Magneto-optical Materials Research Articles

Dual-band topological Bott insulators in Ammann–Beenker-tiling square photonic quasicrystals

Abstract In this work, we study topological states in Ammann-Beenker-tiling photonic quasicrystals made of magneto-optical materials. While conventional topological states in photonic systems with crystalline symmetry are characterized by topological invariants associated with bulk Bloch bands in momentum space, photonic systems in quasicrystal geometries typically lack exact periodicity and translational symmetry. As a result, conventional topological invariants defined in momentum space for photonic crystals, such as Chern number, are not applicable for photonic quasicrystals. Instead, a topological invariant called Bott index defined in real space could be employed for characterizing the topological properties of photonic quasicrystals, which we term as topological Bott insulators. In specific, we investigate the topological properties of photonic quasicrystals made of gyromagnetic dielectric cylinders arranged in a two- dimensional Ammann-Beenker tiling quasicrystalline lattice and find that this system supports dual-band chiral topological edge states, where the topological nature of both bandgaps is unambiguously confirmed by explicit calculations of the Bott index. Our work not only provides new insights on topological states in photonic quasicrystals based on the Ammann-Beenker-tiling, the results may also offer promising potentials for robust multiband photonic devices and applications not constrained by crystalline symmetries.

Study on ion slicing of iron garnet magneto-optic materials for near and mid-infrared on-chip optical isolators.

Achieving high-crystalline-quality, large-size iron garnet magneto-optic (MO) films on silicon substrates remains a critical challenge for CMOS-compatible on-chip non-reciprocal devices like isolators and circulators. In this study, we explored ion slicing on commercial yttrium iron garnet (YIG) crystals, bismuth-doped iron garnet (BIG), and newly developed YIG ceramics. After He+ ion implantation, wafer bonding and annealing, the BIG film on silicon was successfully fabricated, but its thickness and crystalline phase deviated from expectations. The underlying causes of these discrepancies were systematically investigated. In contrast, the YIG single crystals and ceramics showed blistering during annealing, which demonstrates their ion-slicing viability. Based on the magneto-optical constant dispersion relationships of the two materials, the nonreciprocal phase shift (NRPS) gap between BIG film on silicon and YIG film on silicon narrows significantly as the wavelength increases from 1.55 µm to 2.1 µm, dropping from 399% to 26%. As a proof of concept, we proposed a design for silicon-based TM-mode on-chip isolators at 1.5 µm and 2.1 µm using ion-sliced YIG ceramics, where the simulated insertion loss decreased from 2.78 dB to 0.35 dB due to the substantial reduction in material absorption with increasing wavelength. These results underscore the feasibility and promise of YIG ceramic ion slicing as a practical solution for CMOS-compatible on-chip isolators, particularly in the mid-infrared range.

Anderson localization of circularly polarized waves in a gyrotropic superlattice with correlated disorder

In this paper, we explore the transmission of circularly polarized electromagnetic waves through one-dimensional random periodic-on-average photonic crystals containing layers of magneto-optical material in Faraday geometry. Driven by evidence that long-range correlations crucially influence wave localization within certain spectral ranges, our study aims to harness these effects for the development of novel electromagnetic wave filters tunable via a dc magnetic field. We base our study on a model of light propagation through a finite array of alternating dielectric layers with random thickness variations and layers of gyrotropic material of equal thickness. Assuming weak positional disorder, we employ analytical and numerical methods to analyze the inverse localization length and assess filter performance. Our results demonstrate that specific correlated disorder introduced into periodic systems can enhance or suppress the transmissivity for a wave of a given frequency in any desired interval of the magneto-optical parameter q. Additionally, we show that the Anderson localization can be resonantly suppressed when the thickness of each gyrotropic layer accommodates an integer number of half-wavelengths.

Polycrystalline magneto-optical transparent Pr2Zr2O7 pyrochlore ceramic for Faraday rotation.

To design an innovative magneto-optical material aimed at a large Verdet constant coincides with the development trend of state-of-the-art modern optical devices. In this work, a magneto-optical transparent Pr2Zr2O7 ceramic with pyrochlore structure was successfully fabricated by vacuum sintering plus hydrogen reduction for the first time to our knowledge. The two- and three-dimensional images observed on the laser scanning confocal microscopy reveal that the grain-boundary dent depth of the polished Pr2Zr2O7 ceramic is only ∼1.6 µm upon thermal etching at a high temperature of 1675°C for 2 h, showing excellent heat-resistance ability. The crystal-clear Pr2Zr2O7 ceramic has a high in-line transmittance of ∼71.6% at 780 nm (∼92.3% of the theoretical value), a high refractive index of ∼2.1, and an effective magnetic moment of ∼5.28 µ B. In the visible region, the O2- polarizability and optical basicity of the Pr2Zr2O7 compound generally retain constants around 2.37 ± 0.07 Å3 and 0.965 ± 0.015, respectively. The magneto-optical transparent Pr2Zr2O7 ceramic exhibits high Verdet constants of -336 ± 4, -208 ± 3, -108 ± 1, and -48 ± 2 rad·T-1·m-1 at 515, 635, 780, and 1064 nm, respectively, which were ∼1.55-fold higher than the commercial Tb3Ga5O12 single crystal. Our developed new-type ceramic material has excellent potential application for Faraday rotation.

Automated design of nonreciprocal thermal emitters via Bayesian optimization

Nonreciprocal thermal emitters that break Kirchhoff’s law of thermal radiation promise exciting applications for thermal and energy applications. The design of the bandwidth and angular range of the nonreciprocal effect, which directly affects the performance of nonreciprocal emitters, typically relies on physical intuition. In this study, we present a general numerical approach to maximize the nonreciprocal effect. We choose doped magneto-optic materials and magnetic Weyl semimetal materials as model materials and focus on pattern-free multilayer structures. The optimization randomly starts from a less effective structure and incrementally improves the broadband nonreciprocity through the combination of Bayesian optimization and reparameterization. Optimization results show that the proposed approach can discover structures that can achieve broadband nonreciprocal emission at wavelengths from 5 to 40 μm using only a fewer layers, significantly outperforming current state-of-the-art designs based on intuition in terms of both performance and simplicity.

Magneto-optical Goos–Hänchen shift in uniaxial anisotropic epsilon-near-zero metamaterials

Abstract We theoretically investigate the enhancement of the magneto-optical Goos-Hänchen (MOGH) shift in multilayer structure combining uniaxially anisotropic epsilon-near-zero (ENZ) metamaterials with magneto-optical (MO) materials. The structure enables the excitation of magneto-optical surface plasmon resonance (MOSPR) without the need for a prism or grating coupler, thereby achieving a significant enhancement of the MOGH shift. The effect of the thickness of the ENZ layer and the type of magnetic metal in this structure on the MOGH shift is also discussed. Based on the above structure, we design a high-sensitivity miniature biosensor that is used for the detection of hemoglobin concentration in human blood. The maximum sensitivity achievable with this sensor is 1.4×106 λ/RIU. The results of this investigation contribute to the development of biosensors towards miniaturization and integration.

Magneto-optic properties of highly Dy3+-doped GeO2-B2O3 glasses for NIR optical applications

Recent progress in the development of optical communication and high power laser systems in the near-infrared (NIR) region (1.5 to 2.0 μm) increased the demand for magneto-optic (MO) devices employing MO materials. These MO materials are required with lower optical absorption coefficients and higher Verdet constants at spectral range of interest. In this study, two series of highly Dy3+-doped GeO2–B2O3 glasses with and without P2O5 were fabricated using a simple melt-quenching technique. The glasses were characterized through various techniques (Raman, UV–VIS-NIR, and DTA) for the evaluation of characteristic properties with respect to the Dy2O3 concentration and glass composition. Faraday rotation measurement was carried out at 1550 nm to quantify the Verdet constant of the glasses. The Verdet constant values were found to increase linearly with increasing Dy3+ concentration for both the glass systems. The increase in Dy3+ concentration and, consequently, the number of unpaired electrons, leads to an increase in the Verdet constant. The GeO2-B2O3 glass doped with 30 mol% of Dy2O3 exhibited the highest Verdet constant of −5.36 rad/(T⋅m) at 1550 nm. The glasses exhibited a good optical transparency (>70 %) in the region covering from 400 to 2400 nm. The thermo-physical, optical, and polarizability properties of both glass systems were also investigated. The results indicate that the highly Dy3+-doped glasses are attractive for magneto-optics at 1550 nm.

Study of the magnetic and structural properties of the simple perovskite YFeO3

In the present work, an experimental and theoretical study of the synthesis, magnetic, and structural properties of simple perovskite compounds YFeO3 has been investigated based on their potential applications, such as: solid oxide fuel cells, electrochemical sensors, sensor materials, magneto-optical materials, etc. Experimentally, the synthesis was carried out by the sol-gel method. X-rays and Rietveld refinements confirmed the orthorhombic structure. Scanning electron microscopy observations confirmed agglomerates of simple perovskite particles YFeO3. The study of the magnetic phase of the simple perovskite YFeO3 showed that it behaves like a weak ferromagnetic, this property delimits and directs magnetic fields into well-defined trajectories. On the other hand, the structural results, from a theoretical point of view, are in good agreement with the experimental ones. Through energy comparison, a possible competition was identified between the FM and AFM states, where the electronic properties are associated with the directional nature of the hybridization between the p orbitals of oxygen and the t2g states, where the superexchange mechanism prevails with e oxygen as mediator.

Transparent Tb2Ti2O7 ceramics for use in Faraday isolators

Magneto-optical and thermo-optical characteristics of transparent Tb2Ti2O7 ceramics were investigated. The dependence of the index of refraction on the wavelength in the 0.29–2 μm range, the wavelength and temperature dependence of the Verdet constant, as well as the dependence of thermally induced depolarization on laser radiation power were measured. The value of the Verdet constant in Tb2Ti2O7 surpasses that in Tb3Ga5O12 by more than 1.68 times. The thermo-optical characteristic Qeff was estimated to be (1.8–3.7)∙10−8 1/K, which is record small compared to Qeff of the known magneto-optical materials. The small value of Qeff makes Tb2Ti2O7 a highly promising magneto-optical material for Faraday isolators and rotators for high average power lasers.

Utilizing magneto-optical short-range surface plasmon resonance to enhance the performance of optical fiber sensor

Although the short-range surface plasmon resonance (SRSPR) has larger sensitivity than the long-range surface plasmon resonance (LRSPR) excited simultaneously, SRSPR is usually ignored in previous studies of the LRSPR sensors due to its much larger peak width. However, since the resonance peaks of both SRSPR and LRSPR narrow vastly when combined with the magneto-optical (MO) effect, SRSPR has the potential to reach much higher figure-of-merit (FOM) than LRSPR due to its higher sensitivity. In this work, the feasibility of utilizing magneto-optical short-range surface plasmon resonance (MOSRSPR), which is the combination of SRSPR and MO effect, to enhance FOM was verified first by analyzing the prism-based structure sensor with the transfer matrix method (TMM). Then a novel optical fiber sensor based on MOSRSPR was proposed. The structure of the proposed sensor is a D-shaped optical fiber coated with an MgF2 film, a thin silver film, and a MO material film of cerium-doped yttrium-iron garnet (CeYIG). The influence of the structural parameters including the MgF2, silver, and CeYIG film thicknesses was analyzed theoretically by the finite element method (FEM). With the optimal parameters, the FOM of the optical fiber MOSRSPR sensor is greatly enhanced to 10,750 RIU−1 in maximum and 2888 RIU−1 in average, which surpasses all reported optical fiber LRSPR sensors vastly. The miniaturized and simple design of the D-shaped fiber MOSRSPR sensor, coupled with its high performance, offers a significant reference for the study of optical fiber sensors with ultra-high FOM.

Nonreciprocity in transmission mode with planar structures for arbitrarily polarized light [Invited

Approaching thermodynamic limits in light harvesting requires enabling nonreciprocal thermal emission. The majority of previously reported nonreciprocal thermal emitters operate in reflection mode, following original proposals by M. Green [Nano Lett. 12, 5985 (2012)10.1021/nl3034784] and others. In these proposals, cascaded nonreciprocal junctions that re-direct each junction’s emission towards a subsequent one are employed for efficient light-harvesting. Recently, simplified concepts have been proposed in solar photovoltaics and thermophotovoltaics, respectively, that leverage the concept of tandem junctions to approach thermodynamic limits. In these simplified scenarios, polarization-independent nonreciprocal response in transmission mode is required. We propose a pattern-free heterostructure that enables such functionality, using a magneto-optical material embedded between two dissimilar dielectric layers.

Optical fiber sensor based on magneto-optical surface plasmon resonance with ultra-high figure of merit

Compared to surface plasmon resonance (SPR), the sensors based on the magneto-optical SPR (MOSPR) technique have much higher figure-of-merit (FOM). However, there are no reports about applying MOSPR in the optical fiber structure now. In this work, a novel D-shaped optical fiber sensor based on the MOSPR technique is proposed. The D-shaped optical fiber is coated with a thin silver film and a magneto-optical (MO) material film of Cerium-doped Yttrium-Iron garnet (CeYIG). By applying a magnetic field on the sensing region, the magneto-optical Kerr effect (MOKE) of the CeYIG layer and the related MOSPR phenomenon could be excited when appropriate light is transmitted in the proposed optical fiber sensor. The influence of the structural parameters including the residual cladding thickness, silver and MO material film thicknesses are analyzed theoretically by the finite element method (FEM). With the optimal parameters, the sensor achieves the sensitivity of 5304 nm RIU−1. Since the peak width of MOSPR spectra is much narrower than that of the SPR spectra, the FOM of the sensor is largely enhanced to 3864 RIU−1 on average and 13260 RIU−1 in maximum, which surpasses the optical fiber SPR sensors vastly. The miniaturized and simple design of the D-shaped optical fiber MOSPR sensor, coupled with the ultra-high FOM, offers itself great potential in biochemical sensing applications.

Magneto-Optical Ceramics with High Transparency for Highly Sensitive Magnetometer via Quantum Weak Measurement.

Sensitive magnetometer technology is desirable for biomagnetic field detection and geomagnetic field measuring. Signal amplification materials such as magneto-optical crystals or ceramics are crucial for enhancing detection sensitivity, but severe optical scattering and low Verdet constant further limit its application. To develop high-sensitivity magnetometers for quantum weak measurement schemes, we have conducted investigations on the powder calcining dynamics and prepared a series of high-optical-quality (Ho/Dy)2Zr2O7 transparent ceramic samples. The Verdet constant of magneto-optical materials was measured across a continuous wavelength spectrum, exhibiting a peak at 283 ± 5 rad/(T·m). We further established an electron transition mechanism to elucidate the exceptional magneto-optical attributes of dysprosium. In addition, samples demonstrated superior performance in weak-value amplification, reaching a low detectable magnetic field threshold of 3.5 × 10-8 T and continuously worked over 6 h with high stability. Our work developed a highly sensitive magnetometer using optimized magneto-optical ceramics and provided guidance on design, fabrication, and application for magneto-optical ceramics in quantum weak measurement.

Non-reciprocal optical Tamm state in a photonic crystal heterojunction containing Weyl semimetals

In the past decades, optical Tamm states (OTSs) in photonic crystal (PC) heterojunctions containing magneto-optical materials have been widely utilized to achieve non-reciprocal transmission. Nevertheless, as incident angle increases, both forward and backward OTSs in PC heterojunctions containing magneto-optical materials shift toward shorter wavelengths (i.e., blueshift), giving rise to weak non-reciprocities. In this paper, we achieve a non-reciprocal OTS in a PC heterojunction containing Weyl semimetals (WSMs). Interestingly, as incident angle increases, the forward OTS shifts towards longer wavelengths while the backward OTS shifts toward shorter wavelengths. Therefore, the non-reciprocity can be greatly enhanced. The underlying physical mechanism can be explained by the Fabry-Perot resonant condition. Besides, the effects of the axial vector b and the loss of WSM on non-reciprocity are investigated. These results would possess great potential applications in the design of circulators and isolators.

Tunable Near-Field Radiative Heat Transfer between Graphene-Coated Magneto-Optical Metasurfaces.

The highly structured design of metasurfaces greatly facilitates the manipulation of near-field radiative heat transfer (NFRHT). In this study, we incorporate magneto-optical materials into metasurfaces to theoretically explore the mechanism for controlling NFRHT between anisotropic magneto-optical metasurfaces. Our findings indicate that the interaction between the magnetization-induced modes, arising from interband transitions of graphene, and the surface modes of InSb under a magnetic field leads to a transition in the heat transfer spectrum from a dual band to a triple band. The modification of the distribution and magnitude of transmission wave vectors in surface electromagnetic modes by magnetic fields serves to modulate the radiative heat flux. By combining active control by a magnetic field with passive structural design of metasurfaces, the regulation of heat flux can be increased by more than 8-fold compared with the planar configuration. Additionally, the magnetic field amplifies the anisotropy of the photon energy distribution induced by the symmetry breaking of the metasurface structure. This study is anticipated to provide a pathway for achieving flexible tuning of NFRHT by combining active and passive regulation. It also opens up possibilities for multiband information transmission and for improving the performance of energy conversion devices.

Non-reciprocity in a silicon photonic ring resonator with time-modulated regions.

Non-reciprocity and breaking of the time-reversal symmetry is conventionally achieved using magneto-optic materials. However, the integration of these materials with complementary metal-oxide semiconductor (CMOS)-compatible platforms is challenging. Temporal modulation is a well-suited approach for achieving non-reciprocity in integrated photonics. However, existing experimental implementations based on this method in silicon uses traveling-wave modulation in the whole structure or tandem ring or waveguide modulators, and they lead to high insertion loss and large footprint. In this work we achieve, to the best of our knowledge, the first experimental demonstration of non-reciprocity in a compact single silicon photonic ring resonator with time-modulated regions, fabricated with a CMOS-compatible commercial foundry. We demonstrate symmetry breaking of counter-rotating modes in an active silicon photonic ring resonator by applying phase-shifted RF signals to only two small p-i-n junctions on the ring, without employing traveling-wave modulation in the whole structure. The non-reciprocity is caused by the cross-coupling between the counter-rotating modes of the ring, which breaks their degeneracy. By reversing the polarity of the RF phase difference (e.g. (45°,-45°) asymmetric phases) opposite resonance wavelengths are obtained, with a 16-dB contrast between the transmissions of the asymmetric phases and a low insertion loss of 0.6 dB under 27 dBm RF power. We achieve the highest ratio of the asymmetric transmission to the insertion loss, among the state-of-the-art silicon non-reciprocal integrated optical structures based on time varying modulation. The non-reciprocal ring can be used as a magnetic-free, low-loss, compact, and CMOS-compatible integrated optical isolator.

Magnetic field expulsion in optically driven YBa2Cu3O6.48.

Coherent optical driving in quantum solids is emerging as a research frontier, with many reports of interesting non-equilibrium quantum phases1-4 and transient photo-induced functional phenomena such as ferroelectricity5,6, magnetism7-10 and superconductivity11-14. In high-temperature cupratesuperconductors, coherent driving of certain phonon modes has resulted in a transient state with superconducting-like optical properties, observed far above their transition temperatureTc and throughout the pseudogap phase15-18. However, questions remain on the microscopic nature of this transient state and how to distinguish it from a non-superconducting state with enhanced carrier mobility. For example, it is not known whether cuprates driven in this fashion exhibit Meissner diamagnetism. Here we examine the time-dependent magnetic field surrounding an optically driven YBa2Cu3O6.48 crystal by measuring Faraday rotation in a magneto-optic material placed in the vicinity of the sample. For a constant applied magnetic field and under the same driving conditions that result in superconducting-like optical properties15-18, a transient diamagnetic response was observed. This response is comparable in size with that expected in an equilibrium type II superconductor of similar shape and size with a volume susceptibility χv of order -0.3. This value is incompatible with a photo-induced increase in mobility without superconductivity. Rather, it underscores the notion of a pseudogap phase in which incipient superconducting correlations are enhanced or synchronized by the drive.

Tunable quasi-bound states in the continuum in magneto-optical metasurfaces

The enhancement of electromagnetic field with high Q factor in metasurfaces has attracted extensive attention of researchers. Magneto-optical metasurfaces (MOMS) provide approaches with controllable magnetic fields to modulate the optical response, which contributes towards the magneto-optical Kerr effect in nanophotonic devices. However, it is challenging for MOMS to obtain narrow spectra peak with high Q factors. Here, we propose MOMS-supported quasi-bound states in the continuum (quasi-BIC) and theoretically investigate the evolution process from BIC to quasi-BIC. By applying an external magnetic field, the BIC in our proposed metasurface can be transformed into a quasi-BIC with finite Q factor up to 33 620. Meanwhile, the quasi-BIC shows magnetization-related circular dichroism with a vortex point in the polarization graph, based on time-reversal symmetry breaking in the magneto-optical material under the condition of external magnetic field. The quasi-BIC also maintains nearly unity reflection and almost zero absorption, showing a promising future in sensing. Our results enrich the light control mechanisms of MOMS and provide a unique opportunity for applications requiring flexible tunability and high Q factors, such as sensors, laser sources, filters and chiral-related elements.

Polarization-independent nonreciprocal thermal radiation by cylindrical grating structure

Due to the significant application value of nonreciprocal thermal radiation in energy collection and conversion, it has always been an active research topic. The current research on polarization-independent nonreciprocal thermal radiation is characterized by low efficiency and a notable disparity in nonreciprocal efficiency between transverse electric (TE) and transverse magnetic (TM) polarizations. To address this issue, a polarization-independent nonreciprocal thermal radiation based on the two-dimensional grating structure is proposed, which is composed of silicon cylindrical grating ridges, magneto-optical materials InAs, and metal Ag. By the rigorous coupled-wave analysis (RCWA), the nonreciprocal efficiency exceeds 90 % at a wavelength of 11.763 μm under both TE and TM polarizations. Meanwhile, the difference in nonreciprocal efficiency between the two polarizations is less than 1 %. Through an examination of the magnetic field and electromagnetic distribution at resonance peaks that are polarization-independent, as well as an analysis of coupling mode theory, we have uncovered the underlying physical mechanisms responsible for this phenomenon. The device is capable of sustaining optimal performance across a wide range of structural parameters, offering a significant role for manufacturing applications. The research proposed in this article provides a novel approach for designing efficient polarization-independent nonreciprocal thermal radiation.

Effect of non-stoichiometric Ga content on the microstructure and optical transparency of terbium gallium garnet ceramics

The use of Faraday isolators in high-power lasers is currently a high-priority research topic and terbium gallium garnet (Tb3Ga5O12, TGG) transparent ceramic is a magneto-optical material with great potential for applications in the visible and near-infrared ranges. In this paper, the effect of chemical composition adjustment on the microstructure and properties of Tb3Ga5(1+x)O12 ceramics (where x is the non-stoichiometric index corresponding to the percentage difference in the amount of Ga used in the synthesis with respect to its content in the stoichiometric compound) was investigated. Powders obtained from starting materials of different stoichiometry were found to have similar morphology. However, the relative densities of the ceramics after pre-sintering in air increase with increasing Ga content. After the HIP post-treatment, the optimal properties with an in-line transmittance of 78.5 % (1.5 mm thick) at 1064 nm, and the Verdet constant of the ceramics with x = 0.8 % reaches −134.1 rad T−1 m−1 at 633 nm.