Hotspots from nonreciprocal surface waves

Ill-Defined Topological Phases in Local Dispersive Photonic Crystals

In recent years there has been a great interest in topological materials and in their fascinating properties. Topological band theory was initially developed for condensed matter systems, but it can be readily applied to arbitrary wave platforms with few modifications. Thus, the topological classification of optical systems is usually regarded as being mathematically equivalent to that of condensed matter systems. Surprisingly, here we find that both the particle-hole symmetry and the dispersive nature of nonreciprocal photonic materials may lead to situations where the usual topological methods break down and the Chern topology becomes ill defined. It is shown that due to the divergence of the density of photonic states in plasmonic systems the gap Chern numbers can be noninteger notwithstanding that the relevant parametric space is compact. In order that the topology of a dispersive photonic crystal is well defined, it is essential to take into account the nonlocal effects in the bulk materials. We propose two different regularization methods to fix the encountered problems. Our results highlight that the regularized topologies may depend critically on the response of the bulk materials for large k.

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.

Spin-polarized unidirectional cylindrical waveguide in bianisotropic media

In this paper, we analyze a cylindrical waveguide consisting of two layers of bianisotropic material with anti-symmetric magnetoelectric coupling tensors. The analysis is carried out in terms of pseudo-electric and pseudo-magnetic fields which satisfy Maxwells' equations with gyrotropic permittivity and permeability tensors. We show that the rotationally symmetric modes of the waveguide are unidirectional with transverse pseudo-electric and transverse pseudo-magnetic modes propagating in opposite directions. These modes are surface waves whose electromagnetic field is concentrated near the interface between the two anisotropic materials. They follow the contour of the interface even in the case of sharp discontinuities and pass through an obstacle without backscattering if the obstacle does not change the polarization of the wave. Higher-order modes of the waveguide are also investigated. Although these modes are hybrid modes and not, strictly speaking, unidirectional, they practically behave as the rotationally symmetric mode.

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.

Completely stopping microwaves with extremely enhanced magnetic fields

A microwave one-way waveguide of three-dimensional configuration is proposed and investigated theoretically. In this waveguide there exists a complete one-way propagation band, where the mode propagates only in one direction and can be immune to backscattering. By terminating the one-way waveguide with metal slab, one-way propagating waves in this waveguide system can be stopped at the terminal end without any backscattering. Meanwhile, a hotspot with extremely enhanced magnetic-field amplitude is generated in this 3D waveguide system. For an incident microwave pulse, the trapped wave packet can be compressed to deep subwavelength scale besides the magnetic field enhancement. Moreover, the magnetic field enhancement of trapped waves can be further largely increased by tapering laterally the waveguide system. The approach for trapping microwaves has promising applications in magnetic sensing and magnetic non-linearity.

Nonreciprocal waveguiding structures for THz region based on InSb

We have studied theoretically and numerically surface magnetoplasmons in three types of THz guiding structures, namely, InSb/dielectric planar boundary, InSb/air/metal planar waveguide, and symmetric InSb/air/InSb planar waveguide, in the presence of an external magnetic field. We consider the Voigt magneto-optic configuration in which these structures provide a frequency range where only one propagation direction is allowed to support one-way propagation of the surface plasmon polariton, due to nonreciprocity of the structures. To study the dispersion properties associated with unidirectional propagation of magnetoplasmons in finite-size nanostructured waveguides, we have developed an efficient two-dimensional numerical technique based on the magneto-optic aperiodic rigorous coupled-wave analysis. We have shown that the one-way bandwidth can be controlled by an external magnetic field and by the permittivity and thickness of the dielectric guiding layer. To enable numerical simulation, we have utilized the configuration in which the magnetized section of a waveguide is along the direction of propagation sandwiched by the identical waveguide segments without a magnetic field. We have also shown that the one-way bandwidth can be controlled by an external magnetic field and by the permittivity and thickness of the dielectric guiding layer.

Truly trapped rainbow by utilizing nonreciprocal waveguides

The concept of a “trapped rainbow” has generated considerable interest for optical data storage and processing. It aims to trap different frequency components of the wave packet at different positions permanently. However, all the previously proposed structures cannot truly achieve this effect, due to the difficulties in suppressing the reflection caused by strong intermodal coupling and distinguishing different frequency components simultaneously. In this article, we found a physical mechanism to achieve a truly “trapped rainbow” storage of electromagnetic wave. We utilize nonreciprocal waveguides under a tapered magnetic field to achieve this and such a trapping effect is stable even under fabrication disorders. We also observe hot spots and relatively long duration time of the trapped wave around critical positions through frequency domain and time domain simulations. The physical mechanism we found has a variety of potential applications ranging from wave harvesting and storage to nonlinearity enhancement.

Complete trapping of electromagnetic radiation using surface magnetoplasmons

Because the dispersion properties of surface magnetoplasmons (SMPs) in a magnetized plasmonic material closely depend on its cladding dielectric, it is possible to completely trap SMPs in a system consisting of a plasmonic material with cladding of a dielectric heterostructure. By using a semiconductor, our finite element simulation (performed using the software COMSOL) shows that terahertz one-way SMPs in such a system can be completely trapped at the interface of the heterostructure and hence, a focused subwavelength-scale hotspot with dramatically enhanced field is generated. Moreover, a one-way SMP pulse in this system can also be completely trapped, and the wave packet can be compressed into a stable hotspot on the subwavelength scale.

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.