Nonreciprocal plasmonics enables giant enhancement of thin-film Faraday rotation

Enhancing Faraday and Kerr rotations based on the toroidal dipole mode in an all-dielectric magneto-optical metasurface

The magneto-optical Faraday and Kerr effects are widely used in modern optical devices. In this Letter, we propose an all-dielectric metasurface composed of perforated magneto-optical thin films, which can support the highly confined toroidal dipole resonance and provide full overlap between the localized electromagnetic field and the thin film, and consequently enhance the magneto-optical effects to an unprecedented degree. The numerical results based on the finite element method show that the Faraday and Kerr rotations can reach -13.59° and 8.19° in the vicinity of toroidal dipole resonance, which are 21.2 and 32.8 times stronger than those in the equivalent thickness of thin films. In addition, we design an environment refractive index sensor based on the resonantly enhanced Faraday and Kerr rotations, with sensitivities of 62.96 nm/RIU and 73.16 nm/RIU, and the corresponding maximum figures of merit 132.22°/RIU and 429.45°/RIU, respectively. This work provides a new, to the best of our knowledge, strategy for enhancing the magneto-optical effects at nanoscale, and paves the way for the research and development of magneto-optical metadevices such as sensors, memories, and circuits.

Controllable Patterning of Metallic Photonic Crystals for Waveguide-Plasmon Interaction

Waveguide-plasmon polaritons sustained in metallic photonic crystal slabs show fascinating properties, such as narrow bandwidth and ultrafast dynamics crucial for biosensing, light emitting, and ultrafast switching. However, the patterning of metallic photonic crystals using electron beam lithography is challenging in terms of high efficiency, large area coverage, and cost control. This paper describes a controllable patterning technique for the fabrication of an Ag grating structure on an indium-tin oxide (ITO) slab that enables strong photon-plasmon interaction to obtain waveguide-plasmon polaritons. The Ag grating consisting of self-assembled silver nanoparticles (NPs) exhibits polarization-independent properties for the excitation of the hybrid waveguide-plasmon mode. The Ag NP grating can also be annealed at high temperature to form a continuous nanoline grating that supports the hybrid waveguide-plasmon mode only under transverse magnetic (TM) polarization. We tuned the morphology and the periodicity of the Ag grating through the concentration of silver salt and the photoresist template, respectively, to manipulate the strong coupling between the plasmon and the waveguide modes of different orders.

High Verdet Constant Glass for Magnetic Field Sensors

Due to the high transparency, high Verdet constant, as well as easy processing properties, rare-earth ion-doped glasses have demonstrated great potential in magneto-optical (MO) applications. However, the variation in the valence state of rare-earth ions (Tb³⁺ to Tb⁴⁺) resulted in the decreased effective concentration of the paramagnetic ions and thus degraded MO performance. Here, a strategy was proposed to inhibit the oxidation of Tb³⁺ into Tb⁴⁺ as well as improve the thermal stability by tuning the optical basicity of glass networks. Moreover, the depolymerization of the glass network was modulated to accommodate more Tb ions. Thus, a record high effective concentration (14.19 × 10²¹/cm³) of Tb ions in glass was achieved, generating a high Verdet constant of 113 rad/(T·m) at 650 nm. Lastly, the first application of MO glass for magnetic field sensors was demonstrated, achieving a sensitivity of 0.139 rad/T. We hope our work provides guidance for the fabrication of MO glass with high performance and thermal stability and could push MO glass one step further for magnetic sensing applications.

Magneto-Optical Spectroscopy of Short Spin Waves by All-Dielectric Metasurface

The optical method of spin dynamics measurements via the detection of various magneto-optical effects is widely used nowadays. Besides it being a convenient method to achieve time-resolved measurements, its spatial resolution in the lateral direction is limited by a diffraction limit for the probe light. We propose a novel approach utilizing a Mie-resonance-based all-dielectric metasurface that allows for the extraction of a signal of a single submicron-wavelength spin wave from the wide spin precession spectra. This approach is based on the possibility of designing a metasurface that possesses nonuniform magneto-optical sensitivity to the different nanoscale regions of the smooth magnetic film due to the excitation of the Mie modes. The metasurface is tuned to be unsensitive to the long-wavelength spin precession, which is achieved by the optical resonance-caused zeroing of the magneto-optical effect for uniform magnetization in the vicinity of the resonance. At the same time, such a Mie-supporting metasurface exhibits selective sensitivity to a narrow range of short wavelengths equal to its period.

Design of an Er-doped surface plasmon resonance-photonic crystal fiber to improve magnetic field sensitivity

In order to meet the demand for large-scale magnetic field testing, this paper proposes a D-shaped magneto-refractive photonic crystal fiber (MRPCF) based on surface plasmon resonance (SPR) by using the erbium-doped materials. The four different structures of Models A, B, C, and D are designed by changing the diameter, the position, and the number of layers of the air holes, and the corresponding magnetic field sensing characteristics are analyzed. The results show that in the magnetic field range of 5-405 mT, the magnetic field sensitivities of Models A, B, C, and D are 28 pm/mT, 48 pm/mT, 36 pm/mT, and 21 pm/mT, respectively. Meanwhile, the figure of merit (FOM) of the four MRPCF-SPR sensors is investigated, which have FOMs of 4.8 × 10^-4 mT^-1, 6.4 × 10^-4 mT^-1, 1.9 × 10^-4 mT^-1, 0.9 × 10^-4 mT^-1. Model B has higher sensitivity and larger FOM. In addition, the effect of the structural parameters of Model B on the sensing performance is also studied. By optimizing each parameter, the magnetic field sensitivity of the optimized Model B is increased to 53 pm/mT, and its magneto-refractive sensitivity and FOM are 2.27 × 10^-6 RIU/mT and 6.2 × 10^-4 mT^-1, respectively. It shows that the magneto-refractive effect of MRPCF can be effectively enhanced by optimizing the structural design of fiber. The proposed MRPCF is an all-solid-state fiber, which solves the instability problem of the magnetic fluid-filled fiber and reduces the complexity of the fabrication process. The all-solid-state MRPCF can be used in the development of quasi-distributed optical fiber magnetic field sensors and has broad applications in the fields of geological exploration, earthquake and tsunami monitoring, and military navigation.

Magnetic Control of the Plasmonic Chirality in Gold Helicoids

Chiral plasmonic nanostructures have facilitated a promising method for manipulating the polarization state of light. While a precise structural modification at the nanometer-scale-level could offer chiroptic responses at various wavelength ranges, a system that allows fast response control of a given structure has been required. In this study, we constructed uniformly arranged chiral gold helicoids with cobalt thin-film deposition that exhibited a strong chiroptic response with magnetic controllability. Tunable circular dichroism (CD) values from 0.9° to 1.5° at 550 nm wavelength were achieved by reversing the magnetic field direction. In addition, a magnetic circular dichroism (MCD) study revealed that the gap structure and size-related surface plasmon resonance induced MCD peaks. We demonstrated the transmitted color modulation, where the color dynamically changed from green-to-red, by controlling the field strength and polarizer axis. We believe current work broadens our understanding of magnetoplasmonic nanostructure and expands its potential applicability in optoelectronics or optical-communications.

Directional dependent magnetooptical effect and the photonic spin Hall effect in a magnetic Weyl semimetal-based photonic crystal

The ability to generate and manipulate the directional dependent magnetooptical effect and photonic spin Hall effect is essential toward realistic unidirectional optoelectronic devices, but its exploration remains scarce. Here we theoretically identify that the multilayer structure whose unit cell is composed of a new, to the best of our knowledge, emergent magnetic Weyl semimetal layer and two anisotropic dielectric layers has the capability of creating the propagation direction dependent magnetooptical effect and photonic spin Hall effect simultaneously due to its intrinsic lack of space inversion and time reversal symmetries. Specifically, we also realize the continuous manipulation of the magnetooptical effect and photonic spin Hall effect in this structure under two opposite directions by an electrical means, which is contributed by the control of the optical properties in magnetic Weyl semimetals by Fermi energy. Our work enables an alternative strategy to achieve the directional dependent optical as well as magnetooptical effects simultaneously, which provides new perspectives in the fresh field of unidirectional optoelectronics and spin photonics.

Polarization transformation and destructive interference on subwavelength magnetic domains in magneto-plasmonic systems

We demonstrate magneto-optical (MO) polarization transformation due to surface plasmons in CoPt perpendicular magnetic films in the polar Kerr geometry. An extraordinary Kerr rotation angle (θ_K =  ± 88.9°) that almost reaches the upper limit of polarization is produced in the attenuated total reflection (Kretschmann) configuration. P-polarized incident radiation is almost transformed upon reflection to s-polarized radiation, which may be out of phase depending on whether the magnetization of CoPt is up or down. Moreover, the reflected intensity may be drastically modulated by applying an external magnetic field. The reflectivity goes almost to zero in the demagnetized state and increases with increasing external magnetic field. This drastic optical response is attributed to the MO destructive interference produced by the subwavelength magnetic domain structure.

Standalone, CMOS-based Faraday rotation in a silicon photonic waveguide

Nonreciprocity is a fundamental requirement of signal isolation in optical communication systems. However, on chip isolator designs require either post-processing steps or external magnetic biasing, which are impractical for commercial applications. This raises the need for standalone devices which support nonreciprocal functionality using standardized fabrication techniques. Here, we report the first design of an electromagnetic coil surrounding a waveguide which exclusively employed the complementary metal-oxide-semiconductor (CMOS) process flow. The coil supported an electric current up to 14 mA. In simulations, it generated an alternating magnetic flux density up to 1.16 mT inside a strip waveguide and thereby induced a rotation of 50.71 picodegrees for the fundamental transverse-magnetic mode at a wavelength of 1352 nm. Our analysis further revealed methods to increase the rotation by orders of magnitude. It demonstrated the scope of manufacturing processes and serves as a building block for the development of a commercially viable, on-chip optical isolator.

Broadband nonreciprocal spoof plasmonic phase shifter based on transverse Faraday effects

Spoof surface plasmon polaritons (SSPPs) have aroused widespread concern due to their strong ability in field confinement at low frequencies. For miniaturized integrated circuits, there is a pressing need for nonreciprocal spoof plasmonic platforms that provide diode functionalities. In this letter, we report the realization of nonreciprocal phase shifting in SSPPs using the transverse Faraday effect. A plasmonic coupled line is constructed by flipped stacking two corrugated metallic strips, in order to enhance the mode coupling between evanescent waves that carry opposite transverse spin angular momenta. With a transverse magnetized ferrite cladding, the SSPP mode is split into two circularly-polarized ones that show different propagation constants over a broad band. A nonreciprocal phase shifter compatible to standard microstrips is designed to validate the breaking of time-reversal symmetry in SSPPs. Microwave measurement demonstrates a differential phase shift up to 46.2°/cm from 12 GHz to 15 GHz. Owing to the advantages of strong field confinement and contactless ferrite integration, the proposed method enables an alternative pathway for nonreciprocal spoof interconnects.

Acoustic, Phononic, Brillouin Light Scattering and Faraday Wave-Based Frequency Combs: Physical Foundations and Applications

Frequency combs (FCs)-spectra containing equidistant coherent peaks-have enabled researchers and engineers to measure the frequencies of complex signals with high precision, thereby revolutionising the areas of sensing, metrology and communications and also benefiting the fundamental science. Although mostly optical FCs have found widespread applications thus far, in general FCs can be generated using waves other than light. Here, we review and summarise recent achievements in the emergent field of acoustic frequency combs (AFCs), including phononic FCs and relevant acousto-optical, Brillouin light scattering and Faraday wave-based techniques that have enabled the development of phonon lasers, quantum computers and advanced vibration sensors. In particular, our discussion is centred around potential applications of AFCs in precision measurements in various physical, chemical and biological systems in conditions where using light, and hence optical FCs, faces technical and fundamental limitations, which is, for example, the case in underwater distance measurements and biomedical imaging applications. This review article will also be of interest to readers seeking a discussion of specific theoretical aspects of different classes of AFCs. To that end, we support the mainstream discussion by the results of our original analysis and numerical simulations that can be used to design the spectra of AFCs generated using oscillations of gas bubbles in liquids, vibrations of liquid drops and plasmonic enhancement of Brillouin light scattering in metal nanostructures. We also discuss the application of non-toxic room-temperature liquid-metal alloys in the field of AFC generation.

Nonreciprocity and Faraday Rotation at Time Interfaces

Nonreciprocity is critically important in modern wave technologies, yet its general principles and practical implementations continue to raise intense research interest, in particular in the context of broken reciprocity based on spatiotemporal modulation. Abrupt changes in time of the electromagnetic properties of a material have also been shown to replace spatial boundaries, supporting highly unusual wave-matter interactions in so-called time metamaterials. Here, we introduce nonreciprocity for temporal boundaries, demonstrating Faraday polarization rotation in a magnetoplasma with material properties abruptly switched in time. Our findings open new opportunities for time metamaterials, yielding new avenues for nonreciprocity with broad applicability for wave engineering.

All-optical frequency-dependent magnetic switching in metal-insulator-metal stub structures

Many attempts to switch magnetization with optical pulses were based on free-space coupling schemes of circularly polarized light pulses, so-called all-optical helicity-dependent magnetic switching; however, waveguide coupling schemes are promising for on-chip all-optical magnetic switching. Metal-insulator-metal (MIM) stub structures provide a promising platform for highly integrated photonic circuits, thanks to their compact size, on-chip compatibility, and ease of fabrication. We found clockwise and counterclockwise ring-like modes in the MIM stub structure, which can act as effective magnetic fields with two opposite directions due to the inverse Faraday effect. Effective magnetic field spectra inside the MIM stub have dual resonant peaks at which the effective magnetic field intensity reaches its extreme values with opposite signs, corresponding to binary magnetic states. Switching between the binary magnetic states can be achieved by altering the optical pump frequency. The all-optical frequency-dependent magnetic switching in the MIM stub may provide a chip-compatible and ultracompact tool for ultrafast switching of magnetic order.

Magnetically controllable holographic encryption based on a magneto-optical metasurface

As a flexible and compact nanophotonic device, the metasurface exhibits excellent potential in holographic display and optical information encryption. However, most metasurfaces are passive devices due to the limitations of fixed material properties and structural components. Magneto-optical metasurface is a hybrid device that integrates tunable functional material with elaborately designed nanostructures. It can realize dynamic modulation of the properties of light since the permittivity tensor for the magneto-optical material can be changed by applying an external magnetic field. Here, we propose a tunable metasurface composing metallic nanohole arrays with a bismuth-substituted yttrium iron garnet interleave layer and a metallic film underlayer placed on a glass substrate. The magneto-optical metasurface can achieve dynamic switchable holographic display in different polarization channels via magnetic field control based on the optical rotation of magnetic material and the complex amplitude modulation of the elaborately designed nanoholes. This feature provides a novel approach for the construction of an active tunable metasurface, which can improve the information storage capacity and security of the device. This concept is expected to be applied to various dynamic modulation fields, such as magnetically tunable lens, beam shaping, and optical information encryption.

Enhanced Faraday effects of magneto-plasmonic crystals with plasmonic hexagonal hole arrays

Magneto-optical (MO) properties of the bilayed Au/BIG and trilayered Au/BIG/Au magneto-plasmonic crystals (MPCs) were analyzed by the finite-difference time-domain method. In contrast to the low deflection angle and transmission of the smooth thin film, all the heterostructures with perforated holes in the top Au film displayed a similar trend with two strong resonant bands in Faraday rotation and transmittance in the near infrared wavelength range. The bands and electric distribution relative to the component and hole structure were revealed. The MPC with plasmonic hexagonal holes was found to own superior Faraday effects with distinctive anisotropy. The evolution of the resonant bands with the size and period of hexagonal holes, the thickness of different layers, and the incident light polarization was illustrated. The Faraday rotation of the optimized bilayed and trilayered hexagonal MPCs was improved 15.3 and 17.5 times, and the transmittance was enhanced 12.1 and 11.1 folds respectively at the resonant wavelength in comparison to the continuous Au/BIG film, indicating that the systems might find potential application in MO devices.

Magneto-Optical Activity in Nonmagnetic Hyperbolic Nanoparticles

Active nanophotonics can be realized by controlling the optical properties of materials with external magnetic fields. Here, we explore the influence of optical anisotropy on the magneto-optical activity in nonmagnetic hyperbolic nanoparticles. We demonstrate that the magneto-optical response is driven by the hyperbolic dispersion via the coupling of metallic-induced electric and dielectric-induced magnetic dipolar optical modes with static magnetic fields. Magnetic circular dichroism experiments confirm the theoretical predictions and reveal tunable magneto-optical activity across the visible and near infrared spectral range.

Fiber Optic Sensors Based on the Faraday Effect

Some 175 years ago Michael Faraday discovered magnetic circular birefringence, now commonly known as the Faraday effect. Sensing the magnetic field through the influence that the field has on light within the fiber optic sensor offers several advantages, one of them fundamental. These advantages find application in the measurement of electric current at high voltages by measuring the induced magnetic field, thus warranting application for this kind of fiber optic sensor (FOS) in future smart grids. Difficulties in designing and manufacturing high-performance FOSs were greatly alleviated by developments in optical telecommunication technology, thus giving new impetus to magnetometry based on the Faraday effect. Some of the major problems in the processing of optical signals and temperature dependence have been resolved, yet much effort is still needed to implement all solutions into a single commercial device. Artificial structures with giant Faraday rotation, reported in the literature in the 21st century, will further improve the performance of FOSs based on the Faraday effect. This paper will consider obstacles and limits imposed by the available technology and review solutions proposed so far for fiber optic sensors based on the Faraday effect.

High-Performance Asymmetric Optical Transmission Based on a Dielectric-Metal Metasurface

Asymmetric optical transmission plays a key role in many optical systems. In this work, we propose and numerically demonstrate a dielectric-metal metasurface that can achieve high-performance asymmetric transmission for linearly polarized light in the near-infrared region. Most notably, it supports a forward transmittance peak (with a transmittance of 0.70) and a backward transmittance dip (with a transmittance of 0.07) at the same wavelength of 922 nm, which significantly enhances operation bandwidth and the contrast ratio between forward and backward transmittances. Mechanism analyses reveal that the forward transmittance peak is caused by the unidirectional excitation of surface plasmon polaritons and the first Kerker condition, whereas the backward transmittance dip is due to reflection from the metal film and a strong toroidal dipole response. Our work provides an alternative and simple way to obtain high-performance asymmetric transmission devices.

Tunable Transmissive Terahertz Linear Polarizer for Arbitrary Linear Incidence Based on Low-Dimensional Metamaterials

In this work, we propose a structure consisting of three metamaterial layers and a metallic grating layer to rotate the polarization of arbitrary linearly polarized incidence to the y-direction with high transmissivity by electrically tuning these metamaterials. The transfer matrix method together with a harmonic oscillator model is adopted to theoretically study the proposed structure. Numerical simulation based on the finite difference time-domain method is performed assuming that the metamaterial layers are constituted by graphene ribbon arrays. The calculation and simulation results show that the Drude absorption is responsible for the polarization rotation. Fermi level and scattering rate of graphene are important for the transmissivity. For a polarization rotation of around 90°, the thickness of either the upper or lower dielectric separations influences the transmission window. For a polarization rotation of around 45° and 135°, the lower dielectric separations decide the frequency of the transmission window, while the upper dielectric separations just slightly influence the transmissivity.

All-optical nonreciprocity due to valley polarization pumping in transition metal dichalcogenides

Nonreciprocity and nonreciprocal optical devices play a vital role in modern photonic technologies by enforcing one-way propagation of light. Here, we demonstrate an all-optical approach to nonreciprocity based on valley-selective response in transition metal dichalcogenides (TMDs). This approach overcomes the limitations of magnetic materials and it does not require an external magnetic field. We provide experimental evidence of photoinduced nonreciprocity in a monolayer WS2 pumped by circularly polarized (CP) light. Nonreciprocity stems from valley-selective exciton population, giving rise to nonlinear circular dichroism controlled by CP pump fields. Our experimental results reveal a significant effect even at room temperature, despite considerable intervalley-scattering, showing promising potential for practical applications in magnetic-free nonreciprocal platforms. As an example, here we propose a device scheme to realize an optical isolator based on a pass-through silicon nitride (SiN) ring resonator integrating the optically biased TMD monolayer.

Magnetic Control of Optical Reflectance from Metallic Thin Film Using Surface Plasmon Resonance and Faraday Rotation

We demonstrate magnetic control of optical reflectance with no ferromagnetic material via combining the Faraday rotation and the surface plasmon resonance (SPR) in a Kretschman configuration under magnetic fields < 0.5 T. The SPR produces the polarization sensitive reflectance from the Au or Ag thin film coated on a N-BK7 prism in which the Faraday rotation occurs. The gold (Au) or silver (Ag) metal film as a plasmonic film somewhat acts as an incident angle-dependent reflection polarizer that can sensitively sense the polarization change induced by the Faraday rotation that occurs in a prism. We find that combination of Faraday rotation and the surface plasmon can induce a significant magnetic modulation of reflectance normalized with respect to that obtained with no magnetic fields at a specific incident angle of light. The magnetic control of optical reflectance presented may find an application in polarizer-free photonic devices with no ferromagnetic material for magneto-optical modulation.

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.

Nonreciprocal and Topological Plasmonics

Metals, semiconductors, metamaterials, and various two-dimensional materials with plasmonic dispersion exhibit numerous exotic physical effects in the presence of an external bias, for example an external static magnetic field or electric current. These physical phenomena range from Faraday rotation of light propagating in the bulk to strong confinement and directionality of guided modes on the surface and are a consequence of the breaking of Lorentz reciprocity in these systems. The recent introduction of relevant concepts of topological physics, translated from condensed-matter systems to photonics, has not only given a new perspective on some of these topics by relating certain bulk properties of plasmonic media to the surface phenomena, but has also led to the discovery of new regimes of truly unidirectional, backscattering-immune, surface-wave propagation. In this article, we briefly review the concepts of nonreciprocity and topology and describe their manifestation in plasmonic materials. Furthermore, we use these concepts to classify and discuss the different classes of guided surface modes existing on the interfaces of various plasmonic systems.

Self-Isolated Raman Lasing with a Chiral Dielectric Metasurface

The light sources that power photonic networks are small and scalable, but they also require the incorporation of optical isolators that allow light to pass in one direction only, protecting the light source from damaging backreflections. Unfortunately, the size and complex integration of optical isolators makes small-scale and densely integrated photonic networks infeasible. Here, we overcome this limitation by designing a single device that operates both as a coherent light source and as its own optical isolator. Our design relies on high-quality-factor dielectric metasurfaces that exhibit intrinsic chirality. By carefully manipulating the geometry of the constituent silicon metaatoms, we design three-dimensionally chiral modes that act as optical spin-dependent filters. Using spin-polarized Raman scattering together with our chiral metacavity, we demonstrate Raman lasing in the forward direction, while the lasing action is suppressed by over an order of magnitude for reflected light. Our high-Q chiral metasurface design presents a new approach toward compactly isolating integrated light sources by directly tailoring the emission properties of the light source itself.

Terahertz Broadband Polarization Conversion for Transmitted Waves Based on Graphene Plasmon Resonances

We applied the harmonic oscillator model combined with the transfer matrix method to study the polarization conversion for transmitted waves in metallic grating/plasmon-excitation layer/metallic grating structure in the terahertz (THz) region. By comparing the calculated spectra and the simulated (by the finite-difference-time-domain method) ones, we found that they correspond well with each other. Both methods show that the Drude background absorption and the excited plasmon resonances are responsible for polarization conversion. The transmission is close to 0 when the distance between the top/bottom metallic gratings and gated graphene is an integer multiple of half the wavelength of the incident wave (in the dielectrics), at which points the plasmon resonances are greatly suppressed by the destructive interference between the backward/forward electromagnetic waves and that reflected by the top/bottom metallic gratings. Away from these points, the transmission can be higher than 80%. The electron density and the excitation efficiency of the plasmon-excitation layer were found to be important for the bandwidth of the polarization conversion window, while the scattering rate was found to influence mainly the polarization conversion rate. Multi-broadband polarization conversion is realized by exciting plasmon modes between the 0 transmission points in the THz region.

Nonreciprocal elasticity and the realization of static and dynamic nonreciprocity

The realization of the mechanical nonreciprocity requires breaking either the time-reversal symmetry or the material deformation symmetry. The time-reversal asymmetry was the commonly adopted approach to realize dynamic nonreciprocity. However, a static nonreciprocity requires—with no any other option—breaking the material deformation symmetry. By virtue of the Maxwell–Betti reciprocal theorem, the achievement of the static nonreciprocity seems to be conditional by the use of a nonlinear material. Here, we further investigate this and demonstrate a novel “nonreciprocal elasticity” concept. We investigated the conditions of the attainment of effective static nonreciprocity. We revealed that the realization of static nonreciprocity requires breaking the material deformation symmetry under the same kinematical and kinetical conditions, which can be achieved only and only if the material exhibits a nonreciprocal elasticity. By means of experimental and topological mechanics, we demonstrate that the realization of static nonreciprocity requires nonreciprocal elasticity no matter what the material is linear or nonlinear. We experimentally demonstrated linear and nonlinear metamaterials with nonreciprocal elasticities. The developed metamaterials were used to demonstrate that nonreciprocal elasticity is essential to realize static nonreciprocal-topological systems. The nonreciprocal elasticity developed here will open new venues of the design of metamaterials that can effectively break the material deformation symmetry and achieve, both, static and dynamic nonreciprocity.

Enhanced Faraday rotation in proximitized monolayer transition metal dichalcogenides

Monolayer transition metal dichalcogenides (TMDCs) under the application of a magnetic exchange field carry the nontrivial optical Hall conductivity and thus exhibit the nonzero Faraday rotation (FR) angle. However, the tradeoff between the FR angle and transmission hinders their possible applications in magnetic-optical (MO) devices. Here, we theoretically show that a heterostructure of two photonic crystals with proximitized monolayer TMDCs enables the enhancement of the FR angle and transmission simultaneously through the combination of a four-band Hamiltonian model, Kubo formula and transfer matrix method. The MO improvement in the hybrid structure in both the FR angle and transmission is determined by the combined effects from the localized electromagnetic field at the interface between the two photonic crystals and the satisfaction of the phase match condition. Our work opens up an alternative opportunity to use TMDCs in two-dimensional MO fields in the visible frequency range.

Giant nonreciprocal transmission in low-biased gyrotropic metasurfaces

Strong magneto-optical effect with low external magnetic field is of great importance to achieve high-performance isolators in modern optics. Here, we experimentally demonstrate a significant enhancement of the magneto-optical effect and nonreciprocal chiral transmission in low-biased gyrotropic media. A designer magneto-optical metasurface consists of a gyrotropy-near-zero slab doped with magnetic resonant inclusions. The immersed magnetic dopants enable efficient nonreciprocal light-matter interactions at the subwavelength scale, providing a giant macroscopic nonreciprocity and strong robustness against the bias disturbance. Microwave measurements reveal that the metasurface can act as a chiral isolator for circular polarization, with extremely weak intrinsic gyromagnetic activity. We also demonstrate its capability of signal isolation for circularly polarized antennas. Our findings provide an experimental verification of nonreciprocal photonic doping with low static magnetic fields.

All-dielectric magnetic metasurface for advanced light control in dual polarizations combined with high-Q resonances

Nanostructured magnetic materials provide an efficient tool for light manipulation on sub-nanosecond and sub-micron scales, and allow for the observation of the novel effects which are fundamentally impossible in smooth films. For many cases of practical importance, it is vital to observe the magneto-optical intensity modulation in a dual-polarization regime. However, the nanostructures reported on up to date usually utilize a transverse Kerr effect and thus provide light modulation only for p-polarized light. We present a concept of a transparent magnetic metasurface to solve this problem, and demonstrate a novel mechanism for magneto-optical modulation. A 2D array of bismuth-substituted iron-garnet nanopillars on an ultrathin iron-garnet slab forms a metasurface supporting quasi-waveguide mode excitation. In contrast to plasmonic structures, the all-dielectric magnetic metasurface is shown to exhibit much higher transparency and superior quality-factor resonances, followed by a multifold increase in light intensity modulation. The existence of a wide variety of excited mode types allows for advanced light control: transmittance of both p- and s-polarized illumination becomes sensitive to the medium magnetization, something that is fundamentally impossible in smooth magnetic films. The proposed metasurface is very promising for sensing, magnetometry and light modulation applications.

The authors fabricate and investigate the metasurface made of 2D iron-garnet subwavelength nanopillar array on a thin iron-garnet film. It exhibits high quality-factor resonances, leading to a multifold increase in light intensity modulation of the transmitted light with an advantage of P and S polarizations both sensitive to the medium magnetization.

Boosting the figure of merit of refractive index sensing via magnetoplasmon in H-shaped magnetoplasmonic crystals

Nanoscale refractive index (RI) sensors based on plasmonic structures usually suffer from a low figure of merit (FoM) due to the broad linewidth of the resonance peaks. Here, we report a magnetoplasmon-based RI sensing method with high FoM in the designed H-shaped magnetoplasmonic crystals. Instead of the light intensity spectrum, the Faraday signal is detected to analyze the changes of the surrounding RI. Sharp resonance with extremely narrow linewidth is obtained by plotting the reciprocal Faraday rotation near the null point region. Therefore, the FoM is hugely enhanced, and a theoretical value exceeding 1775/RIU is achieved, which is one order of magnitude higher than has ever been reported, to the best of our knowledge, for the RI sensor based on the Faraday effect. The Faraday reversal and the enhanced FoM arise from the Fano resonance. These findings are of potential value for practical high performance biochemical sensors.

Ultrafast tunable integrated Faraday isolator based on optical pumping in a graphene-InSb-graphene structure

We have designed a new class of single-stage Faraday isolators that employs two sheets of graphene. Using nonreciprocal optical materials such as InSb boosts plasmonic coupling between the graphene layers, which leads to a 45 deg polarization rotation as well as increased transmission through the structure. More than 91% transmission is achieved in the THz band (1 THz-7 THz). This design opens up the way in ultrafast integrated magneto-optical nanophotonic devices to realize single-stage isolators with enhanced transmittance in the THz band.

All-Optical Manipulation of Magnetization in Ferromagnetic Thin Films Enhanced by Plasmonic Resonances

In this paper, we report all-optical manipulation of magnetization in ferromagnetic Co/Pt thin films enhanced by plasmonic resonances. By annealing a thin Au layer, we fabricate large-area Au nanoislands on top of the Co/Pt magnetic thin films, which show plasmonic resonances around the wavelength of 606 nm. Using a customized magneto-optical Kerr effect setup, we experimentally observe an 18.5% decrease in the minimum laser power required to manipulate the magnetization, comparing the on- and off-resonance conditions. The results are in very good agreement with numerical simulations. Our research findings demonstrate the possibility to achieve an all-optical magnetic recording with low energy consumption, low cost, and high areal density by integrating plasmonic nanostructures with magnetic media.

Characterisation and Manipulation of Polarisation Response in Plasmonic and Magneto-Plasmonic Nanostructures and Metamaterials

Optical properties of metal nanostructures, governed by the so-called localised surface plasmon resonance (LSPR) effects, have invoked intensive investigations in recent times owing to their fundamental nature and potential applications. LSPR scattering from metal nanostructures is expected to show the symmetry of the oscillation mode and the particle shape. Therefore, information on the polarisation properties of the LSPR scattering is crucial for identifying different oscillation modes within one particle and to distinguish differently shaped particles within one sample. On the contrary, the polarisation state of light itself can be arbitrarily manipulated by the inverse designed sample, known as metamaterials. Apart from polarisation state, external stimulus, e.g., magnetic field also controls the LSPR scattering from plasmonic nanostructures, giving rise to a new field of magneto-plasmonics. In this review, we pay special attention to polarisation and its effect in three contrasting aspects. First, tailoring between LSPR scattering and symmetry of plasmonic nanostructures, secondly, manipulating polarisation state through metamaterials and lastly, polarisation modulation in magneto-plasmonics. Finally, we will review recent progress in applications of plasmonic and magneto-plasmonic nanostructures and metamaterials in various fields.

All-Dielectric Nanophotonics Enables Tunable Excitation of the Exchange Spin Waves

Launching and controlling magnons with laser pulses opens up new routes for applications including optomagnetic switching and all-optical spin wave emission and enables new approaches for information processing with ultralow energy dissipation. However, subwavelength light localization within the magnetic structures leading to efficient magnon excitation that does not inherently absorb light has still been missing. Here, we propose to marriage the laser-induced ultrafast magnetism and nanophotonics to efficiently excite and control spin dynamics in magnetic dielectric structures. We demonstrate that nanopatterning by a 1D grating of trenches allows localization of light in spots with sizes of tens of nanometers and thus launch the exchange standing spin waves of different orders. The relative amplitude of the exchange and magnetostatic spin waves can be adjusted on demand by modifying laser pulse polarization, incidence angle, and wavelength. Nanostructuring of the magnetic media provides a unique possibility for the selective spin manipulation, a key issue for further progress of magnonics, spintronics, and quantum technologies.

Enhanced magnetic modulation of light polarization exploiting hybridization with multipolar dark plasmons in magnetoplasmonic nanocavities

Enhancing magneto-optical effects is crucial for reducing the size of key photonic devices based on the non-reciprocal propagation of light and to enable active nanophotonics. Here, we disclose a currently unexplored approach that exploits hybridization with multipolar dark modes in specially designed magnetoplasmonic nanocavities to achieve a large enhancement of the magneto-optically induced modulation of light polarization. The broken geometrical symmetry of the design enables coupling with free-space light and hybridization of the multipolar dark modes of a plasmonic ring nanoresonator with the dipolar localized plasmon resonance of the ferromagnetic disk placed inside the ring. This hybridization results in a low-radiant multipolar Fano resonance that drives a strongly enhanced magneto-optically induced localized plasmon. The large amplification of the magneto-optical response of the nanocavity is the result of the large magneto-optically induced change in light polarization produced by the strongly enhanced radiant magneto-optical dipole, which is achieved by avoiding the simultaneous enhancement of re-emitted light with incident polarization by the multipolar Fano resonance. The partial compensation of the magneto-optically induced polarization change caused by the large re-emission of light with the original polarization is a critical limitation of the magnetoplasmonic designs explored thus far and that is overcome by the approach proposed here.

Longitudinal magneto-optical effect enhancement with high transmission through a 1D all-dielectric resonant guided mode grating

A significant enhancement of the longitudinal magneto-optical effect is demonstrated numerically and experimentally in transmission, and for small angles of incidence, through a subwavelength resonant structure consisting of a dielectric grating on top of a magneto-optical waveguide. The enhanced polarization rotation is associated with a high transmittance. These low footprint devices may thus be suitable for applications like magnetic field sensors or in non-destructive testing.

Enhancement of Both Faraday and Kerr Effects with an All-Dielectric Grating Based on a Magneto-Optical Nanocomposite Material

We report on the design, fabrication, and characterization of an all-dielectric one-dimensional (1D) resonant device formed by a silicon nitride grating impregnated by a low-index magneto-optical silica-type matrix. This impregnation is realized through the dipping of the 966 nm periodic template in a sol-gel solution previously doped with CoFe₂O₄ nanoparticles, and able to fill the grating slits. By a proper adjustment of the geometrical parameters of such a photonic crystal membrane, simultaneous excitation of transverse electric (TE) and transverse magnetic (TM) polarization resonances is nearly achieved at 1570 nm. This TE/TM phase-matching situation leads to a fivefold enhancement of the Faraday effect in the resonance area with an increased merit factor of 0.32°. Moreover, the device demonstrates its ability to enhance longitudinal and transverse Kerr effects for the other directions of the applied magnetic field. Taking benefits from the ability of the nanocomposite material to be processed on photonic platforms, and despite its quite low magneto-optical activity compared to classical magnetic materials, this work proves that an all-dielectric 1D device can produce a high magneto-optical sensitivity to every magnetic field directions.

Absence of unidirectionally propagating surface plasmon-polaritons at nonreciprocal metal-dielectric interfaces

In the presence of an external magnetic field, the surface plasmon polariton that exists at the metal-dielectric interface is believed to support a unidirectional frequency range near the surface plasmon frequency, where the surface plasmon polariton propagates along one but not the opposite direction. Recent works have pointed to some of the paradoxical consequences of such a unidirectional range, including in particular the violation of the time-bandwidth product constraint that should otherwise apply in general in static systems. Here we show that such a unidirectional frequency range is nonphysical using both a general thermodynamic argument and a detailed calculation based on a nonlocal hydrodynamic Drude model for the metal permittivity. Our calculation reveals that the surface plasmon-polariton at metal-dielectric interfaces remains bidirectional for all frequencies.

The local Drude model predicts that, under certain conditions, surface plasmon polaritons at a metal-dielectric surface have a frequency range where only unidirectional propagation is supported. Here, the authors show that in more realistic non-local models surface plasmon polaritons exhibit bidirectional propagation for all frequencies.

Optical nonreciprocity and slow light in coupled spinning optomechanical resonators

We study the optical transmission characteristics of coupled spinning optomechanical resonators with pump-probe driven lasers. Under the steady-state conditions, we focus on how changing the optical Sagnac effect due to same or opposite spinning directions of the resonators can give rise to non-reciprocal and delayed probe light transmission. We find that coupled resonators can exhibit distinct transmission features, can generate negative group delays (slow as well as fast light) and offer additional control of the probe light transmission as compared to the case of a single spinning resonator. Our results can be useful in achieving chiral light propagation in quantum communication technologies without using traditional magneto-optical means.

Surface plasmon polaritons in thin-film Weyl semimetals

We theoretically investigate surface plasmon polaritons propagating in the thin-film Weyl semimetals. We show how the properties of surface plasmon polaritons are affected by hybridization between plasmons localized at the two metal-dielectric interfaces. Generally, this hybridization results in new mixed plasmon modes, which are called short-range surface plasmons and long-range surface plasmons, respectively. We calculate dispersion curves of these mixed modes for three principle configurations of the axion vector describing axial anomaly in Weyl semimetals. We show that the partial lack of the dispersion and the non-reciprocity can be controlled by fine-tuning of the thickness of the Weyl semimetals, the dielectric constants of the outer insulators, and the direction of the axion vector.

Nanoscale nonreciprocity via photon-spin-polarized stimulated Raman scattering

Time reversal symmetry stands as a fundamental restriction on the vast majority of optical systems and devices. The reciprocal nature of Maxwell’s equations in linear, time-invariant media adds complexity and scale to photonic diodes, isolators, circulators and also sets fundamental efficiency limits on optical energy conversion. Though many theoretical proposals and low frequency demonstrations of nonreciprocity exist, Faraday rotation remains the only known nonreciprocal mechanism that persists down to the atomic scale. Here, we present photon-spin-polarized stimulated Raman scattering as a new nonreciprocal optical phenomenon which has, in principle, no lower size limit. Exploiting this process, we numerically demonstrate nanoscale nonreciprocal transmission of free-space beams at near-infrared frequencies with a 250 nm thick silicon metasurface as well as a fully-subwavelength plasmonic gap nanoantenna. In revealing all-optical spin-splitting, our results provide a foundation for compact nonreciprocal communication and computing technologies, from nanoscale optical isolators and full-duplex nanoantennas to topologically-protected networks.

Here, the authors introduce and study theoretically and numerically a scheme for breaking time-reversal symmetry and achieving nonreciprocity on the nanoscale, using spin-selective stimulated Raman scattering. These results could pave the way for compact nonreciprocal communication and computing technologies.

Enhancing the magneto-optical effects in low-biased gyromagnetic media via photonic doping

Enhancing nonreciprocal light-matter interaction at subwavelength scales has attracted enormous attention due to high demand for compact optical isolators. Here, we propose a significant enhancement of the magneto-optical effect in low-biased gyromagnetic media via photonic doping. Magnetic particles immersed in a gyrotropy-near-zero medium act as dopants that largely modify the macroscopic gyromagnetic effects as well as the gyroelectric ones. Around the resonance frequency, the gyromagnetic activity is largely increased and even exceeds unity, thus providing a photonic band in which the wavenumber of one circularly polarized wave becomes purely imaginary. The sign of gyromagnetic activity flips at two chiral modes, and an equivalent switching of the external bias is revealed. A proof-of-concept low-biased planar isolator is designed with a thickness of only 1/28 wavelength and a degree of isolation achieving as high as 0.94. This methodology is robust against disturbance of the biased magnetic field and can be flexibly extended to other frequencies, thus offering a promising platform to achieve giant optical isolation with infinitesimally intrinsic magneto-optical effects and reduced sizes.

Polarization-insensitive tunable terahertz polarization rotator

A rotation-angle variable polarization rotator is proposed and demonstrated using an all-dielectric metasurface doublet. Such a transmissive device can conveniently rotate the polarization of incident light by any desired angles by mechanically changing the relative angle of the double metasurface layers regardless of the incident state of polarization. Under certain conditions the device acts as a full phase modulator for the circularly polarized incident wave. Hence, it has a promising application in polarization and phase control.

Surface Plasmon Resonance (SPR) to Magneto-Optic SPR

In this editorial, a brief background of the surface plasmon resonance (SPR) principle is discussed, followed by several aspects of magneto-optic SPR (MOSPR) and sensing schemes from the viewpoint of fundamental studies and potential technological applications. New sensitivity metrics are introduced that would allow researchers to compare the performance of SPR and MOSPR-based sensors. Merits of MOSPR over SPR based sensors and challenges faced by MOSPR sensors in terms of their practical use and portability are also considered. The editorial ends with potential new configurations and future prospects. This work is considered highly significant to device engineers, graduate and undergraduate students, and researchers of all levels involved in developing new classes of bio-devices for sensing, imaging, environmental monitoring, toxic gas detection, and surveying applications to name a few.

RGB Magnetophotonic Crystals for High-contrast Magnetooptical Spatial Light Modulators

Magnetooptical spatial light modulators (MOSLMs) are photonic devices that encode information in photonic waveforms by changing their amplitude and phase using magnetooptical Faraday or Kerr rotation. Despite the progress on both MO materials and switching methods, significant improvements on materials engineering and SLM design are needed for demonstrating low-power, multicolor, analog and high-contrast MOSLM devices. In this study, we present design rules and example designs for a high-contrast and large figure-of-merit MOSLM using three-color magnetophotonic crystals (MPC). We demonstrate for the first time, a three-defect MPC capable of simultaneously enhancing Faraday rotation, and high-contrast modulation at three fundamental wavelengths of red, green and blue (RGB) within the same pixel. We show using 2D finite-difference time-domain simulations that bismuth-substituted yttrium iron garnet films are promising for low-loss and high Faraday rotation MOSLM device in the visible band. Faraday rotation and loss spectra as well as figure-of-merit values are calculated for different magnetophotonic crystals of the form (H/L)^p/(D/L)^q/(H/L)^p. After an optimization of layer thicknesses and MPC configuration, Faraday rotation values were found to be between 20–55° for losses below 20 dB in an overall thickness less than 1.5 µm including three submicron garnet defect layers. The experimental demonstration of our proposed 3-color MOSLM devices can enable bistable photonic projectors, holographic displays, indoor visible light communication devices, photonic beamforming for 5 G telecommunications and beyond.

Magnetoplasmonic nanograting geometry enables optical nonreciprocity sign control

We experimentally demonstrate a disruptive approach to control magnetooptical nonreciprocal effects. It has been known that the combination of a magneto-optically (MO) active substrate and extraordinary transmission (EOT) effects through deep-subwavelength nanoslits of a noble metal grating, leads to giant enhancements of the magnitude of the MO effects that would normally be obtained on just the bar substrate. This was demonstrated both in the transmission configuration, where the OET is directly observed, as well as in reflection configuration, where an increase of a transmitted power results in a decrease in reflected power. We show here that even more than just an enhancement, the MO effects can also undergo a sign reversal by achieving a hybridization of the different types of resonances at play in these EOT nanogratings. By tuning the geometrical profile of the grating's slits, one can engineer - for a fixed wavelength and fixed magnetization - the transverse MO Kerr effect (TMOKE) reflectivity of such a magnetoplasmonic system to be enhanced, extinguished or inversely enhanced. We have fabricated gold gratings with varying nanoslit widths on a Bi-substituted gadolinium iron garnet and experimentally confirmed such a behavior using a customized magneto-optic Mueller matrix ellipsometer. This demonstration allows new design paradigms for integrated nonreciprocal circuits and biochemical sensors with increased sensitivity and reduced footprint.