Magneto-optical Materials Research Articles

Liquid Crystalline Magneto-Optically Active Peralkylated Azacoronene.

Organic Faraday rotators have gained significant attention in recent years as a promising alternative to traditional inorganic magneto-optical (MO) materials as a result of their lower cost, superior mechanical properties, and potential for large-scale deployment. This interest is peaked by the fact that a number of high symmetry, rigid, strongly optically absorbing organic chromophores display Verdet constants an order of magnitude higher than commercial inorganic Faraday rotators. Critical to the development of new generations of organic materials is the ability to organize them in optimal structures for optical coupling/measurements. We report herein the synthesis of a dodecyl-substituted hexapyrrolohexaazacoronene (C12-HPHAC) displaying discotic liquid crystalline (LC) properties and large Faraday rotation. Thin films with a redox mixed C12-HPHAC/C12-HPHAC+2 composition display a discotic columnar LC phase, are stable to air and moisture in the solid and solution states, and achieve a maximum Verdet constant of 3.36 × 105 deg T-1 m-1 at 700 nm. This result is consistent with Serber's model of magnetic circular birefringence and displays one of the largest reported Verdet constants for organic materials in the UV-Vis range. The LC phase aligns the molecules and leads to gains in Verdet constants of up to 105% through the favorable orientation of the molecules' magnetic and electric transition dipole moments with respect to the applied magnetic field.

Growth and characterization of the KDy3F10 and Na0.38Dy0.62F2.24 fluoride crystals for the Faraday devices

The latest research on magneto-optical materials for Faraday devices has been repeatedly highlighting the benefits of using the crystalline media with the negative value of the optical anisotropy parameter (OAP). The reason is that these media have proven to be very efficient in eliminating the degrading impact of the thermal-stress-induced birefringence which occurs in the Faraday devices that are operated with high average power light. In this paper, we report on the successful growth of OAP-negative fluoride crystals – the KDy3F10 (KDF) and Na0.38Dy0.62F2.24 (NDF) – and on the characterization of the material properties allowing to assess the benefits of using these crystals for the Faraday devices. Unlike the recently commercialized OAP-negative KTb3F10 (KTF) crystal, these crystals exhibit high transparency at the mid-IR wavelengths (>1500 nm), for which there are currently no OAP-negative crystals available. And although the Verdet constant at these wavelengths is not exceptionally high (≤12 rad/Tm in the absolute value) it is sufficient for the practical implementation of a Faraday device based on these crystals using the currently available permanent magnets. In addition, the thermal conductivity of the KDF crystal is advantageously ∼24% higher than that of the state-of-the-art KTF crystal.

Magneto-optical spatial light modulator driven by current-induced domain wall motion for holographic display applications.

We have developed a magneto-optical spatial light modulator (MO-SLM) with a 10 k × 5 k pixel layout and with a pixel pitch horizontally of 1 µm and vertically of 4 µm. An MO-SLM device pixel has a magnetic nanowire made of Gd-Fe magneto-optical material whose magnetization was reversed by current-induced magnetic domain wall motion. We successfully demonstrated the reconstruction of holographic images, showing large viewing zone angles as wide as 30 degrees and visualizing different depths of the objects. These characteristics are unique to holographic images, providing physiological depth cues which may play a vital role in three-dimensional (3D) perception.

Large reciprocal magneto-optical effect induced by all-dielectric resonant gratings based on a magnetic nanocomposite

The beneficial combination of micro- and nano-patterned surfaces with magneto-optical materials was investigated over the recent years. Due to their resonant behavior, these structures are commonly used to enhance the non-reciprocal magneto-optical effects. In this paper, a novel kind of magneto-optical intensity effect is enhanced with an all-dielectric grating patterned on a magnetic nanocomposite layer. This nanocomposite is made of CoFe2O4 nanoparticles (NPs) embedded in a silica matrix by sol–gel technique. The demonstrated magneto-optical intensity effect is reciprocal and it is observed with transverse magnetic field, for both polarization (TE and TM) and small angles of incidence. Such effect is not explained by the classical appearance of off-diagonal elements in the permittivity tensor of the magneto-optical material under magnetic field. However, it can be attributed to a magneto-induced reciprocal modification of the diagonal elements. Furthermore, this effect strongly depends on the NPs orientation inside the magneto-optical film and can originate from the magnetostrictive property of the magnetic CoFe2O4 NPs.

Comparative investigation of 1D photonic crystal of magneto-optical and electro-optical materials with a nanocomposite of three/four-phase mixtures

Comparative investigation of 1D photonic crystal of magneto-optical and electro-optical materials with a nanocomposite of three/four-phase mixtures

A review on magneto-optical ceramics for Faraday isolators

As a promising magneto-optical material applied in Faraday isolators, magneto-optical ceramics possess excellent comprehensive properties and have attracted much attention these years. Herein, we review the fabrication and properties of magneto-optical ceramics including garnet, sesquioxide, and A₂B₂O₇ceramics. Some of the ceramics have been proved to possess applicable performance, while further studies are still needed for most of the magneto-optical ceramics. Aiming at the application for isolators, the research status, existing problems, and development trends of magneto-optical ceramics are shown and discussed in this review.

Strong nonreciprocal radiation for extreme small incident angle

The perfect nonreciprocal radiation effect for extreme small incident angle is studied. The proposed scheme consists of a patterned magneto-optical (MO) material on a continuous MO film backed with a metallic mirror and coated with a metallic film. Perfect emission and extreme small absorption are achieved at resonance, which results in the realization of near-complete nonreciprocal radiation for an incident angle of only 1o. Such phenomenon is attributed to the excitation of guided mode resonance (GMR) in the MO material, which is revealed by investing the magnetic field intensity distribution and confirmed by the impedance matching theory. In addition, the near-complete nonreciprocal radiation performance remains excellent in a wide range of structure parameters, which lowers the costs of production. Last, the scheme is also employed to investigate the possibility for realizing enhanced nonreciprocity with small applied magnetic field and dual-band nonreciprocal radiation. It is believed the proposed scheme is promising in improving the energy conversion in solar cells, thermophotovoltaics as well as thermal routers and thermal diodes.

Chiral photonic topological states in Penrose quasicrystals.

Electromagnetic topological edge states typically are created in photonic systems with crystalline symmetry and these states emerge because of the topological feature of bulk Bloch bands in momentum space according to the bulk-edge correspondence principle. In this work, we demonstrate the existence of chiral topological electromagnetic edge states in Penrose-tiled photonic quasicrystals made of magneto-optical materials, without relying on the concept of bulk Bloch bands in momentum space. Despite the absence of bulk Bloch bands, which naturally defiles the conventional definition of topological invariants in momentum space characterizing these states, such as the Chern number, we show that some bandgaps in these photonic quasicrystals still could host unidirectional topological electromagnetic edge states immune to backscattering in both cylinders-in-air and holes-in-slab configurations. Employing a real-space topological invariant based on the Bott index, our calculations reveal that the bandgaps hosting these chiral topological edge states possess a nontrivial Bott index of ±1, depending on the direction of the external magnetic field. Our work opens the door to the study of topological states in photonic quasicrystals.

Multi-band and wide-angle nonreciprocal thermal radiation

Violating Kirchhoff's radiation law through magneto-optical materials or spatiotemporal (Floquet) metamaterials can open a new door for engineering thermal radiation by breaking the widely-accepted equal constraint of spectral absorptivity (α) and emissivity (ϵ). Most existing work only reports the unequal α and ϵ spectra in one or two bands and within limited angles. This significantly limits the practical applications like the nonreciprocal thermophotovoltaics. In this work, we present a general machine-learning-kernel-based algorithm framework, based on which we achieve four-band nonreciprocal thermal radiation via the magneto-optical (MO) materials. The realization of multi-band nonreciprocity is mainly attributed to the coupling effect of magneto-optical effect and the excitation of cavity modes with different orders, which can be confirmed by investigating the magnetic field distribution. In addition, it is found that dual-band/multi-band strong nonreciprocal thermal radiation can be realized in a wide range of incident angles (15°-85°). The number of bands and range of angle can be further enhanced by modulating the number of layers, structures, materials, and applied magnetic field. The present work offers a general design roadmap for nonreciprocal thermal radiation, and can be extended for designing metamaterials beyond thermal metamaterials.

Fabrication and performance optimization of novel (Tb0.9975R0.0025)3Sc2Al3O12 (R= Pr, Ho, Dy, Eu) magneto-optical transparent ceramics

Magneto-optical materials with high Verdet constants at high extinction ratios are desirable for the Faraday isolators. However, current terbium-based crystals have encountered several issues, such as low Verdet constants (TGG crystals), difficult growth (TSAG crystals), and low utilization of raw materials, which limit the development of the Faraday isolators. Here, R0.0025TSAG (R= Pr, Ho, Dy, Eu) transparent ceramics are prepared. The ceramics exhibit high optical transparency above 74.0% in the 525–1565 nm range and a high relative density of 98.9%. In particular, the transmittance of R0.0025TSAG ceramics at 1064 nm is approximately 81.6%, 81.6%, 79.8%, and 80.6%, respectively. The Verdet constants of R0.0025TSAG ceramics at the measurement wavelength are greater than those of the TSAG ceramic. The compelling magneto-optical performance of the Dy0.0025TSAG ceramic includes a high Verdet constant of 161.8 ± 3 rad·m−1·T−1 at 635 nm. The Verdet constant of the Dy0.0025TSAG ceramic is still superior to that of TSAG ceramic in the same temperature range. The extinction ratios of the R0.0025TSAG (R= Pr, Ho, Dy) ceramics exceed 40 dB. The figure of merit points out the superior characteristics of R0.0025TSAG ceramics. Moreover, R0.0025TSAG (R= Pr, Ho, Dy) ceramics with a thickness above the centimeter level are successfully prepared. This work indicates that the Dy0.0025TSAG ceramic can promote the miniaturization of magneto-optical isolators and can be applied in high-power lasers.

High-rectification near-field radiative thermal diode using Weyl semimetals

Thermal diodes, which allow heat transfer in a preferential direction while being blocked in a reverse direction, have numerous applications in thermal management, information processing, energy harvesting, etc. Typical materials of thermal diodes in previous works include phase-change and magneto-optical materials. However, such thermal diodes depend highly on specific working temperatures or external magnetic fields. In this work, we propose a near-field radiative thermal diode (NFRTD) based on two Weyl semimetal (WSM) nanoparticles (NPs) mediated by a WSM planar substrate, which works without an external magnetic field and with flexible temperatures. Numerical results show that the maximum rectification ratio of NFRTD can be up to 2673 when the emitter is 200 K and receiver is 180 K, which exceeds the maximum value reported in some previous works by more than 10 times. The underlying physical mechanism is the strong coupling of the localized plasmon modes in the NPs and nonreciprocal surface plasmon polaritons in the substrate. In addition, we calculate the distribution of the Green's function and reflection coefficient to investigate nonreciprocal energy transfer in NFRTDs. Finally, we discuss the effects of momentum separation on the rectification performance of the NFRTD. This work demonstrates the great potential of WSMs in thermal rectification and paves a path for designing high-performance NFRTDs.

Discretized wavevectors in bulk magneto-optics

We introduce a wavevector discretization formulation for bulk magneto-optics. The natural discretization described herewith results in a nonlocal complex amplitude superposition of polarization states. Our analysis reveals the emergence of a multiplicity of discretized modes within elliptical and linear polarization pairs. This mode multiplicity is not due to confinement or resonance effects. Rather, it arises from the variance in refractive index for different beam-propagation directions in magneto-optic materials. Theoretical and numerical solutions leading to this result are presented herewith, together with computational verification of the theory applied to bismuth-substituted iron garnets. These results may lead to applications to quantum phenomena with single photons while maintaining a simpler classical architecture.

On-chip Ce:YIG/Si Mach-Zehnder optical isolator with low power consumption.

The integrated optical isolator is an essential building block in photonic integrated chips. However, the performance of on-chip isolators based on the magneto-optic (MO) effect has been limited due to the magnetization requirement of permanent magnets or metal microstrips on MO materials. Here, an MZI optical isolator built on a silicon-on-insulator (SOI) without any external magnetic field is proposed. A multi-loop graphene microstrip operating as an integrated electromagnet above the waveguide, instead of the traditional metal microstrip, generates the saturated magnetic fields required for the nonreciprocal effect. Subsequently, the optical transmission can be tuned by varying the intensity of currents applied on the graphene microstrip. Compared with gold microstrip, the power consumption is reduced by 70.8%, and temperature fluctuation is reduced by 69.5% while preserving the isolation ratio of 29.44 dB and the insertion loss of 2.99 dB at1550 nm.

Interface-Mediated YIG/Ce:YIG Bilayers Structure with Enhanced Magneto-optical Properties via Pulsed Laser Deposition

For the realization of integrated nonreciprocal photonic devices for optical communication, on-chip monolithic integration of magneto-optical materials onto silicon remains a challenge. Implemented with pulsed laser deposition (PLD), a MgO interfacial layer is introduced to overcome the material incompatibilities between a silicon substrate and magneto-optical thin films of yttrium iron garnet (YIG). With a thickness of ∼40 nm, the amorphous interfacial layer of MgO can effectively inhibit interdiffusion across YIG/Si and promote the growth of a high-density, high-phase-purity polycrystalline garnet structure in the bilayer of YIG/Ce:YIG. Such modified chemistry and microstructure in YIG/Ce:YIG lead to enhanced magneto-optical properties, including an ∼38% increase in Faraday rotation and an ∼15% increase in saturation magnetization, as well as an ∼20% increase in infrared (IR) transmission. Offering dual functions of diffusion barrier and structure template, the MgO layer demonstrated herein suggests a new remedy solution to heterogeneous interfaces in advanced thin film devices.

Characteristics and Recent Development of Fluoride Magneto-Optical Crystals

Magneto-optical materials are the fundamental component of Faraday isolators; therefore, they are significantly important for solid-state laser systems. Fluoride magneto-optical crystals such as CeF3, KTb3F10 and LiTbF4 exhibit advantages of wide transmittance range, high optical homogeneity, smaller thermal lensing and weaker thermal induced depolarization effect, and thus are promising candidates for Faraday isolators in high-power solid-state lasers. Recent progress in crystal growth and characterizations of these fluoride magneto-optical crystals are introduced. Possible applications of Faraday isolators based on various fluoride crystals are discussed, especially for solid-state lasers in the ultraviolet (UV) or infrared (IR) spectral region.

Optical properties and Faraday magneto-optical effects of highly transparent novel Tb2Zr2O7 fluorite ceramics

A novel magneto-optical transparent Tb2Zr2O7 ceramic with a high in-line transmittance of ∼76.2% at 635 nm (∼98.2% of the theoretical transmittance of perfect Tb2Zr2O7 single crystal) was successfully fabricated by vacuum sintering from its solid-phase reactive fluorite particle. In the visible region, the Tb2Zr2O7 bulk material was found to have a high refractive index of around 2.10 ± 0.03 and an optical basicity of 0.985 ± 0.015 while the electronic polarizability of the O2− anion in the Tb2Zr2O7 system generally remained constant at 2.45 ± 0.06 Å3. The Faraday effect results revealed that the transparent Tb2Zr2O7 ceramic had Verdet constants of −400 ± 4, −260 ± 4, and −62 ± 2 rad·T−1·m−1 at 515, 635, and 1064 nm, respectively, which were roughly 1.9-fold higher than the commercial Tb3Ga5O12 single crystal. Our outcomes demonstrated that the developed optical functional Tb2Zr2O7 ceramic has potential application as an excellent magneto-optical material.

Modeling of Enhanced Polar Magneto-Optic Kerr Effect by Surface Plasmons in Au Bowtie Arrays

The weak magneto-optical (MO) signal of traditional MO materials is indeed an important issue for their further practical applications. Although many strategies have been proposed to improve the MO effect, hybridization with noble metal nanostructures is a promising route in recent years due to the high localized-surface plasmon resonances (LSPR) effect. A new magneto-optical surface plasmon resonance (MOSPR) structure hybrid with Au bowtie arrays is proposed to increase the measuring range of the polar magneto-optical Kerr effect (PMOKE) and the quality factor through the LSPR effect. It is verified by a numerical simulation of the finite element method (FEM). The optimized parameters were found by modulating the shape and geometric dimensions. Owing to the significant LSPR from the Au bowties, a PMOKE amplification signal spectrum with narrow linewidth, and a high amplitude with high-sensing performance was achieved. Compared with the bare magnetic film alone, by optimizing the relevant parameters of the LSPR structure, the maximum signal increases 3255 times, and the quality factor can be greatly improved, which would provide important guidance and help for the practical application of MO devices.

Nonreciprocal optical properties of magneto-optical film doped with metal particles based on the effective medium theory

In search of next-generation optical information functional materials, magneto-optical microstructures have attracted great attention since they can break Kirchhoff’s law and produce higher photoelectric conversion efficiency. Theoretical studies using finite-difference time-domain and transfer matrix methods have been performed to investigate the optical properties of magneto-optical microstructures. However, these methods are computationally intensive and require periodic conditions, which may not be satisfied with most fabricated samples. The equivalent medium algorithm is improved to make it suitable for the equivalent of magneto-optical materials. Based on the improved equivalent medium theory (EMT), a magneto-optical InSb film structure doped with Au particles (D-InSb) is designed. The effective dielectric functions of the D-InSb layer for transfer matrix waves are obtained from the Bruggeman approximation. Thin-film optics formulas incorporating the anisotropic wave propagation in uniaxial media are employed to calculate the nonreciprocal absorptance of the D-InSb film. The effect of geometric parameters, such as filling ratio and number of layers, is investigated. In addition to modeling the directional radiative properties at various angles of incidence, the hemispherical properties are also calculated to understand the light absorption. The results of our study can provide methods and ideas for the design of solar cells, infrared absorbers, and optical isolators.

Broadband Nonreciprocal Thermal Emission

The reciprocity between thermal emission and absorption in materials that satisfy the Lorentz reciprocity places a fundamental constraint on photonic energy conversion and thermal management. For approaching the ultimate thermodynamic limits in various photonic energy conversions and achieving nonreciprocal radiative thermal management, broadband nonreciprocal thermal emission is desired. However, existing designs of nonreciprocal emitters are narrowband. Here, we introduce a gradient epsilon-near-zero magneto-optical metamaterial for achieving broadband nonreciprocal thermal emission. We start by analyzing the nonreciprocal thermal emission and absorption in a thin layer of epsilon-near-zero magneto-optical material atop a substrate. We use temporal coupled-mode theory to elucidate the mechanism of nonreciprocal emission in the thin-film emitter. We then introduce a general approach for achieving broadband nonreciprocal emission by using a gradient epsilon-near-zero magneto-optical metamaterial. We numerically demonstrate broadband nonreciprocal emission in gradient-doped semiconductor multilayer, as well as in a magnetic Weyl semimetal multilayer with gradient chemical potential. Our approach for achieving broadband nonreciprocal emitters is useful for developing broadband nonreciprocal devices for energy conversion and thermal management.

Crystallization of high gyrotropy garnets with decreasing thermal processing budgets as analyzed by electron backscatter diffraction

Rare-earth iron garnets with large magnetic gyrotropy, made with reduced thermal budgets, are ideal magneto-optical materials for integrated isolators. However, reduced thermal budgets impact Faraday rotation by limiting crystallization, and characterization of crystallinity is limited by resolution or scannable area. Here, electron backscatter diffraction (EBSD) is used to measure crystallinity in cerium substituted yttrium- and terbium-iron garnets (CeYIG and CeTbIG) grown on planar Si, crystallized using one-step rapid thermal processes, leading to large Faraday rotations > −3500 °/cm at 1550 nm. Varying degrees of crystallinity are observed in planar Si and patterned Si waveguides, and specific dependences of crystallite size are attributed to the nucleation/growth processes of the garnets and the lateral dimensions of the waveguide. On the other hand, a low thermal budget alternative–exfoliated CeTbIG nanosheets–are fully crystalline and maintain high Faraday rotations of −3200 °/cm on par with monolithically integrated thin film garnets.