonlinear Magneto-Optical Bragg Gratings

In magneto-optical gratings the magnetic dipolar coupling superimposed on the electric dipolar one introduces nonreciprocity and spectral discrimination between circular polarization states, measured by a Zeeman-like splitting of the photonic Bragg resonances. In a nonlinear regime the degree of nonreciprocity is modified by the photoinduced interplay of these splittings and their Stark-like shifts. We predict novel magneto-optical modulation schemes for switching between orthogonal circular polarization states of transmission or reflection operated by means of an intense linearly polarized optical pulse train.

Modeling of magneto-optical properties of periodic nanostructures

Optical and magneto-optical (MO) response from nanometer-size grating is calculated using improved Fourier modal method. The MO grating is modeled by an anisotropic MO layer of effective medium. The complex refractive indices and MO parameter of the effective layer are compared with simple effective medium approximation. Dependence of the optical and MO parameters on the filling factor of the grating is presented. MO grating with period in the sub-wavelength scale could be used as an effective medium with desired optical and MO properties.

Characterization of permalloy wires by optical and magneto-optical spectroscopy

Spectroscopic ellipsometry, reflectometry and magnetooptical (MO) spectroscopy in the zero and first diffraction orders are used to analyze submicron ultrathin MO gratings. Two different theoretical approaches, a rigorous analysis and an approximate method, are used to simulate the optical and MO response of the gratings. The numerical analysis suggests the presence of native oxide layers on the substrate and capping. A strong spectral anomaly observed in the MO response is identified by the integration of both theoretical approaches. The possibility to monitor the quality and topography of shallow gratings with high sensitivity is concluded.

Diffraction hysteresis loop modeling in magneto-optical gratings

Magneto-optical gratings are investigated by means of a rigorous method of diffraction from gratings. We investigate the behavior of the transverse magneto-optical Kerr effect (TMOKE) with respect of the height of the grating. Our results are compared with a previously published work by two of us, for instance using the perturbation Rayleigh method (PRM). It is shown that changing the height of the grating does not affect the hysteresis loop obtained for normalized efficiencies.

Near-field diffraction by a magneto-optical grating in a ferromagnetic film

Theoretical analysis of the near-field magneto-optical (MO) diffraction of light propagating through a ferromagnetic film by a domain structure has been carried out. The results are expressed in terms of the classical theory of diffraction gratings, namely the effective perturbation of the index of refraction and generalised Bragg condition, extended to the near-field case. The near-field case is characterised by a wider variety of types of diffraction, which includes diffraction by in-plane polarisation, both anisotropic and isotropic. It has been shown, analytically and numerically, that the effective layer of a ferromagnetic film that actively contributes to the MO diffraction is severely limited by the Bragg condition, being of the order of d/4, where d is the period of the diffraction grating. Any further increase in the thickness leads only to additional optical losses due to the light absorption. This can explain the lack of resolution reported in the near-field MO experiments where relatively thick garnet films were used.

Diffraction by a relief magneto-optic grating: diagrammatic approach

We describe a straightforward analytical solution for the problem of diffraction of light by a periodic surface structure engraved on a ferromagnetic material in the transverse Kerr magneto-optical effect (TMOKE) geometry. The theoretical approach is based on a double perturbative expansion which allows an interpretation in terms of multiscattering Feynman-type diagrams. It has been shown, both analytically and numerically, that the Leontovich approximation is valid for most practically interesting situations, which leads to amazingly simple final expressions.

Oblique-coordinate-system-based Chandezon method for modeling one-dimensionally periodic, multilayer, inhomogeneous, anisotropic gratings

A formulation of the Chandezon method for modeling one-dimensionally periodic, multilayer, inhomogeneous, anisotropic diffraction gratings is presented. The formulation is based on an oblique Cartesian coordinate system so that blazed surface-relief profiles and slanted volume permittivity variations can be treated simultaneously. The permittivity tensors of the anisotropic materials can be arbitrary, but their spatial variations are assumed to have the same period as the surface corrugations. Some of the previous authors’ works in the above individual aspects of the subject are improved on in regard to clarity, simplicity, and generality. Most important, the theory of Fourier factorization is applied throughout the analysis to ensure fast convergence of the numerical method when the permittivity tensors or the derivatives of the grating’s surface-profile functions have discontinuities. Numerical examples, including those of two types of multilayer magneto-optic gratings, are provided.

Measurements of the magneto-optic Kerr effect and the extraordinary Hall effect on grooved glass substrates coated with amorphous TbFeCo

Kerr loops measured in the 0th, −1st, and −2nd diffraction order beams on deep magneto-optic gratings show constructive and destructive interference of the beams reflected from grooves and lands as a function of effective phase depths and relative widths of grooves and lands. Hall-effect measurements yielded coercivities of grooves, lands, and sidewalls at different angles of the applied field. Sidewall reversals appear clearly in Kerr loops measured in the various diffracted beams, with signal amplitude depending on groove depth and the angle between the sidewall and the diffracted beam.

Near-field theoretical study of a magneto-optical grating

We propose a theoretical description of a near-field observation of a magneto-optical grating. We consider a scanning tunnelling optical microscopy (STOM) experiment where the sample is a thin isotropic magnetic material. We choose a transverse geometry: the magnetization is perpendicular to the plane of incidence and parallel to the grating grooves. For calculating the detected signal we use a perturbative diffraction theory of a multilayer anisotropic structure. The formalism has been tested by modelling a far-field diffraction experiment. Then we calculate STOM near-field signal above the grating and compare optical images and magneto-optical images.

Magneto-optical beam splitter for atoms.

We report an experimental demonstration of diffraction of ${\mathrm{He}}^{\mathrm{*}}$ atoms from a magneto-optical grating. The grating was produced by the interaction of three-level atoms with a light field of spatially varying polarization and a magnetic field. For a light shift matched to the Zeeman shift, a two-peaked diffraction pattern was observed, corresponding to a momentum splitting of 42\ensuremath{\Elzxh}k. The effect of changing the polarizations of the light field was investigated. The diffraction from the magneto-optical grating is compared directly with the diffraction of two-level atoms from a standing light wave and it is shown, that the magneto-optical interaction leads to more efficient coupling into high order momentum states.

Field control of magneto-optic gratings

The Faraday or Kerr rotation of the plane of polarization of light incident on adjacent magnetic domains has opposite sense. Thus light transmitted through a film containing stripe domains or reflected from it is diffracted, as by a grating. The period of such a grating can be controlled by the application of a uniform magnetic field. We report on experimental and theoretical work that explores the range of grating field control under quasistatic conditions, using real films, specifically