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Spatio-temporal permittivity modulation can simultaneously impart wave vector and frequency shifts during an indirect photonic transition process where nonreciprocal responses are accomplished. Here, we combine the nonreciprocity with exceptional points (EPs) by employing line parameter trajectory to a spatio-temporally modulated waveguide. The instantaneous state evolutions on direction-dependent Riemann sheets are thus different in forward and backward directions. Light propagates through the structure is nonreciprocal. Forwardly, an arbitrary incident wave will convert to a combination of the two waveguide modes with same intensity. Backwardly, the waves almost entirely convert to one specific mode. The conversion efficiency is more than 80% and robust to the line design. The operating wavelength is in the telecommunications band, conducive to integration with other chips. Compared to the widely used loop path, the line path has the merit of easy preparation in experiments as only one parameter is changed. The research sheds light on the optical devices such as isolator and amplifier.

Abstract In recent years, topological photonics inspired by electric topological insulators has promoted extensive research on robust electromagnetic (EM) wave manipulation and new wave‐functional devices. Optical resonators can significantly confine EM waves and are the basic building blocks for constructing diverse topological structures under a tight binding mechanism. As an artificial “magnetic atom,” the split‐ring‐resonator (SRR) is one of the most attractive optical resonators. SRRs provide an excellent and flexible platform for constructing various topological structures with complex coupling distributions, uncovering abundant topological properties, and innovating practical devices. Here, the realization and fundamental EM responses of the SRR are briefly introduced. Compared to conventional EM resonance elements, the coupling between SRRs depends not only on the coupling distance but also on the orientation angle of the slits. The recent achievements in various low‐dimensional photonic topological structures composed of SRRs are summarized. Furthermore, this review explains the underlying physical principles and discusses progress in topological devices with SRRs, including wireless power transfer, sensing, and switching. Finally, this review provides an overview of the future of SRR topological structures and their impact on the development of novel topological systems and devices.

Non-Hermitian skin effect (NHSE), where huge modes are accumulated at system boundaries, offers new possibility for steering the transport and localization of light by non-Hermiticity. Here, the direction-dependent NHSE is proposed in a photonic waveguide array via spatially complex modulation, where the skin modes tend to localize at different boundaries for opposite propagation directions. We utilize complex modulation to arouse anisotropic coupling between symmetric and anti-symmetric modes in multimode waveguides and further match the refractive index of adjacent waveguides. In this way, a non-Hermitian Su–Schrieffer–Heeger (SSH) lattice that supports NHSE is achieved. In particular, the anisotropic coupling is highly unidirectional. For forward direction, it allows mode conversion from antisymmetric modes to symmetric modes. However, the process is forbidden for backward direction. As a result, the skin modes tend to locate at lower boundary for forward propagation but the localization direction is reversed for backward injection. Our results provide a potential platform to investigate NHSE on photonic chips and may find applications in non-magnetic unidirectional devices.

We theoretically investigate the time-domain dynamics in a non-Hermitian electric circuit network with anti-parity-time (anti-PT) symmetry, which is consisted of amplified resonant LRC circuits connected by resistors. The effective coupling strength and on-site potential can be flexibly tuned via controlling the resistance value and resonant frequencies of LRC circuits, respectively. Therein, we are able to realize all three symmetry phases of the system, including anti-PT symmetric, intermediate, and broken phases, which are distinguished from their band structures. We show the time evolution of the envelope of voltage is analogous to discrete diffraction as a single injection stimulates more and more channels over time. Specifically, the system exhibits constant intensity diffraction at the boundary between intermediate and broken phases as the voltages at different channels are of the same value during the evolution. The reason lies in the dispersionless band structure near exceptional points (EPs) and the related linear energy amplification. The results enrich the study of discrete diffraction and are helpful to further exploration of topological phenomena in anti-PT-symmetric systems.

Recently, topological chains have attracted extensive attention because of their simple structure, rich physics and important applications. In this work, we theoretically and experimentally uncover that the abundant topological phases of periodic trimer chain composed of one kind of meta-atom, namely split-ring resonators (SRRs), can be flexibly controlled by tunning the rotation angle of SRRs. On the one hand, we study the rotation controlled phase transition between two topological distinguished trimer chains with inversion symmetry. The generation of symmetric edge states can be easily controlled in this phase transition. On the other hand, the topological phases of the trimer chain broken inversion symmetry is demonstrated. Especially, the rotation controlled asymmetric edge states are observed in this process. So, rotation provides a new degree of freedom to manipulate edge states in the trimer chain composed of SRRs. The results in this work not only provide a flexible way to observe controlled edge states, but also provide a good research platform for designing other topological models with complex coupling distributions.

We study the plasmonic second-order topological modes in nanowire waveguides which are covered by monolayer graphene. The waveguide arrays are arranged in the kagome lattice. In a triangle-shaped lattice, the topological modes are localized at the corner of the triangle by tuning the spatial spacing between the different waveguides. The corner modes also depend on the corner shape, which only exist at one corner in a rhombic lattice. In addition to the corner modes, both structures also support the topological edge mode. We show that the corner modes experience a smaller modal wavelength, longer propagation distance, and smaller mode volume than the edge modes. The study may be utilized to explore the topological bound modes at the nanoscale.

We studied the topological properties of an extended Su–Schrieffer–Heeger (SSH) model composed of a binary waveguide array with alternating real and imaginary couplings. The topological invariant of the periodic structures remained quantized with chiral symmetry even though the system was non-Hermitian. The numerical results indicated that phase transition arose when the absolute values of the two couplings were equal. The system supported a topological zero mode at the boundary of nontrivial structures when chiral symmetry was preserved. By adding onsite gain and loss to break chiral symmetry, the topological modes dominated in all supermodes with maximum absolute value of imaginary energy. This study enriches research on the SSH model in non-Hermitian systems and may find applications in optical routers and switches.

We explored exceptional points (EPs) in one dimensional non-Hermitian photonic crystals incorporated with a defect. The defect was asymmetric with respect to the center. Two EPs could be derived by modulating the normalized frequency and the gain-loss coefficient of defect. The reflection coefficient complex phase changed dramatically around EPs, and the change in complex phase was π at EPs. The electric field of EPs was mainly restricted to the defect, which can induce a giant Goos−Hänchen (GH) shift. Moreover, we found a coherent perfect absorption-laser point (CPA-LP) in the structure. A giant GH shift also existed around the CPA-LP. The study may have found applications in highly sensitive sensors.

We investigate the topological bound modes of surface plasmon polaritons (SPPs) in a graphene pair waveguide array. The arrays are with uniform inter-layer and intra-layer spacings but the chemical potential of two graphene in each pair are different. The topological bound modes emerge when two arrays with opposite sequences of chemical potential are interfaced, which are analogous to Jackiw-Rebbi modes with opposite mass. We show the topological bound modes can be dynamically controlled by tuning the chemical potential, and the propagation loss of topological bound modes can be remarkably reduced by decreasing the chemical potential. Thanks to the strong confinement of graphene SPPs, the modal wavelength of topological bound modes can be squeezed as small as 1/70 of incident wavelength. The study provides a promising approach to realizing robust light transport beyond diffraction limit.

An MXene material absorbs visible and IR light which makes a MXene-based solar absorber an ideal absorber. Here, we propose a high-absorption broadband absorber based on an array of MXene composite cylinder ring structures. The structure designed in this article fully utilizes the MXene material's large surface area to volume ratio, and in the wavelength range of 300-5000 nm, the average absorption efficiency is as high as 98.44%, and the energy absorption rate in the AM 1.5 solar radiation spectrum is 98.76%. The absorption characteristics of the absorber are analyzed by using the finitedifference time-domain (FDTD) method. The electric and magnetic field patterns indicate that the high absorption performance is attributed to the coupling effect of surface plasmon resonance and gap surface plasmon resonance. Furthermore, the absorber exhibits insensitivity to the polarization angle and demonstrates high absorption efficiency even at large incidence angles. Within a certain manufacturing tolerance range, the absorber can still maintain its broadband absorption characteristics. The absorber also shows a high thermal emissivity of 98.5% when the temperature is 1750 K. The findings offer a theoretical foundation for the development of absorption metamaterials for solar energy harvesting elements. [ABSTRACT FROM AUTHOR]

Exceptional points (EPs) are branch point singularities of self-intersecting Riemann sheets, and they can be observed in a non-Hermitian system with complex eigenvalues. It has been revealed recently that dynamically encircling EPs by adiabatically changing the parameters of a system composed of lossy optical waveguides could lead to asymmetric (input-output) mode transfer. However, the length of the waveguides had to be considerable to ensure adiabatic evolution. Here we demonstrate that the parameters can change adiabatically along a smaller encircling loop by utilizing moving EPs, leading to significant shortening of the structures compared to fixed EPs. Meanwhile, the mode transmittance is remarkably improved and the transfer efficiency persists at ~90%. Moving EPs are very promising for applications such as highly integrated broadband optical switches and convertors operating at telecommunication wavelengths. [ABSTRACT FROM AUTHOR]