Theory of wave propagation in magnetized near-zero-epsilon metamaterials: evidence for one-way photonic states and magnetically switched transparency and opacity

Probing Topological Thermal Flux in Equilibrium Using Electron Beams

Near nonreciprocal media at finite temperature, fluctuating near fields exhibit imbalanced thermal populations in opposite directions, generating equilibrium topological thermal fluxes that circulate the media. While the existence of these fluxes remains unconfirmed, we propose exploiting their interaction with free electron beams for detection. We establish a general framework to quantify thermal flux at any location near an object of arbitrary shape. This reveals unexplored properties of thermal flux spectra depending on their orientation. Further, we connect the electron scattering rate to the equilibrium thermal flux. As a specific example, electrons encountering a planar surface's perpendicular thermal flux preferentially scatter transversely. This measurable scattering distribution, i.e., via angle-resolved electron microscopy, allows us to recover the thermal flux spectrum. Additionally, electron interactions with equilibrium thermal fluxes surrounding local structures offer a novel approach to generating electron vortex beams.

Improved performance of temperature sensors based on the one-dimensional topological photonic crystals comprising hyperbolic metamaterials

This paper seeks to progress the field of topological photonic crystals (TPC) as a promising tool in face of construction flaws. In particular, the structure can be used as a novel temperature sensor. In this regard, the considered TPC structure comprising two different PC designs named PC1 and PC2. PC1 is designed from a stack of multilayers containing Silicon (Si) and Silicon dioxide (SiO2), while layers of SiO2 and composite layer named hyperbolic metamaterial (HMM) are considered in designing PC2. The HMM layer is engineered using subwavelength layers of Si and Bismuth Germinate, or BGO (Bi 4 Ge 3 O 12 ). The mainstay of our suggested temperature sensor is mainly based on the emergence of some resonant modes inside the transmittance spectrum that provide the stability in the presence of the geometrical changes. Meanwhile, our theoretical framework has been introduced in the vicinity of transfer matrix method (TMM), effective medium theory (EMT) and the thermo-optic characteristics of the considered materials. The numerical findings have extensively introduced the role of some topological parameters such as layers' thicknesses, filling ratio through HMM layers and the periodicity of HMM on the stability or the topological features of the introduced sensor. Meanwhile, the numerical results reveal that the considered design provides some topological edge states (TESs) of a promising robustness and stability against certain disturbances or geometrical changes in the constituent materials. In addition, our sensing tool offers a relatively high sensitivity of 0.27 nm/°C.

Tunable non-reciprocal waveguide using spoof plasmon polariton coupling to a gaseous magnetoplasmon

We experimentally demonstrate non-reciprocal (one-way) waveguiding in a microstrip transmission line tailored to support the propagation of spoof plasmon polaritons. Time-reversal symmetry is broken by coupling the microstrip fields to a magnetized gaseous plasma discharge column thereby exciting non-reciprocal magnetoplasmons at the interface between the plasma and a surrounding quartz envelope. The magnetic bias introduces asymmetry in the dispersion of the surface plasmon polaritons at the gaseous plasma-dielectric interface, resulting in a breaking of the bidirectionality of the wave propagation in the microstrip. The isolation generated at conditions of modest magnetic bias is measured to be nearly 60 dB, and tunable by varying the plasma density through the voltage applied to the discharge. The advantage of using magnetized gaseous plasmas to produce this unidirectional waveguide structure is that it can be turned on or off at rates limited by the production and recombination of the plasma.

One-way light flow by spatio-temporal modulation

The unidirectional flow of electrons that takes place in a conventional electronic diode has been a cornerstone in the development of the field of electronics. Achieving an equivalent one-way flow for light has been a long-standing problem. While a number of concepts have been suggested recently, attaining a unidirectional flow of light in a two-port system (e.g., a waveguiding configuration) is still challenging. Here, we present what we believe to be a novel approach for breaking reciprocity and achieving one-way flow of light. Taking a nanoplasmonic waveguide as an example, we show that a combination of time-dependent interband optical transitions, when in systems exhibiting a backward wave flow, can yield light transmission strictly in one direction. In our configuration, the energy flow is unidirectional: light is fully reflected in one direction of propagation, and is unperturbed in the other. The concept can find use in a range of applications including communications, smart windows, thermal radiation management, and solar energy harvesting.

Optomagnonics in Dispersive Media: Magnon-Photon Coupling Enhancement at the Epsilon-near-Zero Frequency

Reaching strong light-matter coupling in solid-state systems has long been pursued for the implementation of scalable quantum devices. Here, we put forward a system based on a magnetized epsilon-near-zero (ENZ) medium, and we show that strong coupling between magnetic excitations (magnons) and light can be achieved close to the ENZ frequency due to a drastic enhancement of the magneto-optical response. We adopt a phenomenological approach to quantize the electromagnetic field inside a dispersive magnetic medium in order to obtain the frequency-dependent coupling between magnons and photons. We predict that, in the epsilon-near-zero regime, the single-magnon single-photon coupling can be comparable to the magnon frequency for a small magnetic volume and perfect mode overlap. For state-of-the-art illustrative values, this would correspond to achieving the single-magnon strong coupling regime, where the coupling rate is larger than all the decay rates. Finally, we show that the nonlinear energy spectrum intrinsic to this coupling regime can be probed via the characteristic multiple magnon sidebands in the photon power spectrum.

Nonreciprocal and Non-Hermitian Material Response Inspired by Semiconductor Transistors

Here, inspired by the operation of conventional semiconductor transistors, we introduce a novel class of bulk materials with nonreciprocal and non-Hermitian electromagnetic response. Our analysis shows that material nonlinearities combined with a static electric bias may lead to a linearized permittivity tensor that lacks the Hermitian and transpose symmetries. Remarkably, the material can either dissipate or generate energy, depending on the relative phase of the electric field components. We introduce a simple design for an electromagnetic isolator based on an idealized "MOSFET-metamaterial" and show that its performance can in principle surpass conventional Faraday isolators due to the material gain. Furthermore, it is suggested that analogous material responses may be engineered in natural media in nonequilibrium situations. Our solution determines an entirely novel paradigm to break the electromagnetic reciprocity in a bulk nonlinear material using a static electric bias.

Inelastic Scattering of Electron Beams by Nonreciprocal Nanotructures

Probing optical excitations with high resolution is important for understanding their dynamics and controlling their interaction with other photonic elements. This can be done using state-of-the-art electron microscopes, which provide the means to sample optical excitations with combined meV-sub-nm energy-space resolution. For reciprocal photonic systems, electrons traveling in opposite directions produce identical signals, while this symmetry is broken in nonreciprocal structures. Here, we theoretically investigate this phenomenon by analyzing electron energy-loss spectroscopy (EELS) and cathodoluminescence (CL) for structures consisting of magnetically biased InAs as an instance of gyrotropic nonreciprocal material. We find that the spectral features associated with excitations of InAs films depend on the electron propagation direction in both EELS and CL, and can be tuned by varying the applied magnetic field within a relatively modest subtesla regime. The magnetic field modifies the optical field distribution of the sampled resonances, and this in turn produces a direction-dependent coupling to the electron. The present results pave the way to the use of electron microscope spectroscopies to explore the near-field characteristics of nonreciprocal systems with high spatial resolution.

Physical Violations of the Bulk-Edge Correspondence in Topological Electromagnetics

In this Letter, we discuss two general classes of apparent violations of the bulk-edge correspondence principle for continuous topological photonic materials, associated with the asymptotic behavior of the surface modes for diverging wave numbers. Considering a nonreciprocal plasma as a model system, we show that the inclusion of spatial dispersion (e.g., hydrodynamic nonlocality) formally restores the bulk-edge correspondence by avoiding an unphysical response at large wave numbers. Most importantly, however, our findings show that, for the considered cases, the correspondence principle is physically violated for all practical purposes, as a result of the unavoidable attenuation of highly confined modes even if all materials are assumed perfect, with zero intrinsic bulk losses, due to confinement-induced Landau damping or nonlocality-induced radiation leakage. Our work helps clarifying the subtle and rich topological wave physics of continuous media.

All-optical logic gates based on unidirectional surface plasmon polaritons

Optical logic gates are important elements in optical computing and optical circuits. In this study, all-optical logic gates, including OR, XOR, XNOR, NAND, and NOT gates, are obtained in a composite structure composed of gyroelectric material and metal based on the interference and absorption of surface waves. In the composite structure, a Y-shaped channel through which surface plasmon polaritons propagate unidirectionally is formed. Through the ingenious one-way surface plasmon polaritons, backscattering and side-scattering generated by the composite structure and influencing the surrounding devices are prohibited effectively. The precisely controlled optical phase difference ensures the realization of five different logic gates with high contrast ratios. The maximum contrast ratios for XOR, XNOR, NAND, and NOT gates are 31.64 dB, 43.47 dB, 31.47 dB, and 43.47 dB, respectively. The output logic states "1"or "0" obtained by different logic inputs have nearly the same light intensity. Our research results provide the possibility for constructing all-optical integrated logic devices.

Analog of giant magnetoimpedance in magnetized ε-near-zero plasma

In this Letter, we show that magnetized plasma with properly designed parameters at plasma frequency suffers a large magnetoimpedance change under a moderate external magnetic field. Such an interesting feature can be described as the analog of giant magnetoimpedance (GMI), which is confirmed by detailed calculation. GMI devices based on magnetized ε-near-zero plasma do not require microfabrication, and they are capable of working at tunable frequency, even in terahertz frequency range; more importantly, they can be dynamically controlled by environmental parameters such as temperature and pressure.

Topologically-protected one-way leaky waves in nonreciprocal plasmonic structures

We investigate topologically-protected unidirectional leaky waves on magnetized plasmonic structures acting as homogeneous photonic topological insulators. Our theoretical analyses and numerical experiments aim at unveiling the general properties of these exotic surface waves, and their nonreciprocal and topological nature. In particular, we study the behavior of topological leaky modes in stratified structures composed of a magnetized plasma at the interface with isotropic conventional media, and we show how to engineer their propagation and radiation properties, leading to topologically-protected backscattering-immune wave propagation, and highly directive and tunable radiation. Taking advantage of the non-trivial topological properties of these leaky modes, we also theoretically demonstrate advanced functionalities, including arbitrary re-routing of leaky waves on the surface of bodies with complex shapes, as well as the realization of topological leaky-wave (nano)antennas with isolated channels of radiation that are completely independent and separately tunable. Our findings help shedding light on the behavior of topologically-protected modes in open wave-guiding structures, and may open intriguing directions for future antenna generations based on topological structures, at microwaves and optical frequencies.

Electric-current-induced unidirectional propagation of surface plasmon-polaritons

Nonreciprocity and one-way propagation of optical signals are crucial for modern nanophotonic technology, and typically achieved using magneto-optical effects requiring large magnetic biases. Here we suggest a fundamentally novel approach to achieve unidirectional propagation of surface plasmon-polaritons (SPPs) at metal-dielectric interfaces. We employ a direct electric current in metals, which produces a Doppler frequency shift of SPPs due to the uniform drift of electrons. This tilts the SPP dispersion, enabling one-way propagation, as well as zero and negative group velocities. The results are demonstrated for planar interfaces and cylindrical nanowire waveguides.

Interface States and Interface-Bulk Correspondence of One-dimensional Hyperbolic Metamaterials

We investigate the interface state on one-dimensional hyperbolic metamaterial (1DHMM). Initially, we analyze the plasmonic band structure of binary 1DHMM and analytically determine its band crossing condition. Then, we scrutinize the existence of an interface state in the plasmonic band gap of 1DHMM on three types of interfaces: dielectric/1DHMM, metal/1DHMM, and 1DHMM/1DHMM. We find that the band crossing dramatically influences the existence of an interface state. We also show a rigorous relation between the existence of the interface state of 1DHMM in the plasmonic band gap and the wave admittance in the plasmonic band region. More importantly, this relation not only holds for binary 1DHMM but also can be generalized to any 1DHMM with inversion symmetry. We also characterize the interface state by the transverse spin angular momentum and reveal the transverse spin flipping of the interface state.

Optical bistability in core-shell magnetoplasmonic nanoparticles with magnetocontrollability

We propose a mechanism to actively tune optical bistable behavior with the external magnetic field in nonlinear coated nanospheres with a magneto-optical (MO) shell and nonlinear metallic core. We show that such nanostructures can exhibit typical bistable phenomena near surface plasmon resonant wavelengths, which can be modified through the external magnetic fields B. We demonstrate numerically that the optical bistability exists only when the volume fraction η of the metallic core is larger than a critical one η_c. Moreover, the bistable behavior is found to be dependent on the incident polarization state as well as the external magnetic field. The application of an external magnetic field does not only increase (or decrease) the upper/lower threshold fields but also changes the critical volume fractions. Such nanostructures with magneto-controllable optical bistability may be designed for us as nonlinear optical nanodevices, such as optical nanoswitches, nanosensors and so on.

The effects of three-dimensional defects on one-way surface plasmon propagation for photonic topological insulators comprised of continuum media

We have investigated one-way surface plasmon-polaritons (SPPs) at the interface of a continuum magnetoplasma material and metal, in the presence of three-dimensional surface defects. Bulk electromagnetic modes of continuum materials have Chern numbers, analogous to those of photonic crystals. This can lead to the appearance of topologically-protected surface modes at material interfaces, propagating at frequencies inside the bandgap of the bulk materials. Previous studies considered two-dimensional structures; here we consider the effect of three-dimensional defects, and show that, although backward propagation/reflection cannot occur, side scattering does take place and has significant effect on the propagation of the surface mode. Several different waveguiding geometries are considered for reducing the effects of side-scattering, and we also consider the effects of metal loss.

One-way helical electromagnetic wave propagation supported by magnetized plasma

In this paper we reveal the presence of photonic one-way helical surface states in a simple natural system- magnetized plasma. The application of an external magnetic field to a bulk plasma body not only breaks time-reversal-symmetry but also leads to separation of Equi-Frequency Contour surfaces (EFCs) to form topologically nontrivial gaps in k space. Interestingly, these EFCs support topologically protected surface states. We numerically investigate an interface between magnetized plasma, using a realistic model for parameter dispersion, and vacuum, to confirm the existence of one-way scatter-immune helical surface states. Unlike previous proposals for achieving photonic one-way propagation, our scheme does not require the use of artificial structures and should therefore be simple to implement experimentally.

Topological properties of linear circuit lattices

Motivated by the topologically insulating circuit of capacitors and inductors proposed and tested by Jia et al. [arXiv:1309.0878], we present a related circuit with fewer elements per site. The normal mode frequency matrix of our circuit is unitarily equivalent to the hopping matrix of a quantum spin Hall insulator, and we identify perturbations that do not backscatter the circuit's edge modes. The idea behind these models is generalized, providing a platform to simulate tunable and locally accessible lattices with arbitrary complex spin-dependent hopping of any range. A simulation of a non-Abelian Aharonov-Bohm effect using such linear circuit designs is discussed.

Magneto-Plasmonics and Resonant Interaction of Light with Dynamic Magnetisation in Metallic and All-Magneto-Dielectric Nanostructures

A significant interest in combining plasmonics and magnetism at the nanoscale gains momentum in both photonics and magnetism sectors that are concerned with the resonant enhancement of light-magnetic-matter interaction in nanostructures. These efforts result in a considerable amount of literature, which is difficult to collect and digest in limited time. Furthermore, there is insufficient exchange of results between the two research sectors. Consequently, the goal of this review paper is to bridge this gap by presenting an overview of recent progress in the field of magneto-plasmonics from two different points of view: magneto-plasmonics, and magnonics and magnetisation dynamics. It is expected that this presentation style will make this review paper of particular interest to both general physical audience and specialists conducting research on photonics, plasmonics, Brillouin light scattering spectroscopy of magnetic nanostructures and magneto-optical Kerr effect magnetometry, as well as ultrafast all-optical and THz-wave excitation of spin waves. Moreover, readers interested in a new, rapidly emerging field of all-dielectric nanophotonics will find a section about all-magneto-dielectric nanostructures.

Topological photonic phase in chiral hyperbolic metamaterials

Recently, the possibility of achieving one-way backscatter immune transportation of light by mimicking the topological properties of certain solid state systems, such as topological insulators, has received much attention. Thus far, however, demonstrations of nontrivial topology in photonics have relied on photonic crystals with precisely engineered lattice structures, periodic on the scale of the operational wavelength and composed of finely tuned, complex materials. Here we propose a novel effective medium approach towards achieving topologically protected photonic surface states robust against disorder on all length scales and for a wide range of material parameters. Remarkably, the nontrivial topology of our metamaterial design results from the Berry curvature arising from the transversality of electromagnetic waves in a homogeneous medium. Our investigation therefore acts to bridge the gap between the advancing field of topological band theory and classical optical phenomena such as the spin Hall effect of light. The effective medium route to topological phases will pave the way for highly compact one-way transportation of electromagnetic waves in integrated photonic circuits.

Electrically controlled one-way photon flow in plasmonic nanostructures

Photonics is frequently regarded as a potential pathway for substituting current solid-state electronics and as a promise for higher-speed all-optical computing. The fundamental challenges facing nanophotonics and electronics of the future are nanoscale on-chip integration of electronics and photonics with an efficient electric field tuning of light propagation, dynamic access to the light sources and material parameters of the system, as well as isolation of optical signals analogous to that in electronics. Here we suggest a paradigm for a monolithically integrated electronic control over the light propagation in nanoscale plasmonic waveguides. We theoretically demonstrate that magnetic field induced by the direct electric current flowing in metallic constituents of plasmonic nanostructures alters the material parameters and thus the optical signal flow. We use this principle for the design of an electrically controlled subwavelength optical isolator.

Frequency-tunable circular polarization beam splitter using a graphene-dielectric sub-wavelength film

Manipulating the circular polarization of light is of great importance in chemistry and biology, as chiral molecules exhibit different physiological properties when exposed to different circularly polarized waves. Here we suggest a graphene/dielectric-stacked structure, which has both the properties of an epsilon-near-zero material and the high Hall conductivity of graphene. The proposed sub-wavelength structure demonstrates efficient manipulation of circular polarization properties of light. In a quite broad frequency range and at a large oblique incidence angle, the present magnetically active structure is transparent for one circularly polarized wave, and opaque for another. Such an effect can be further tuned by changing the magnitude of the applied magnetic field and chemical potential of graphene.