Surface Magnon Polaritons Research Articles

Geometric Similarities and Topological Phases in Surface Magnon Polaritons.

Surface polaritons have proven to be uniquely capable of controlling light-matter interactions. Here we explore surface magnon polaritons in low-loss ferrimagnetic semiconductors, with a focus on their topological phases. We propose several surface magnon polariton devices, including microwave resonators that can strongly enhance magnetic fields and low-loss interconnects joining waveguides with vastly different impedances. Our work can facilitate the exploration of topological phases in polaritons and the development of topological microwave devices for quantum sensing and information processing.

Surface magnon-polaritons at a dielectric/graphene/gyromagnetic interface in a perpendicular applied magnetic field

We present a theoretical study of the surface magnon-polaritons at an interface formed by vacuum and a gyromagnetic medium (that can be either ferromagnetic or antiferromagnetic), when there is a graphene layer deposited between the media at the interface and a magnetic field is applied perpendicular to the interface. The retarded-mode dispersion relations are calculated by considering a superposition of transverse magnetic and transversal electric electromagnetic waves in both media. Our results reveal the appearance of the surface magnon-polariton modes (with frequencies typically of a few GHz) that do not exist in the absence of graphene at the interface. Also, a typical magnon-polariton dispersion relation with damping is revealed, including a resonant frequency that depends on the applied magnetic field. The effects of varying the doping levels, which modify the Fermi energies in the graphene, and varying the perpendicular applied magnetic field are presented, revealing a strong influence exerted by the presence of graphene on the surface magnon-polariton modes. Other effects include the control of the slope of the dispersion curves (with respect to the in-plane wave vector) for the modes as the Fermi energies of the graphene sheet are changed and the distinctive localization properties for the emerging surface modes.

Ghost surface polaritons at oblique antiferromagnetic surface

We investigated hybrid-polarization surface magnon polaritons at the oblique surface of antiferromagnet (AF) in an external magnetic field pointed along the AF easy axis, where the easy axis does not lie in the AF surface. According to the direction of surface-polariton propagation, there are two specific geometries. In geometry-I, where the direction of propagation is vertical to the easy axis, we found two hybrid-polarization surface polaritons (HSPs), which are highly nonreciprocal. They are one ordinary hybrid-polarization surface polariton (called a HSP still) and one ghost surface polariton (GSP-I). GSP-I can exist not only inside but also outside the AF reststrahlen frequency bands and originates from the external magnetic field. In Geometry-II with the easy axis in the propagation plane, we predicted another ghost surface polariton (GSP-II). GSP-II is a TE surface wave and is reciprocal. Another difference between the two GSPs is that the Poynting vector of GSP-I is of oscillation-attenuation but that of GSP-II monotonously decays with the distance from the surface and GSP-II exists only in the reststrahlen band without external magnetic field. This study further enlarges the family of surface polariton and widens the window of polariton application.

Twist-Induced Near-Field Radiative Heat Switch in Hyperbolic Antiferromagnets

We study the twisted control of the near-field radiative heat transfer between two hyperbolic antiferromagnetic insulators under external magnetic fields. We show that the near-field heat flux can be affected by both the twist angle $\theta$ and the magnitude of the applied magnetic field with different broken symmetries. Irrespective of twist angle, the external magnetic field causes the radiative heat flux to change nonmonotonically, and the minimum heat flux can be found with the magnetic fields of about 1.5 T. Such nonmonotonic behavior is due to the fact that the magnetic field can radically change the nature of the magnon polaritons with time reversal symmetry breaking. The field not only affects the topological structure of surface magnon polaritons, but also induces the volume magnon polaritons that progressively dominate the heat transfer as the field increases. We further propose a twist-induced thermal switch device with inversion symmetry breaking, which can severely regulate radiative heat flux through different magnetic fields. Our findings account for a characteristic modulation of radiative heat transfer with implications for applications in dynamic thermal management.

Use of 4×4 transfer matrix method in the study of surface magnon polaritons via simulated attenuated total reflection measurements on the antiferromagnetic semiconductor MnF2

The surface magnon polaritons (SMPs) present in thin film antiferromagnet semiconductors can be extremely confined, having most of their energy distributed within the magnetic medium. For extremely thin films these SMPs can therefore be well described using a quasimagnetostatic treatment. In this work Berreman's $4\ifmmode\times\else\texttimes\fi{}4$ transfer matrix method (TMM) is used to study the SMPs supported by thicker films, beyond the magnetostatic approximation. Focus is placed on the antiferromagnet semiconductor ${\mathrm{MnF}}_{2}$ for which attenuated total reflection measurements are modeled, probing the hyperbolic dispersion of the medium. The dispersion relations from both the TMM and the analytical quasimagnetostatic approximation are compared. For thicker films, the coupling efficiencies into the SMP energy channels are analyzed as a function of air gap distances. The TMM is used to probe the SMP dispersion for realistic experimental configurations with modifications including type of substrate, film thickness, and crystal axis orientation. Rich phenomena are observed such as strong SMP-waveguide mode coupling and SMPs with negative refraction.

Spatial shifts of reflected beams from surface polaritons in antiferromagnets

Goos–Hänchen (GH) and Imbert–Fedorov (IF) shifts of reflected beams and the effects of two Dyakonov surface magnon polaritons (DSMPs) on these shifts are studied in the Otto configuration with an antiferromagnetic substrate. A modified dispersion equation of the DSMPs is proposed to recognize GH and IF shifts related to the DSMPs. Reflected beam shift spectra are simulated for a F e F 2 substrate. Shift features around the two DSMPs are observed by frequency scanning with a fixed incident angle. The GH-shift amplitude corresponding to either DSMP is situated between the immediately adjacent maximum and minimum of a GH-shift curve. The shift-amplitude maximum of an IF-shift curve corresponds to the relevant DSMP accurately. GH and IF shifts also are given by frequency scanning along dispersion curves.

Experimental realization of topologically protected unidirectional surface magnon polaritons on ceramic YIG ferrites

Topologically protected, unidirectional surface magnon polaritons (SMPs) guided along the interface between air and ferrimagnetic yttrium iron garnet slab structures, biased with an external static magnetic field in the Voigt configuration, are realized experimentally by measuring the S-parameters between two small loop magnetic dipole antennas that launch and receive the SMP. Transmission is compared in the two cases where an abrupt change in the form of a step is present and absent in the interface. We find that transmission of the SMP peaks near the same level in each case, a sign of its topological protection. In addition, the dependence of isolation on slab thickness is investigated for a specified antenna configuration.

Twist-Induced Near-Field Thermal Switch Using Nonreciprocal Surface Magnon-Polaritons

We explore that two ferromagnetic insulator slabs host a strong twist-induced near-field radiative heat transfer in the presence of twisted magnetic fields. Using the formalism of fluctuational electrodynamics, we find the existence of large twist-induced thermal switch ratio in large damping condition and nonmonotonic twist manipulation for heat transfer in small damping condition, associated with the different twist-induced effects of nonreciprocal elliptic surface magnon-polaritons, hyperbolic surface magnon-polaritons, and twist-non-resonant surface magnon-polaritons. Moreover, the near-field radiative heat transfer can be significantly enhanced by the twist-non-resonant surface magnon-polaritons in the ultra-small damping condition. Such twist-induced effect is applicable for other kinds of anisotropic slabs with timereversal symmetry breaking. Our findings provide a way to twisted and magnetic control in nanoscale thermal management and improve it with twistronics concepts.

Magnon-polaritons in graphene/gyromagnetic slab heterostructures

We present a theoretical study for the surface magnon-polaritons in structures formed by graphene layer(s) on an insulating gyromagnetic medium (that can be either ferromagnetic or antiferromagnetic) surrounded by vacuum. We consider different doping levels to vary the Fermi energies in the graphene, including both semi-infinite and slab magnetic samples. Our results reveal a strong influence, exerted by the presence of graphene, on the surface magnon-polariton modes. The effects include control of the group velocities for the modes as the Fermi energies of the graphene sheet are varied, modified nonreciprocal and reciprocal mode propagation properties depending on the type of magnetic material, and distinct localization properties for the emerging surface modes.

Laser-induced regular nanostructure chains within microgrooves of Fe-based metallic glass

Femtosecond laser-induced surface structures have been extensively investigated over decades, however, their formation is not well controlled especially for the practical use in the photonic field. We here report the highly regular nanostructure formation within the grooves of the Fe-based metallic glass, upon irradiation of two temporally delayed femtosecond laser beams. For the case of micrometer-sized groove, multiple chains of spindle-like nanostructures are developed, with the spatial periods of 667 nm and 180 nm for the chain and spindle distributions, respectively. Their arrangement properties can be modulated by varying laser parameters. On the other hand, for the case of submicrometer-sized groove, there is only one chain of spindle-like nanostructures formed inside with the improved regular appearance. Two physical effects, including the plasmonic field generation from the groove edges and the surface magnon polariton excitation within the groove, are considered responsible for the phenomena, which consequently result in the spatial interlocking of two types of ripples. The theoretical analyses match well the experimental observations.

Controlling spins with surface magnon polaritons

Polaritons in metals, semimetals, semiconductors, and polar insulators can allow for extreme confinement of electromagnetic energy, providing many promising opportunities for enhancing typically weak light-matter interactions such as multipolar radiation, multiphoton spontaneous emission, Raman scattering, and material nonlinearities. These extremely confined polaritons are quasielectrostatic in nature, with most of their energy residing in the electric field. As a result, these ``electric'' polaritons are far from optimized for enhancing emission of a magnetic nature, such as spin relaxation, which is typically many orders of magnitude slower than corresponding electric decays. Here, we take concepts of ``electric'' polaritons into magnetic materials, and propose using surface magnon polaritons in negative magnetic permeability materials to strongly enhance spin relaxation in nearby emitters. Specifically, we provide quantitative examples with ${\mathrm{MnF}}_{2}$ and ${\mathrm{FeF}}_{2}$, enhancing spin transitions in the THz spectral range. We find that these magnetic polaritons in 100-nm thin films can be confined to lengths over 10 000 times smaller than the wavelength of a photon at the same frequency, allowing for a surprising 12 orders of magnitude enhancement in magnetic dipole transitions. This takes THz spin-flip transitions, which normally occur at timescales on the order of a year, and forces them to occur at sub-ms timescales. Our results suggest an interesting platform for polaritonics at THz frequencies, and more broadly, a way to use polaritons to control light-matter interactions.

Non-reciprocity and topology in optics: one-way road for light via surface magnon polariton

We show how non-reciprocity and topology are used to construct an optical one-way waveguide in the Voigt geometry. First, we present a traditional approach of the one-way waveguide of light using surface polaritons under a static magnetic field. Second, we explain a recent discovery of a topological approach using photonic crystals with the magneto-optical coupling. Third, we present a combination of the two approaches, toward a broadband one-way waveguide in the microwave range.

A corrugated perfect magnetic conductor surface supporting spoof surface magnon polaritons.

In this paper, we demonstrate that spoof surface magnon polaritons (SSMPs) can propagate along a corrugated perfect magnetic conductor (PMC) surface. From duality theorem, the existence of surface electromagnetic modes on corrugated PMC surfaces are manifest to be transverse electric (TE) mode compared with the transverse magnetic (TM) mode of spoof surface plasmon plaritons (SSPPs) excited on corrugated perfect electric conductor surfaces. Theoretical deduction through modal expansion method and simulation results clearly verify that SSMPs share the same dispersion relationship with the SSPPs. It is worth noting that this metamaterial will have more similar properties and potential applications as the SSPPs in large number of areas.

Surface polaritons with arbitrary magnetic and dielectric materials: new regimes, effects of negative index, and superconductors

A surface magnon-polariton can be excited by both p- and s-polarized light if at least one of the layers is a magnetic material. We present general expressions of the tangential wave vectors of s- and p-polarized light at an interface of two media. Analysis reveals additional new regimes of surface polariton resonances with magnetic materials for s- and p-polarized light. The tangential wave vectors are found to be equal in magnitude to the normal wave vectors at surface polariton resonances. The spatial distributions of the fields at resonant enhancement and the spectra of the tangential wave vectors are studied for different dielectric permittivities and magnetic permeabilities of the two media. If one of the media has dispersive dielectric function and permeability function, additional surface polariton resonance peaks appear for both s- and p polarizations. For a medium with a superconductor, the tangential component increases asymptotically at lower frequencies, providing subwavelength capability at the terahertz regime.

NEW CLASS OF SURFACE MAGNON POLARITONS IN ENANTIOMERIC ANTIFERROMAGNETIC STRUCTURES

A novel class of surface magnon polaritons supported in identical enantiomeric antiferromagnetic structures is presented. The surface waves arise due to bianisotropy. The existence of two distinct surface modes with unusual dispersion and polarization properties is predicted. The role of losses is investigated and the propagation length of the surface waves is determined.

Theory of electromagnetic modes of a magnetic superlattice in a transverse magnetic field: An effective-medium approach.

We derive effective-medium expressions, suitable for calculation of the properties of long-wavelength electromagnetic modes, for a magnetic superlattice in a magnetic field perpendicular to the interfaces. These are applied to a calculation of the dispersion relations for bulk and surface magnon polaritons of magnetic/nonmagnetic and magnetic/magnetic superlattices for both ferromagnetic and antiferromagnetic structures. Although we find no magnetostatic surface waves in such structures, in agreement with previous calculations, we predict retarded surface modes which are virtual. Such virtual modes have no magnetostatic limit.

Analysis of Nonlinear Excitation and Detection of Surface Magnon Polaritons

A theoretical investigation is made of the excitation and detection of surface magnon polaritons in YIG film through nonlinear optical mixing method. The treatment is carried out by total field approach. We suggest some experimental conditions under which the surface magnon polaritons might be excited and detected.

Studies of surface polaritons on magnetic material

The authors have performed numerical calculations for the attenuated total reflection spectra of surface magnon polaritons (SMP) on YIG slabs in the Voigt configuration. It seems that the ATR method can serve as a tool for probing the SMP. A calculation scheme for the SMP on a semi-infinite YIG in the Faraday geometry is presented. The possibility that SMP may occur in a ferromagnetic metal such as nickel is also discussed. The results differ from those obtained by Hartstein and co-workers (1973) who proposed that no surface polaritons exist on ferromagnetic metals.

ATR mode of surface magnon polaritons on YIG

The first discussion on ATR mode of surface magnon polaritons on semi- infinite YIG is presented for the Otto configuration (polyethylene-air-YIG geometry). Damping and the effect of retardation are included in the analysis. The thickness of the air layer appropriate for ATR measurements is determined.