Magneto-optical Materials Research Articles

Magnetically controllable metasurface and its application.

The dynamic control of the metasurface opens up a vital technological approach for the development of multifunctional integrated optical devices. The magnetic field manipulation has the advantages of sub-nanosecond ultra-fast response, non-contact, and continuous adjustment. Thus, the magnetically controllable metasurface has attracted significant attention in recent years. This study introduces the basic principles of the Faraday and Kerr effect of magneto-optical (MO) materials. It classifies the typical MO materials according to their properties. It also summarizes the physical mechanism of different MO metasurfaces that combine the MO effect with plasmonic or dielectric resonance. Besides, their applications in the nonreciprocal device and MO sensing are demonstrated. The future perspectives and challenges of the research on MO metasurfaces are discussed.

Fabrication and performance evaluation of novel transparent ceramics RE:Tb3Ga5O12 (RE = Pr, Tm, Dy) toward magneto-optical application

Transparent ceramics are at the heart of modern magneto-optical materials providing promising opportunities for Faraday isolators. 1.0 at% RE:Tb3Ga5O12 (rare earth (RE) = Pr, Tm, Dy) transparent ceramics were successfully prepared by air sintering and sequential HIP technique using the coprecipitated powders as the raw material. All the powders have shown to be a pure cubic terbium gallium garnet (TGG) phase and exhibit good dispersion. Additionally, a change could not be observed in particle shape with the different doped ions. After the two-step sintering, all the obtained ceramics have good optical quality, and the in-line transmittances at 1070 nm are higher than 80%. Moreover, no secondary phase can be detected from the microstructures. However, the pores which remain entrapped in the ceramics can be noted. The Verdet constant of ceramic samples is optimized by RE doping, and the Verdet constant at 632.8 nm is about −143 rad·T−1·m−1, which is about 5% higher than that of TGG ceramics. Finally, the thermo-optical properties of 1.0 at% RE:TGG transparent ceramics are compared. The annealed TGG ceramic showed the best thermo-optical properties, and the thermally induced depolarization of 1.0 at% Ce:TGG and 1.0 at% Tm:TGG was inferior to that of annealed TGG ceramic.

Temperature dependent refractive index and dielectric modulation of paramagnetic magneto-optic PrF3 crystal with terahertz time-domain spectroscopy

PrF3 crystals are excellent paramagnetic magneto-optical materials. In this paper we report on the PrF3 complex refractive index and the complex permittivity in terahertz (THz) frequency range. Measurements have been performed using transmission time domain terahertz spectroscopy (THz-TDS). The influence of temperature on the complex refractive index and the complex permittivity was also investigated. To the best of authors knowledge this dependency have not been reported thus far. We show that complex refractive index increases with temperature (50 K–170 K), so do the values of complex permittivity and loss tangent. A relatively small (~0.02) increase of loss tangent value is obtained for all frequencies (0.4THz-1.4 THz) at 170 K contrary to 50 K. It's worth mentioning the permittivity of PrF3 exhibits better thermal stability than CeF3 between 0.75 THz and 0.9 THz. Our research shows PrF3 is an excellent candidate for optoelectronic tunable application in THz frequency, especially between 0.75 THz and 0.9 THz when the temperature varies dramatically.

First-principles study on magneto-optical effects in the ferromagnetic semiconductorsY3Fe5O12andBi3Fe5O12

The magneto-optical (MO) effects not only are a powerful probe of magnetism and electronic structure of magnetic solids but also have valuable applications in high-density data-storage technology. Yttrium iron garnet (Y$_3$Fe$_5$O$_{12}$) (YIG) and bismuth iron garnet (Bi$_3$Fe$_5$O$_{12}$) (BIG) are two widely used magnetic semiconductors with strong magneto-optical effects and have also attracted the attention for fundamental physics studies. In particular, YIG has been routinely used as a spin current injector. In this paper, we present a thorough theoretical investigation on magnetism, electronic, optical and MO properties of YIG and BIG, based on the density functional theory with the generalized gradient approximation plus onsite Coulomb repulsion. We find that both semiconductors exhibit large MO effects with their Kerr and Faraday rotation angles being comparable to that of best-known MO materials such as MnBi. Especially, the MO Kerr rotation angle for bulk BIG reaches -1.2$ ^{\circ}$ at photon energy $\sim2.4$ eV, and the MO Faraday rotation angle for BIG film reaches -74.6 $ ^{\circ}/\mu m$ at photon energy $\sim2.7$ eV. Furthermore, we also find that both valence and conduction bands across the MO band gap in BIG are purely spin-down states, i.e., BIG is a single spin semiconductor. These interesting findings suggest that the iron garnets will find valuable applications in semiconductor MO and spintronic nanodevices. The calculated optical conductivity spectra, MO Kerr and Faraday rotation angles agree well with the available experimental data. The main features in the optical and MO spectra of both systems are analyzed in terms of the calculated band structures especially by determining the band state symmetries and the main optical transitions at the $\Gamma$ point in the Brillouin zone.

Optical isolation using microring modulators

Optical isolators, while commonplace in bulk and fiber optical systems, remain a key missing component in integrated photonics. Isolation using magneto-optic materials has been difficult to integrate into complementary metal–oxide–semiconductor (CMOS) fabrication platforms, motivating the use of other paths to effective non-reciprocity such as temporal modulation. We demonstrate a non-reciprocal element comprising a pair of microring modulators and a microring phase shifter in an active silicon photonic process, which, in combination with standard bandpass filters, yields an isolator on-chip. Isolation up to 13 dB is measured with a 3 dB bandwidth of 2 GHz and insertion loss of 18 dB. We also show transmission of a 4 Gbps optical data signal through the isolator while retaining a wide-open eye diagram. This compact design, in combination with increased modulation efficiency, could enable modulator-based isolators to become a standard ‘black-box’ component in integrated photonics CMOS foundry platform component libraries.

Collective lattice resonances: Plasmonics and beyond

Engineering nanostructures with exceptionally high-Q resonances mediated by the Fano-type hybridization between discrete states associated with the periodicity of the structure and broadband resonances excited on constituent scatterers are the emerging field in optics and photonics. These collective lattice resonances (CLRs) attracted a lot of attention in recent years due to a number of their exciting applications in sensing, optical filtering, structural color printing, fluorescence enhancement, nanoscale lasing, and nonlinear optics, which resulted in a rapidly growing number of fundamental and experimental studies. CLRs have been discovered for arrays of plasmonic metal nanoparticles with strong electric dipole resonances nearly four decades ago. Thereafter, the scope of CLRs has gradually extended to all-dielectric and magneto-optical nanoparticles, 2D materials and other types of constituents, which has broadened the range of CLRs applicability and enriched their properties. We provide a comprehensive review of the recent progress in this field with a special emphasis on advances far beyond plasmonics.

Homogenization of 1-D Magneto-Photonic Crystals by Using 4 × 4 Characteristic Matrices

Based on the Herpin theorem used to derive the equivalent parameters of a symmetric periodic multilayer of isotropic materials, we extend the method to consider 1-D photonic crystals, including magneto-optic anisotropic materials combined with thin films composed of dielectric or metallic materials. We demonstrate that it is possible to obtain the equivalent non-local components of the dielectric tensor associated with the magneto-photonic crystal that contains information on the band structure, and the EM fields interacting with these systems, and compare with those obtained by using the Rytov formalism valid only for the low-frequency regime.

Nanoplasmonic magneto-optical isolator [Invited

We introduce a nanoplasmonic isolator that consists of a cylindrical resonator placed close to a metal-dielectric-metal (MDM) waveguide. The material filling the waveguide and resonator is a magneto-optical (MO) material, and the structure is under an externally applied static magnetic field. We theoretically investigate the properties of the structure and show that the cavity mode without MO activity splits into two modes when the MO activity is present. In addition, we find that the presence of the MDM waveguide leads to a second resonance due to the geometrical asymmetry caused by the existence of the waveguide. We also show that, when MO activity is present, the cavity becomes a traveling wave resonator. Thus, the transmission of the structure depends on the direction of the incident light, and the proposed structure operates as an optical isolator.

Influence of structure parameters of optical current sensor with iron core on temperature distribution

The accuracy of optical current sensors in power systems will be affected by the interference of the external environment. Among them, temperature has a significant influence on the accuracy of optical current sensors, which is one of the main factors that restrict the practicality of optical current sensors. In this paper, the temperature field distribution of the optical current sensor with iron core is calculated by simulation. The influence of its structural parameters on the temperature field distribution is studied. It is compared with the temperature field distribution of the optical current sensor of the commonly used magnetic ring structure. It is found that under the same current, the temperature of the magneto-optical material with the core optical current sensor is much lower than the optical current mutual inductance of the common magnetic ring structure. The geometric structure parameters of the optical current sensor with iron core have a great influence on its temperature field distribution. The temperature field distribution can be improved by optimizing the structure parameters, thereby improving the measurement accuracy. This article can provide a basis for the design optimization of optical current sensors.

Fabrication and Magneto-Optical Property of (Dy0.7Y0.25La0.05)2O3 Transparent Ceramics by PLSH Technology

(Dy0.7Y0.25La0.05)2O3 magneto-optical transparent ceramics were successfully fabricated by pressureless sintering in reductive H2 atmosphere (PLSH). The raw powder of (Dy0.7Y0.25La0.05)2O3 was synthesized by a modified self-propagating high-temperature synthesis (SHS) and sintered to transparent ceramics at 1400–1600 °C in a flowing H2 atmosphere, showing good sinterability of the as-synthesized raw powder. The magneto-optical Verdet constant of (Dy0.7Y0.25La0.05)2O3 transparent ceramics was measured to be −191.57 rad/(T·m) at a wavelength of 632.8 nm. In this magneto-optical material of (Dy0.7Y0.25La0.05)2O3, relative cheaper Dy and Y were used to replace Tb, and the low cost and good magneto-optical property showed the advantage of application on Faraday isolators (FIs) and Faraday rotators (FRs).

Unidirectional transmission in two-dimensional photonic crystal composed of parity-time symmetric and magneto-optical materials

In this study, a one-way electromagnetic waveguide consisting of magneto-optical (MO) and parity-time (PT)-symmetric materials was constructed. Due to the composite boundary conditions, this waveguide simultaneously has two unidirectional dispersive curves and two different unidirectional mechanisms. One unidirectional mechanism is breaking the time-reversal symmetry of MO materials, and the other unidirectional mechanism is asymmetric complex eigenvalues in the local PT symmetrical configuration. The designed structure provides an effective approach to control the directions of electromagnetic waves.

Integrated optical isolators using electrically driven acoustic waves.

We propose and investigate the performance of integrated photonic isolators based on non-reciprocal mode conversion facilitated by unidirectional, traveling acoustic waves. A triply-guided waveguide system on-chip, comprising two optical modes and an electrically-driven acoustic mode, facilitates the non-reciprocal mode conversion and is combined with spatial mode filters to create the isolator. The co-guided and co-traveling arrangement enables isolation with no additional optical loss, without magnetic-optic materials, and with low power consumption. The approach is theoretically evaluated with simulations predicting over 20 dB of isolation and 2.6 dB of insertion loss with a 370 GHz optical bandwidth and 1 cm device length. The isolator uses only 1 mW of electrical drive power, an improvement of 1-3 orders of magnitude over the state of the art. The electronic drive and lack of magneto-optic materials suggest the potential for straightforward integration with drive circuits, including in monolithic CMOS electronic-photonic platforms, enabling a fully contained 'black box' optical isolator with two optical ports and DC electrical power.

Nonvolatile magneto-optical switches integrated with a magnet stripe array.

Nonvolatile optical switches are promising components for low-power photonic integrated circuits with multiple functionalities. In this study, we experimentally demonstrate magneto-optical switches integrated with a magnet array. Optical switches in both microring and Mach-Zehnder configurations are fabricated on a high-quality single-crystalline magneto-optical material Ce:YIG. The switched state is alternated by a current-induced magnetic field from an integrated electromagnet and remained without any external power supply owing to the nonvolatile magnetization of thin-film magnets. Subsequently, the arbitrary level control of optical transmission is demonstrated by changing the magnetization state of integrated thin-film magnets with a current applied in the microring gate switch, and a maximum switching ratio over 25 dB is achieved in the Mach-Zehnder switch. The latching operation is presented with a 1-µs pulsed voltage.

Surface lattice resonance-based magneto-plasmonic switch in NiFe patterned nano-structure

In this work, a 2D magneto-plasmonic grating structure combining materials with ferromagnetic and plasmonic properties is demonstrated. NiFe composite ferromagnetic material, as an active medium with tunable physical properties, and Au metal, as a plasmonic excitation layer, were the materials of choice. Here, we have experimentally investigated the active control of the plasmonic characteristics in Au/NiFe bilayer by the action of an external magnetic field, as well as the switching effect of the system. The active plasmonic control, can be achieved by the magnetization switching of the ferromagnetic material, opening a new path in the development of active plasmonic devices. To our best knowledge, this is the first demonstration of such a magneto-optical plasmonic switch based on the coupling of plasmons with magneto-optical active materials, in which the response time was estimated to be in the range of microseconds.

Femtosecond Laser-Induced Vanadium Oxide Metamaterial Nanostructures and the Study of Optical Response by Experiments and Numerical Simulations.

Surface patterning is a popular approach to produce photonic metasurfaces that are tunable when electro-optic, thermo-optic, or magneto-optic materials are used. Vanadium oxides (VyOx) are well-known phase change materials with many applications, especially when used as tunable metamaterial photonic structures. Particularly, VO2 is a well-known thermochromic material for its near-room-temperature phase transition from the insulating to the metallic state. One-dimensional (1D) VO2 nanograting structures are studied by numerical simulation, and the simulation results reveal that the VO2 nanograting structures could enhance the luminous transmittance (Tlum) compared with a pristine flat VO2 surface. It is worth mentioning that Tlum is also polarization-dependent, and both larger grating height and smaller grating periodicity give enhanced Tlum, particularly at TE polarization in both insulating (20 °C) and metallic (90 °C) states of VO2. Femtosecond laser-patterned VO2 films exhibiting nanograting structures with an average periodicity of ≈500-700 nm have been fabricated for the first time to enhance thermochromic properties. Using X-ray photoelectron spectroscopy, it is shown that at the optimum laser processing conditions, VO2 dominates the film composition, while under extra processing, the existence of other vanadium oxide phases such as V2O3 and V2O5 increases. Such structures show enhanced transmittance in the near-infrared (NIR) region, with an improvement in NIR and solar modulation abilities (ΔTNIR = 10.8%, ΔTsol = 10.9%) compared with a flat VO2 thin film (ΔTNIR = 8%, ΔTsol = 10.2%). The slight reduction in transmittance in the visible region is potentially due to the scattering caused by the imperfect nanograting structures. This new patterning approach helps understand the polarization-dependent optical response of VO2 thin films and opens a new gateway for smart devices.

Boosting optical nonreciprocity: surface reconstruction in iron garnets

Bi-substituted iron garnet films are extensively used in the fabrication of nonreciprocal devices in optical telecommunications. The miniaturization of these devices for on-chip integration requires the development of more efficient magneto-optic materials than presently available. Recent evidence has emerged of large near-surface enhancements in the magneto-optic response in these materials. However, their operative mechanisms at the atomic and electronic levels are not as yet understood. We report significant differences in the ionic structure between surface and bulk in bismuth-substituted iron garnet materials. It is found that the unit cell is elongated normal to the surface, thus enlarging the separation between F e 3 + ions. These ions play a central role in the magneto-optic response of this material. A marked displacement of Fe ions creates gaps at the surface that are populated in the bulk. Concomitantly, surface- and bulk-sensitive measurements of spin-polarized 3d F e 3 + states show significant differences in the magnitude of L 2 edge x-ray magnetic circular dichroism, as well as differences in L 3 edge dichroism, which, in the presence of spin-orbit coupling in 3d states, can be assigned to high-energy states. An increase in magnetic circular dichroism correlates with larger Faraday rotation. These findings provide a deeper understanding of the role of the surface in the electronic transitions to excited F e 3 + 3d states, responsible for these nonreciprocal phenomena. Together with the surface reconstruction underlying these effects reported here, they provide a useful tool for the further development of improved materials technologies to advance the integration of nonreciprocal devices in optical circuits.

Magnetic switching of Kerker scattering in spherical microresonators

Abstract Magneto-optical materials have become a key tool in functional nanophotonics, mainly due to their ability to offer active tuning between two different operational states in subwavelength structures. In the long-wavelength limit, such states may be considered as the directional forward- and back-scattering operations, due to the interplay between magnetic and electric dipolar modes, which act as equivalent Huygens sources. In this work, on the basis of full-wave electrodynamic calculations based on a rigorous volume integral equation (VIE) method, we demonstrate the feasibility of obtaining magnetically-tunable directionality inversion in spherical microresonators (THz antennas) coated by magneto-optical materials. In particular, our analysis reveals that when a high-index dielectric is coated with a magneto-optical material, we can switch the back-scattering of the whole particle to forward-scattering simply by turning off/on an external magnetic field bias. The validity of our calculations is confirmed by reproducing the above two-state operation, predicted by the VIE, with full-wave finite-element commercial software. Our results are of interest for the design of state-of-the-art active metasurfaces and metalenses, as well as for functional nanophotonic structures, and scattering and nanoantennas engineering.

Magneto-optical spin Hall effect of light and its application in refractive index detection

The spin Hall effect of light (SHEL) is a photonic version of the spin Hall effect in electronic systems, and it has been a focused research area in the recent ten years. However, due to the lack of effective methods dynamic modulation of spin-dependent splitting, the practical application of SHEL has remained a challenge, still urgently waiting to be resolved. In recent years, SHEL has been applied to the precise measurement and sensing of various physical quantities. Now, how to make SHEL measurement flexible is the research direction of researchers The magneto-optical spin Hall effect of light (MOSHEL) is a useful attempt to implement a dynamic transformation of SHEL. The spin–orbit coupling or non-reciprocal phase shift caused by magnetic fields in different directions applied to magneto-optical materials can be flexibly and timely controlled. This paper reports our studies on the magneto-optical effect in multilayer structure. For the first time, the refractive index of very low concentration sample solution is sensed through the method of weak measurement under the condition of transverse magneto-optical Kerr effect. The ultra-high sensitivity of 2.9×104μm/RIU was measured under the optimum parameters. This is of great significance in advancing the practical application of MOSHEL.

Signal evolution of an optical buffer based on the nonreciprocal silicon-on-insulator waveguide

Optical buffer can not only solve the competition problem of the same port, but also provide an adjustable buffer time for nodes to process the frame header. In this study, an optical buffer using the nonreciprocal waveguide composed of magneto-optical material was introduced. The dispersion equations and nonreciprocal propagation characteristics of surface magneto-plasmons in gyroelectric and gyromagnetic semiconductor waveguides under the external magnetic field were studied. By using the C-band gyroelectric semiconductor, the surface magneto-plasmons in the nonreciprocal frequency region could be localized near the interface between dielectric and metal in the waveguide system. The signal evolution, effective buffer time and readout operation in the buffer were also investigated.

Non-reciprocal polarization rotation using dynamic refractive index modulation

One of the most prominent classes of non-reciprocal devices relies upon the effect of non-reciprocal polarization conversion, such as those observed in Faraday isolators. This effect is usually achieved with the use of magneto-optical materials. Here, we introduce a waveguide type optical isolator based on non-reciprocal polarization conversion, without the use of magneto-optical materials. Our isolator is based on spatial-temporal dynamic refractive index modulation, which is more readily amenable for on-chip integration. We numerically demonstrate our design with both first-principle multi-frequency electromagnetic simulations and the vectorial coupled mode theory formalism.