Magneto-optical Materials Research Articles

Nonreciprocal transmission characteristics in double-cavity double-optomechanical system

<sec>Optical non-reciprocal devices such as the isolators are quite important components in optical systems. To realize the non-reciprocal transmission of the light, the Lorenz reciprocity theorem must be broken first and the main method is that Faraday magnetic rotation effect is used to change the polarization state of the signal through magneto-optical materials. However, this method is difficult to achieve on-chip integration. So using optomechanical system is presented to overcome the difficulty.</sec><sec>In order to improve the isolation characteristics of the device, a double-cavity double-optomechanical system, which is coupled to two optical modes by two mechanical oscillators with two different optomechanical coupling strengths, is proposed. Driven by the red detuning field in such a system, the non-reciprocal phenomenon can be realized by regulating the phase difference, and the direction of light transmission and isolation can be determined as well. This property is determined by the quantum interference effect between the optomechanical coupling strengths and the couplings of the optical cavity modes. The method is that the relative operators are represented by their average value plus their relative fluctuations, and then according to the input-output relationship the transmission amplitude and the isolation rate are obtained.</sec><sec>We mainly discuss the distribution of the isolation rate as a function of the optomechanical coupling strength. The results are that the combined action of two mechanical modes can make the system have higher fault tolerance rate. The other mechanical mode can make the system achieve a large isolation rate at two specific frequencies and the reverse transmission in the resonant frequency signals at the same time.</sec>

Weyl semimetal mediated epsilon-near-zero hybrid polaritons and the induced nonreciprocal radiation.

Polaritonic excitation and management in ultra-thin polar crystals has recently received significant attention and holds new promise for epsilon-near-zero (ENZ) modes. However, manipulation of the ENZ mode via anisotropic magneto-optic (MO) material remains elusive. Herein, we provide an effective strategy for constructing an ENZ polar thin film with dependence on Weyl semimetals (WSM). The thermal radiation of the proposed device is explored with electromagnetic (EM) simulations that utilize the anisotropic rigorous coupled-wave analysis (aRCWA) method. Strong coupling of the ENZ mode to WSM polaritons has been demonstrated, and the structural parameters hold tolerance on the order of hundreds of nanometers, which is highly favorable for low-cost fabrication and high-performance application. By changing both the azimuthal angle (ϕ) and angle of incidence (θ), the nonreciprocity (η) can be effectively influenced. The distribution of η is symmetrical with ϕ = 180°, η = 0 when ϕ = 90° and ϕ = 270°. The mechanism of this proposal is owing to the hybrid polaritons supported by the polar thin film and nonreciprocal radiation of WSM, which is validated by examining the amplitude distribution of the magnetic field. The nonreciprocal emitter described herein allows simultaneous control of spectral distribution and polarization of radiation, which will facilitate the active design and application of mid-infrared (MIR) thermal emitters.

Manipulating fluorescence by photo-switched spin-state conversions in an iron(ii)-based SCO-MOF.

Manipulating fluorescence by photo-switched spin-state conversions is an attractive prospect for applications in smart magneto-optical materials and devices. The challenge is how to modulate the energy transfer paths of the singlet excited state by light-induced spin-state conversions. In this work, a spin crossover (SCO) FeII-based fluorophore was embedded into a metal-organic framework (MOF) to tune the energy transfer paths. Compound 1 {Fe(TPA-diPy)[Ag(CN)2]2}·2EtOH (1) has an interpenetrated Hofmann-type structure, wherein the FeII ion is coordinated by a bidentate fluorophore ligand (TPA-diPy) and four cyanide nitrogen atoms and acts as the fluorescent-SCO unit. Magnetic susceptibility measurements revealed that 1 underwent an incomplete and gradual spin crossover with T1/2 = 161 K. Photomagnetic studies confirmed photo-induced spin state conversions between the low-spin (LS) and high-spin (HS) states, where the irradiation of 532 and 808 nm laser lights converted the LS and HS states to the HS and LS states, respectively. Variable-temperature fluorescence spectra study revealed an anomalous decrease in emission intensity upon the HS → LS transition, confirming the synergetic coupling between the fluorophore and SCO units. Alternating irradiation of 532 and 808 nm laser lights resulted in reversible fluorescence intensity changes, confirming spin state-controlled fluorescence in the SCO-MOF. Photo-monitored structural analyses and UV-vis spectroscopic studies demonstrated that the photo-induced spin state conversions changed energy transfer paths from the TPA fluorophore to the metal-centered charge transfer bands, ultimately leading to the switching of fluorescence intensities. This work represents a new prototype compound showing bidirectional photo-switched fluorescence by manipulating the spin states of iron(ii).

Reporting a novel visible near-infrared {Tb3}[Ga0.1Sc1.9](Al3)O12 single crystal for Faraday isolators

A {Tb3}[Ga0.1Sc1.9](Al3)O12 crystal is designed and investigated for the first time. This novel single crystal exhibits excellent magneto-optical properties and is very promising for visible near-infrared Faraday applications.

All-fiber optical nonreciprocity based on parity-time-symmetric Fabry-Perot resonators

Nonreciprocal light transmission in an all-fiber platform is critical in modern optical communication systems, which can avoid the packaging and integration process required in current devices based on magneto-optical or nonlinear materials. Here we propose and demonstrate an all-fiber device with remotely tunable isolation ratio and switchable isolation direction by constructing two mutually coupled Fabry-Perot (FP) resonators with identical geometry and balanced gain and loss. By controlling the pumping power, strong optical nonreciprocity is achieved due to gain saturation nonlinearity that is enhanced by the broken parity-time symmetry. Nonreciprocal light transmission with an isolation ratio of 8.58 dB at 1550 nm and an insertion loss of 2.5 dB is demonstrated. The isolation bandwidth is 125 MHz, which is determined by the bandwidths of the two coupled FP resonators. The proposed approach provides an all-fiber solution for a remotely tunable and optically controlled isolator, which may find applications in software-defined optical networks.

Effects of VZn and Hi interstitials with different valence states and alkaline earth metal doping on carrier lifetime, activity, and oxidation–reduction reaction of ZnO

We applied the generalized gradient approximation plane wave ultrasoft pseudopotential method based on the framework of density functional theory in this work to study the influences of Be/Mg/Ca single doping and the coexistence of Zn vacancies with different valence states and H interstitial on the magnetic and photocatalytic properties of ZnO. Results showed that the formation energy of all doping systems decreases with the increase in the valence state of Zn vacancies, and the doping is easy. The binding energies of all doped systems under O-rich conditions are lower than those under Zn-rich conditions, and the doped systems are more stable under O-rich conditions. The Zn34MO36 (VZn1−, M = Be/Mg/Ca), Zn34MHiO36 (VZn1−, M = Be/Mg/Ca), and Zn34MHiO36 (VZn0, M = Mg/Ca) systems show magnetism. The source of the magnetism of the doped system is generated by the itinerant electrons of unpaired spin polarization of the O atom near VZn. The study found that the Zn34MgHiO36(VZn2−) system has the longest carrier lifetime, relatively large electric dipole moment, significant red shift of the absorption spectrum, and strong redox ability. The Zn34MgHiO36 (VZn2−) system can be used as a photocatalyst for H2 production. The result of this work has theoretical reference value for the design and preparation of new magneto-optical functional materials.

Polarimetric analysis of thermal emission from both reciprocal and nonreciprocal materials using fluctuation electrodynamics

Coherent thermal emission for a given polarization has been observed in many metamaterials with micro/nanostructures. Emissivity is typically obtained based on the equivalence between absorptivity and emissivity according to Kirchhoff's law; however, such a relationship may be invalid for nonreciprocal media. This study aims at developing a more general approach, without the constraint of optical reciprocity, that is applicable to magneto-optical materials and magnetic Weyl semimetals. A polarimetric analysis is theoretically carried out based on fluctuation electrodynamics to provide a complete description of the emissivity for all polarization states. The Stokes parameters are calculated from the coherency matrix for a multilayered system, which may include anisotropic media and even nonreciprocal materials. It is demonstrated that thermal emission may be circularly or linearly polarized in selected directions and frequencies. The findings of this work are consistent with the modified Kirchhoff's law provided by several groups in recent years, and therefore, justify the appropriateness of both the direct and indirect methods. This study will help the design of desired thermal emitters for energy harvesting and thermal control.

Temperature-Monitored Fibre Optic Current Sensor Using Channelled-Spectrum Analysis

The fibre optic current sensor demonstrated here uses the intrinsic temperature and wavelength dependence of the Verdet constant of a terbium gallium garnet (TGG) magneto-optic material and the two micro-optic linear polarizers attached, to simultaneously extract the values of temperature and the optical Faraday rotation (induced by the presence of the magnetic field due an electric current on a conductor) without any extra optical component attached to the optical sensor head. The simultaneous measurement is achieved by illuminating the sensor head with a broadband optical source and by careful signal processing of the originated channelled-spectrum, compensate the sensor’s temperature dependence.

Integrated passive nonlinear optical isolators

Fibre and bulk optical isolators are widely used to stabilize laser cavities by preventing unwanted feedback. However, their integrated counterparts have been slow to be adopted. Although several strategies for on-chip optical isolation have been realized, these rely on either integration of magneto-optic materials or high-frequency modulation with acousto-optic or electro-optic modulators. Here we demonstrate an integrated approach for passively isolating a continuous-wave laser using the intrinsically non-reciprocal Kerr nonlinearity in ring resonators. Using silicon nitride as a model platform, we achieve single ring isolation of 17–23 dB with 1.8–5.5-dB insertion loss, and a cascaded ring isolation of 35 dB with 5-dB insertion loss. Employing these devices, we demonstrate hybrid integration and isolation with a semiconductor laser chip. The intrinsic Kerr nonlinearity in ring resonators is exploited to demonstrate passive isolation of a continuous-wave laser. Up to 35-dB isolation with 5-dB insertion loss was achieved on-chip.

Impact of Sm ions on microstructure and magneto-optical effects of Y2O3 ceramics obtained by arc plasma melting

Faraday materials based on transparent ceramics are regarded for high power magneto-optical isolators. Pure yttria ceramics with high transmittance were prepared using the raw oxide powders. To enhance the magneto–optical performance of the ceramics, samarium oxide were introduced to yttrium oxide matrix as a paramagnetic ions. The Sm-Y2O3 Faraday ceramic was melted above 2700 °C for 10 min. It is noted that as-prepared ceramic presents the highest in-line transmittance above 61.0% at the high spectrum range of wavelengths from 350 to 1110 nm. Physicochemical properties of the sample were characterized by means of numerous techniques. No secondary phase was detected from the sample using X-ray diffraction. The Verdet constant of Sm-Y2O3 ceramic at 532 nm is as high as 20 rad/m*T, which is higher than that of pure Y2O3 ceramic. The good optical quality and magneto-optical properties make Sm-Y2O3 ceramic an attractive magneto-optical material.

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.

Effect of WO3 on physical and magneto-optical properties of TeO2–La2O3–WO3 glasses for magneto-optics application

The ternary TeO2–La2O3–WO3 glasses with a formula of (100 − y − x)TeO2–yLa2O3–xWO3 (y = 5 and 10 mol%; x = 10 to 25 mol%) were made by the melting and casting method for high Verdet constant diamagnetic magneto-optical (MO) materials in the application of MO devices operating at 1550 nm. The compositional effects of WO3 and La2O3 contents on key glass properties of thermal stability, Knoop hardness, optical transmission, and Verdet constant, were systematically studied for both series of glasses. High transparency level (T: >70 %) in the region from 0.4 to 5.2 μm and a high refractive index (n: 2.05 at 1550 nm) were obtained in the studied glasses. The tellurite glasses with 5 mol% of La2O3 exhibited higher thermal stability and refractive indices than those with 10 mol% La2O3. The 85TeO2–5La2O3–10WO3 glass exhibited a large Verdet constant value of 3.95 rad/(T·m) at 1550 nm among the studied glasses. These results show that the presented TeO2–La2O3–WO3 glasses have considerable potential as diamagnetic MO materials for MO device applications operating at 1550 nm.

Fabrication of 4 at.% La3+ ion doped (TbxLu0.96−x)2O3 transparent ceramics by using NC-PLSH technology and characterisation of their magneto-optical properties

Nanocrystalline powders and pressureless sintering (NC-PLSH) were used in a H2 atmosphere to fabricate 4 at.% La3+ doped (TbxLu0.96−x)2O3 magneto-optical transparent ceramics. La3+ was utilised as a sintering aid, and Lu3+ was used as a phase stabiliser. A component of the (TbxLu0.96−xLa0.04)2O3 magneto-optical material was designed and investigated based on powder sinterability and green compact sintering. Transparent (TbxLu0.96−xLa0.04)2O3 ceramics were obtained by using a one-step sintering process in H2 atmosphere, without the need for subsequent hot isostatic pressing. The in-line transmittance of the as-polished 4 at.% La3+:(Tb0.8Lu0.16)2O3 ceramic was 79% at 1400 nm. The measured Verdet constant at wavelengths of 633 nm and 1064 nm was 354 rad/(T·m) and 100.2 rad/(T·m), respectively. The required length of the (Tb0.8Lu0.16La0.04)2O3 ceramics was 63% less than that of Tb3Ga5O12 ceramics.

Magnetic and fractional parametric control of Goos-Hänchen shifts in the anisotropic yttrium-iron-garnet film surrounded by isotropic fractal dielectric half-spaces

This article focuses on the controllability of reflected and transmitted Goos-Hänchen shifts for a planar anisotropic yttrium-iron-garnet film. The yttrium-iron-garnet film, being magneto-optical material, is magnetized along the z-direction. For richer control of the Goos-Hänchen shifts and the characteristics of reflectivity/transmitivity, the externally magnetized film is surrounded by isotropic fractal dielectric half-spaces. The tuning of Goos-Hänchen shifts from positive to negative and vice versa, has been obtained by varying the values of incidence frequency in the microwave regime, incidence angle, fractional dimensional parameter, external magnetic field, and slab thickness.

Bi-Substituted Ferrite Garnet Type Magneto-Optic Materials Studied at ESRI Nano-Fabrication Laboratories, ECU, Australia

Since 2007, at the Electron Science Research Institute (ESRI) nano-fabrication laboratories, Edith Cowan University, Australia, we have devoted research efforts to the synthesis and characterization of bismuth-containing ferrite-garnet-type thin-film magneto-optic (MO) materials of different compositions. We report on the growth and characteristics of radio frequency (RF) magnetron sputtered bismuth-substituted iron-garnet thin films. We study the process parameters associated with the RF magnetron sputter deposition technique and investigate the results of optimizing process parameters. To achieve the best MO properties, we employ a few unique techniques, such as co-sputtered nanocomposite films and all-garnet multilayer structures, as well as the application of oxygen plasma treatment to amorphous garnet layers immediately following the deposition process. We demonstrated a remarkable enhancement in the MO properties of Bi-containing ferrite-type garnet thin-film materials, including record-high MO figures of merit and improved conventional and unconventional hysteresis loops of Faraday rotation. Previously unpublished research results on the forward-looking applications of magnetic garnet coatings applied to microparticles of advanced luminescent materials are reported. In the context of developing the next-generation ultra-fast optoelectronic devices, such as light intensity switches and modulators, high-speed flat panel displays, and high-sensitivity sensors, it is important to consider the desirable optical, magnetic, and magneto-optic properties that are found in highly bismuth-substituted iron garnet thin-film materials of various composition types.

Nonreciprocal propagation of surface electromagnetic waves in structures comprising magneto-optical materials

This paper investigates theoretically the effect of nonreciprocity on surface electromagnetic waves (SEWs) propagating in magneto-optical structures in mutually opposite directions, i.e., on waves with tangential wave numbers $k$ of opposite sign. Namely, of our interest is a correlation between the numbers of forward ($k>0$) and backward ($k<0$) propagating SEWs, which manifests itself as a limit on the total number of SEWs. The peculiarity of this limit is that the maximum total number of forward- and backward-propagating SEWs is less than the sum of the maximum numbers of SEWs for each of the opposite directions taken individually. We derive a relation between the electromagnetic surface impedances relevant to forward- and backward-propagating waves. Afterward, by using general properties of these impedances valid regardless of the crystallographic symmetry, we analyze the roots of dispersion equations without solving them explicitly. Various types of magneto-optical structures are considered. In particular, it is shown that, in the bicrystal formed of two half-infinite magneto-optical media, the maximum total number of forward- and backward-propagating SEWs is two at a given value of $|k|$, whereas in each of the mutually opposite directions two SEWs can exist. For example, if two forward-propagating SEWs are known to exist, then no backward-propagating SEW exists. In the case where a metal film is inserted between two different half-infinite magneto-optical media, the maximum number of SEWs in one direction is four but the maximum of the total number of forward- and backward-propagating SEWs is six rather than eight.

Faraday effect in CoPt-CaF2 nanogranular films with hard magnetic property for magnetic-field-free faraday devices

Magneto-optical Faraday devices, such as optical isolators, require a structure to apply a magnetic field to the incorporated magneto-optical material. The application of a magnetic field hinders the miniaturization and integration of these devices. Nanogranular films show Faraday rotation angles up to 40 times larger than Bi-YIG in the optical communication band (1,550 nm). Nanogranular films are submicron-to several-micrometer-thick and contribute to the miniaturization and integration of optical devices. Here, we introduce (Co-Pt)–(CaF2) nanogranular films exhibiting a magnetic-field-free magneto-optical Faraday effect, which are Co3Pt alloy that are hard magnets with residual magnetization. These nanogranular films exhibit the Faraday effect without requiring a magnetic field owing to their residual magnetization.

Novel compact magnetless isolator based on a magneto-optical garnet material

A compact magnetless isolator for optical communication systems based on a ring resonator with an outer layer made of silicon and an inner layer made of a magneto-optical material that does not require an external magnet to keep its magnetization saturated is suggested. Three-dimensional computational simulations of the device performed with the full-wave electromagnetic solver COMSOL Multiphysics show that the insertion loss and isolation levels are about −1.9 dB and −23 dB, respectively, thus confirming the feasibility of the isolator. An analytical model of the device based on the temporal coupled-mode theory method has been formulated and there is a good agreement between the analytical and simulation results. Since it does not require a magnetization scheme based on permanent magnets or electromagnets as conventional isolators do, the presented isolator is much more compact, with a footprint at least one order of magnitude smaller in comparison with other isolator designs known from the literature, and this feature makes the suggested device an ideal candidate for optical circuits with very high integration density.

Broadband Integrated Optical Waveguide Current Sensor Based on a Magneto-Optical Thin Film

Broadband transient current measurement is a basic research method in many engineering fields. Optical integrated current sensor has the advantages of small size, anti electromagnetic interference and high precision, which has become a major trend of current monitoring. However, the existing optical current sensing methods have great challenges in broadband, wide amplitude and volume miniaturization. In this paper, a new high-performance magneto-optical energy coupling sensing method based on silicon photons and magneto-optical materials is studied, and an integrated optical waveguide current sensor based on a magneto-optical thin film and a double-ring assisted Mach–Zehnder interferometer is proposed. The energy coupling between the micro-ring resonator and the magneto-optical thin film is achieved through the magneto-optical nonreciprocity effect, and the current information is obtained by modulating the output light intensity with the Mach–Zehnder interferometer. The experimental results show that the sensor has a good linearity and a large dynamic measurement range. The measurement frequency range is from10 Hz to 13.56 MHz, and the sensitivity is 2.183 mV/A. This work seeks to promote the development of optical broadband current sensors on chips.

Magneto-optical Kerr effect and magnetoelasticity in a weakly ferromagnetic RuF4 monolayer

Considerable research interest has been attracted to noncollinear magnetic structures for their intriguing physics and promising applications. In this work, based on relativistic density functional theory, we reveal the interesting magnetic order and relevant properties in monolayer ${\mathrm{RuF}}_{4}$, which can be exfoliated from its bulk phase. Although the spins on Ru ions are almost antiferromagnetically aligned between nearest neighbors, weak ferromagnetism is generated because of the antisymmetric Dzyaloshinskii-Moriya interaction as well as the single-ion anisotropy. A prominent magneto-optical Kerr effect can be observed for this antiferromagnet, similar to those of regular strong ferromagnets. In addition, a uniaxial strain can induce a ferroelastic switching together with the in-plane rotation of spin direction, giving rise to a strong intrinsic magnetoelasticity. Our work not only suggests an alternative direction for two-dimensional magnetic materials, but also provides hints to future devices based on antiferromagnetic magnetoelastic or magneto-optical materials.