Magneto-optical Materials Research Articles

Determination of the Faraday rotation perpendicular to the optical axis in uniaxial CeF3 crystal by using the Generalized-High Accuracy Universal Polarimeter

Many single crystals have been developed and commercialized for optical isolators. However, optical isolator materials have been limited to isotropic crystals or to the isotropic direction (optic axis) of anisotropic crystals. This study investigates the wavelength dependences of linear birefringence, linear dichroism, Faraday rotation and magnetic-circular dichroism in a single crystal rare-earth fluoride, namely CeF3. Measurements were made in the direction parallel and perpendicular to the optic axis under an applied magnetic field. The magnetic field was generated by Nd-Fe-B magnets installed in the generalized-high accuracy universal polarimeter (G-HAUP). The first application of G-HAUP to a magneto-optical material is presented. In the CeF3 crystal, the Verdet constants along directions parallel and perpendicular to the optic axis were positive over the measured wavelength region (300–680 nm), and their magnitudes were nearly equal. The success in the accurate measurement on Faraday rotation along anisotropic directions has opened the way to study on optical isolators along the direction other than optic axis.

Nonreciprocal transmission using a multilayer magneto-optical dispersive material with defect

A one-dimensional nonreciprocal structure comprising two asymmetric multilayers filled with magneto-optical dispersive media and a defect layer is designed. The nonreciprocal optical Tamm states (OTSs) are excited at the interface of the two asymmetric multilayer structure with an opposing external magnetic field. With the excitation of the nonreciprocal OTSs, the numerical results show that the waves can be transmitted and be reflected in one way. These nonreciprocal properties are affected by the layers thickness of the defect layer, the angles of incidence, and the externally applied magnetic fields. Owing to the inherent dispersive property of the magneto-optical media, our investigations show promising potential for the practical application of nonreciprocal structures.

Advanced Materials and Device Architectures for Magnetooptical Spatial Light Modulators

AbstractFaraday and Kerr rotations are magnetooptical (MO) effects used for rotating the polarization of light in transmission and reflection from a magnetized medium, respectively. MO effects combined with intrinsically fast magnetization reversal, which can go down to a few tens of femtoseconds or less, can be applied in magnetooptical spatial light modulators (MOSLMs) promising for nonvolatile, ultrafast, and high‐resolution spatial modulation of light. With the recent progress in low‐power switching of magnetic and MO materials, MOSLMs may lead to major breakthroughs and benefit beyond state‐of‐the‐art holography, data storage, optical communications, heads‐up displays, virtual and augmented reality devices, and solid‐state light detection and ranging (LIDAR). In this study, the recent developments in the growth, processing, and engineering of advanced materials with high MO figures of merit for practical MOSLM devices are reviewed. The challenges with MOSLM functionalities including the intrinsic weakness of MO effect and large power requirement for switching are assessed. The suggested solutions are evaluated, different driving systems are investigated, and resulting device architectures are benchmarked. Finally, the research opportunities on MOSLMs for achieving integrated, high‐contrast, and low‐power devices are presented.

Highly Linear and Magnetless Isolator Based on Weakly Coupled Nonreciprocal Metamaterials

A novel highly linear magnetless isolator based on weakly coupled nonreciprocal metamaterials is proposed in this article. Nonreciprocal devices, such as isolators and circulators, are widely used in modern microwave, optics. Nowadays, nonreciprocity is exclusively achieved with anisotropic magneto-optic material. Unfortunately, this material needs permanent magnetic bias and is typically bulky, which remains incompatible with technology for integrated circuit. In this article, we reveal that strong isolation can be achieved even under the weak coupling between the transmission line and magnetless nonreciprocal metamaterials (MNMs). Compared with previous magnetless nonreciprocal approaches, in this article, as the wave propagates from port 2 to port 1, the interaction between the incoming wave and nonlinear devices can be neglected due to the weak coupling, yielding low loss (|S12| = −1.03dB) and high linearity $({P}_{{1dB}}{>{17} {dBm}}$ , ${{OIP}}_{{3}}{=33.5 {dBm})}$ . As the wave propagates from port 2 to port 1, 36.2 – dB isolation can be achieved under the loop resonance. Furthermore, the frequency reconfiguration of MNMs has been investigated for the first time, leading to broadband and multiband isolators.

Application-Specific Oxide-Based and Metal-Dielectric Thin-Film Materials Prepared by Radio Frequency Magnetron Sputtering.

We report on the development of several different thin-film functional material systems prepared by radio frequency (RF) magnetron sputtering at Edith Cowan University nanofabrication labs. While focusing on the RF sputtering process optimizations for new or the previously underexplored material compositions and multilayer structures, we disclose several unforeseen material properties and behaviours. Among these are an unconventional magnetic hysteresis loop with an intermediate saturation state observed in garnet trilayers, and an ultrasensitive magnetic switching behaviour in garnet-oxide composites (GOC). We also report on the unusually high thermal exposure stability observed in some nanoengineered metal–dielectric multilayers. We communicate research results related to the design, prototyping, and practical fabrication of high-performance magneto-optic (MO) materials, oxide-based sensor components, and heat regulation coatings for advanced construction and solar windows.

Optically driven effective Faraday effect in instantaneous nonlinear media

Faraday rotation is arguably the most widely studied nonreciprocal phenomenon, observable in the form of polarization rotation as waves travel in magneto-optical materials. It is at the basis of the realization of various forms of isolators and circulators. However, magneto-optical materials are bulky and difficult to integrate, and inherently associated with losses. Here we leverage nonlinear phenomena in conventional optical crystals to mimic Faraday rotation in a resonant cavity excited with two pumps with opposite circular polarizations and slightly detuned frequencies. The effect can be observed in basic resonant structures with no need for material resonances or tailored dispersion, arbitrary frequency of the pumps, and low loss, yielding an interesting path towards the realization of compact magnet-free nonreciprocal optical elements.

Near-complete violation of Kirchhoff's law of thermal radiation with a 0.3 T magnetic field.

The capability to overcome Kirchhoff's law of thermal radiation provides new opportunities in energy harvesting and thermal radiation control. Previously, design towards demonstrating such capability requires a magnetic field of 3T, which is difficult to achieve in practice. In this work, we propose a nanophotonic design that can achieve such capability with a far more modest magnetic field of 0.3 Tesla, a level that can be achieved with permanent magnets. Our design uses guided resonance in low-loss dielectric gratings sitting on a magneto-optical material, which provides significant enhancement on the sensitivity to the external magnetic field.

Lateral optical force on linearly polarized dipoles near a magneto-optical surface based on polarization conversion

Novel lateral optical forces acting on dipoles near surfaces have been investigated in the past few years: circularly polarized dipoles experience lateral optical forces when in proximity to a surface due to the recoil of directionally excited modes. Recent work shows that even linearly polarized dipoles may experience lateral forces when nonreciprocal substrates are used, due to the asymmetric propagation of modes in the surface. We theoretically show that a linearly polarized particle emitting in close proximity to a magneto-optical substrate may experience a lateral optical force even if the external magnetic field is normal to the surface plane. The polarization conversion of the magneto-optic material introduces a gradient in the quasistatic fields reflected from the surface, resulting in a lateral optical force, and also gives surface plasmons a hybrid polarization character, which alters the surface plasmon directional excitation from the dipole, resulting in an optical recoil force. We envisage potential applications in nanomechanical devices, since similar magnetoplasmonic architectures have already been developed experimentally.

Dynamic suppression of Rayleigh backscattering in dielectric resonators

The ultimate limits of performance for any classical optical system are set by sub-wavelength fluctuations within the host material, which may be frozen-in or even dynamically induced. The most common manifestation of such subwavelength disorder is Rayleigh light scattering, which is observed in nearly all waveguiding technologies today and can lead to both irreversible radiative losses as well as undesirable intermodal coupling. While it has been shown that backscattering from disorder can be suppressed by breaking the time-reversal symmetry in magneto-optic and topological insulator materials, common optical dielectrics possess neither of these properties. Here, we demonstrate an optomechanical approach for dynamically suppressing Rayleigh backscattering within dielectric resonators. We achieve this by locally breaking the time-reversal symmetry in a silica resonator through a Brillouin scattering interaction that is available in all materials. Near-complete suppression of Rayleigh backscattering is experimentally confirmed through two independent measurements—the elimination of a commonly seen normal-mode splitting or “doublet” effect and by measurement of the reduction in intrinsic optical loss. Additionally, a reduction of the back-reflections caused by disorder is also observed. Our results provide new evidence that it is possible to dynamically suppress Rayleigh backscattering within any optical dielectric medium using time-reversal symmetry breaking, for achieving robust light propagation in spite of scatterers or defects.

Analysis, Optimization, and Characterization of Magnetic Photonic Crystal Structures and Thin-Film Material Layers

The development of magnetic photonic crystals (MPC) has been a rapidly evolving research area since the late 1990s. Magneto-optic (MO) materials and the techniques for their characterization have also continually undergone functional and property-related improvements. MPC optimization is a feature-rich Windows software application designed to enable researchers to analyze the optical and magneto-optical spectral properties of multilayers containing gyrotropic constituents. We report on a set of computational algorithms which aim to optimize the design and the optical or magneto-optical spectral analysis of 1D MPC, together with a Windows software implementation. Relevant material property datasets (e.g., the spectral dispersion data for the refractive index, absorption, and gyration) of several important optical and MO materials are included, enabling easy reproduction of the previously published results from the field of MPC-based Faraday rotator development, and an effective demonstration-quality introduction of future users to the multiple features of this package. We also report on the methods and algorithms used to obtain the absorption coefficient spectral dispersion datasets for new materials, where the film thickness, transmission spectrum, and refractive index dispersion function are known.

Phase transformation process of Tb2O3 at elevated temperature

Phase transformation process of magneto-optical material Tb2O3 at elevated temperature has been investigated in different atmosphere. When heating in air, Tb2O3 is firstly oxidized to TbO1.81 at 500 °C. Finally, TbO1.81 is respectively deoxidized to TbO1.714 at 600 °C and Tb2O3 at 988 °C. It is remarkable that the most stable phase at room temperature is not Tb2O3, but TbO1.81. While sintering in vacuum, the reversible phase transition of Tb2O3 above 1600 °C makes Tb2O3 crack from inside to outside. These findings provide a deep understanding of controlling and avoiding the phase transformations of Tb2O3, which has great guiding significance to preparing Tb2O3 materials.

Transmission characteristics of linearly polarized light in reflection-type one-dimensional magnetophotonic crystals

The propagation properties of linearly polarized light in reflection-type one-dimensional magnetophotonic crystals are studied by using the 4 × 4 transmission matrix method. The structure models of reflection-type one-dimensional magnetophotonic crystals are designed, the magnetic field direction control characteristics of reflection spectrum and Kerr rotation angle are discussed, and the effect of applied magnetic field direction and strength on reflection spectrum and Kerr rotation angle are analyzed. The results show that the non-diagonal elements in the dielectric constant of magneto optical materials change when the angle φ between applied magnetic field and optical path changes, the reflectivity and Kerr rotation angle decrease when the angle φ increases; when the applied magnetic field strength changes, the reflectivity and Kerr rotation angle increase when the applied magnetic field strength increases; by adjusting the angle φ and strength of the applied magnetic field, the rotation angle of Kerr can be adjusted to 45°, and a more flat reflection spectrum can be obtained by designing the appropriate structure.

A planar waveguide in terbium gallium garnet crystal produced by carbon ion implantation

Terbium-gallium-garnet (TGG) crystals have attracted considerable attention as one of the most suitable magneto-optical materials to produce optical isolators. In this work, we report an improved TGG planar waveguide fabricated by the carbon-ion implantation. Considering that the previously reported carbon-implanted TGG waveguide with a dose of 5.0 × 1014 ions cm−2 cannot propagate light waves, we increase the fluence to 1.0 × 1015 ions cm−2 and collect the near-field light intensity profile by the end-face coupling measurement. By simulating the energy loss distribution and analyzing the reconstructed refractive index profile, we figure out the formation theory of the planar waveguides. Moreover, we observe the guided modes in the dark-mode spectrum and further verify the confinement of light in the TGG waveguide. The investigation on planar waveguides in TGG crystals will be beneficial to the progress of micro-isolators in the integrated optical systems.

Tunable filter and modulator with controlled bandwidth and wide dynamic range based on planar thin films structure.

Tunable narrowband spectral filters with high throughput and wide dynamic range are in high demand for many applications, however, a cost is usually associated with the filter narrowing either in the dynamic range, in the throughput or the manufacturability. Here it is shown for the first time that using the coupling between waveguide and lossy modes (LMs) in lossy ultrathin films through thin film coupling layer it is possible to obtain a reflection peak with controllable width (sub-Angstroms till tens of nm) and tunability over wide spectral range (>500nm in the visible and near infrared). The excitation of broadband LM is enabled using an ultrathin absorptive layer with high imaginary to real part ratio of the dielectric constant (i.e 6nm of Cr). The wider dynamic range and higher contrast are observed more with TE polarization than TM. The tuning is achieved by incidence angle scan of few degrees or by modulating the waveguide layer from the visible till the near infrared and in principle it can be designed to operate in any spectral range. Such a thin waveguide layer can allow tuning at ultrahigh speed using conventional electrooptic, magnetooptic, piezoelectric or thermooptic materials using relatively low external fields. The tuning sensitivity and range depend strongly on the waveguide layer thickness and the refractive index mismatch between the waveguide and the coupling layer. Under small index mismatch new peaks are seen via Rabi type splitting with gaps as high as 700nm or more, thus exhibiting ultrahigh tuning with negative sensitivity.

Near-infrared carbon-implanted waveguides in Tb3+-doped aluminum borosilicate glasses

Ion implantation has played a unique role in the fabrication of optical waveguide devices. Tb3+-doped aluminum borosilicate (TDAB) glass has been considered as an important magneto-optical material. In this work, near-infrared waveguides have been manufactured by the (5.5 + 6.0) MeV C3+ ion implantation with doses of (4.0 + 8.0) × 1013 ions·cm-2 in the TDAB glass. The modes propagated in the TDAB glass waveguide were recorded by a prism-coupling system. The finite-difference beam propagation method (FD-BPM) was carried out to simulate the guiding characteristics of the TDAB glass waveguide. The TDAB glass waveguide allows the light propagation with a single-mode at 1.539 μm and can serve as a potential candidate for future waveguide isolators.

Magnetism and magneto-optical effects in bulk and few-layer CrI3: a theoretical GGA + U study

The latest discovery of ferromagnetism in atomically thin films of semiconductors Cr2Ge2Te6 and CrI3 has unleashed numerous opportunities for fundamental physics of magnetism in two-dimensional (2D) limit and also for technological applications based on 2D magnetic materials. To exploit these 2D magnetic materials, however, the mechanisms that control their physical properties should be thoroughly understood. In this paper, we present a comprehensive theoretical study of the magnetic, electronic, optical and magneto-optical (MO) properties of multilayers (monolayer (ML), bilayer (BL) and trilayer) as well as bulk CrI3, based on the density functional theory with the generalized gradient approximation plus on-site Coulomb repulsion scheme. Interestingly, all the structures except the BL, are found to be single-spin ferromagnetic semiconductors. They all have a large out-of-plane magnetic anisotropy energy (MAE) of ∼0.5 meV/Cr, in contrast to the significantly thickness-dependent MAE in multilayers of Cr2Ge2Te6. These large MAEs suppress transverse spin fluctuations and thus stabilize long-range magnetic orders at finite temperatures down to the ML limit. They also exhibit strong MO effects with their Kerr and Faraday rotation angles being comparable to that of best-known bulk MO materials. The shape and position of the main features in the optical and MO spectra are found to be nearly thickness-independent although the magnitude of Kerr rotation angles increases monotonically with the film thickness. Magnetic transition temperatures estimated based on calculated exchange coupling parameters, calculated optical conductivity spectra, MO Kerr and Faraday rotation angles agree quite well with available experimental data. The calculated MAE as well as optical and MO properties are analyzed in terms of the calculated orbital-decomposed densities of states, band state symmetries and dipole selection rules. Our findings of large out-of-plane MAEs and strong MO effects in these single-spin ferromagnetic semiconducting CrI3 ultrathin films suggest that they will find valuable applications in semiconductor MO and spintronic nanodevices.

Characterizing magnetization reversal processes of GdFeCo film in the vicinity of the spin reorientation transition temperature

An amorphous GdFeCo film has a spin reorientation transition (SRT) with the temperature varying from 230 K to 300 K. Its magnetization reversal processes have been characterized by the hysteresis loops and first-order reversal curve (FORC) in the vicinity of the SRT temperature. Along with the SRT from out-of-plane to in-plane, the total irreversible components decrease with increasing temperature from 84% (250 K) to 13% (300 K) and the reversal process gradually changes from the pinning-type dominated reversal (250 K) to the nucleation type reversal (270 K), and then to the uniform rotation (300 K). The identification of the magnetization reversal process nearby SRT is important to understand the magnetic properties in the magneto-optical recording materials GdFeCo.

Simulation of magneto-optical properties of magnetic photonic crystal waveguides

We propose a kind of magnetic photonic crystal (MPC) slab waveguide made from magneto-optical material, namely bismuth iron garnet. The finite difference beam propagation method is employed for numerical calculations, which is the most widely used propagation technique for modeling integrated and photonic devices. The mode conversion and the influence of corresponding parameters are studied theoretically. As a numerical example, a TE-TM mode conversion waveguide-type optical isolator based on this designed MPC is described. An effective away to enhance the Faraday rotation at the 1.55-μm telecommunication wavelength is shown.

Effect of substrate defects on LIDT of (BiTm)3(GaFe)5O12 films grown by LPE

Recently, demands for magneto-optical (MO) materials with high laser-induced damage thresholds (LIDT) increased rapidly with the continuous development of high power lasers. In order to study the effect of substrate defects on LIDT of (BiTm)3(GaFe)5O12 films, in this study, (BiTm)3(GaFe)5O12 films without gadolinium gallium garnet (GGG) substrates and (BiTm)3(GaFe)5O12/GGG films were prepared by liquid-phase epitaxial (LPE) and mechanical lapping and polishing method. The optical and magneto-optical properties, crystal structure, LIDT and laser-induced damage morphologies of the prepared films were investigated. Experimental results pointed out that epitaxial (BiTm)3(GaFe)5O12 films have very high crystal and surface quality. A discontinuous defective layer with thickness of about 0–500 nm is located between GGG substrates and epitaxial films. Laser-induced damage tests and damage morphologies demonstrate that the discontinuous defective layers have an important effect on LIDT of epitaxial films, and LIDT of epitaxy films can be significantly improved by removing GGG substrates and discontinuous defective layers.

On-chip magnetic-free optical multi-port circulator based on a locally linear parity–time symmetric system

This paper introduces a new type of magnetic-free optical circulator without utilizing a Faraday rotator, polarization beam splitter, and magneto-optic material for the first time, to the best of our knowledge. The designed circulator operates linearly and relies on the parity-time symmetric (PTS) system. The property of the non-Hermitian system (the linear PTS system) plays the key role in the suggested optical circulator so that it can simultaneously support N-ports. The proposed device is integrated, broadband, and operates in the optical telecommunication frequency band. A great value of isolation rate of 47dB is achieved. The proposed circulator can pave the way for optical integrated circuits and optical networks.