Search

Topological photonic edge states are conventionally formed at the interface between two domains of topologically trivial and nontrivial photonic crystals. Recent works exploiting photonic quantum Hall and quantum valley Hall effects have shown that large-area topological waveguide states could be created in a three-layer topological heterostructure that consists of a finite-width domain featuring Dirac cone sandwiched between two domains of photonic crystals with opposite topological properties. In this work, we show that a new kind of large-area topological waveguide states could be created employing the photonic analogs of quantum spin Hall effect. Taking the well-used Wu-Hu model in topological photonics as an example, we show that sandwiching a finite-width domain of photonic crystals featuring double Dirac cone between two domains of expanded and shrunken unit cells could lead to the emergence of large-area topological helical waveguide states distributed uniformly in the middle domain. Importantly, we unveil a power-law scaling regarding to the size of the bandgap within which the large-area helical states reside as a function of the width of the middle domain, which implies that these large-area modes in principle could exist in the middle domain with arbitrary width. Moreover, pseudospin-momentum locking unidirectional propagations and robustness of these large-area waveguide modes against sharp bends are explicitly demonstrated. Our work enlarges the photonic systems and platforms that could be utilized for large-area-mode enabled topologically waveguiding., close to published version

Effective degree of freedom (EDOF) of a multiple-input-multiple-output (MIMO) system represents its equivalent number of independent single-input-single-output (SISO) systems, which directly characterizes the communication performance. Traditional EDOF only considers single polarization, where the full polarized components degrade into two independent transverse components under the far-field approximation. However, the traditional model is not applicable to complex scenarios especially for the near-field region. Based on an electromagnetic (EM) channel model built from the dyadic Green's function, we first calculate the EM EDOF to estimate the performance of an arbitrary MIMO system with full polarizations in free space. Then, we clarify the relations between the limit of EDOF and the optimal number of sources/receivers. Finally, potential benefits of near-field MIMO communications are demonstrated with the EM EDOF, in which the contribution of the longitudinally polarized source is taken into account. This work establishes a fundamental EM framework for MIMO wireless communications., Comment: 5 pages, 5 figures

Spatial division multiplexing using conventional orbital angular momentum (OAM) has become a well-known physical layer transmission method over the past decade. The mode-group (MG) superposed by specific single mode plane spiral OAM (PSOAM) waves has been proved to be a flexible beamforming method to achieve the azimuthal pattern diversity, which inherits the spiral phase distribution of conventional OAM wave. Thus, it possesses both the beam directionality and vorticity. In this paper, it’s the first time to show and verify a novel PSOAM MG based multiple-in-multiple-out (MIMO) communication link (MG-MIMO) experimentally in a line-of-sight (LoS) scenario. A compact multi-mode PSOAM antenna is demonstrated experimentally to generate multiple independent controllable PSOAM waves, which can be used for constructing MGs. After several proof-of-principle tests, it has been verified that the beam directionality gain of MG can improve the receiving signal-to-noise (SNR) level in an actual system. Meanwhile, the vorticity can provide another degree of freedom (DoF) to reduce the spatial correlation of MIMO system. Furthermore, a tentative long-distance transmission experiment operated at 10.2 GHz has been performed successfully at a distance of 50 m with a single-way spectrum efficiency of 3.7 bits/s/Hz/stream. The proposed MG-MIMO may have potential in the long-distance LoS back-haul scenario.

This paper presents an efficient synthesis method for generating vortex beams based on donut-shaped orbital angular momentum (OAM) metasurfaces. It not only allows one to synthesize the vortex beam with an arbitrary combination of OAM modes and arbitrary mode energy distribution, but also avoids the undesired phase singularities in the conventional OAM metasurfaces. Three specific examples are implemented to verify the effectiveness of the synthesis method including single-mode, multi-mode, and equal-amplitude vortex beams. Based on the proposed constraint conditions, a donut-shaped metasurface is designed to generate a high-purity single-mode vortex beam. To generate high-performance multi-mode beams, a shape-related tailoring is further introduced to avoid the undesired phase singularities of the multi-mode OAM metasurfaces. Finally, the synthesized equal-amplitude vortex beams are generated and verified experimentally over a wide frequency range, which demonstrates robust and precise control of the vortex beams.

An orbital angular momentum (OAM) antenna based on chiral harmonic diffraction of spoof surface plasmon polaritons (SSPP) is demonstrated in this paper. Due to the diffraction of the helical wire, SSPP is transformed into the high-purity OAM mode. The operation frequency is determined by SSPP, and the topological charge is manipulated by modulating the harmonic order. Additionally, this system achieves a frequency-controlled beam-scanning characteristic in a wide bandwidth from 7.8 GHz to 12.6 GHz, and the measurement data agrees well with the theoretical analysis and the simulation results. Furthermore, the OAM antenna validated in this work is compact, easy-fabricated, and robust. The underlying principle opens up a new train of thought for generating OAM mode and paves the versatile applications involving wireless communication

This paper presents a novel miniaturized almost all-angle-insensitive strong coupled frequency selective surface (SC-FSS) with all-polarization-insensitive multi-band by introducing the different interlayer coupling patches. Two templates of a single-passband and a dual-passband with unit cell less than 0.03λ are adopted to bring in two stopbands on both ends of each passband. Furthermore, the corresponding equivalent circuit models are developed to explain the physical mechanism and formulate relevant design equations. Finally, a prototype of the proposed dual-passband structure is fabricated and measured to verify the results of full wave simulation and the theoretical analysis. The experiment results show that there are a lower passband working at 2 GHz and a higher passband at 5.8 GHz with a lower stopband at 4 GHz and a higher at 7.5 GHz at the ends of above passbands respectively, which exhibits the stable performance against the variation of the incident angles (from 0° to 86°) for both TE and TM polarizations. The measurement results demonstrate that the proposed multi-band SC-FSS is a good candidate for designing a high-performance antenna radome for solving EMI problems.

In this work, numerical solution to a general electromagnetic (EM) system is studied using a formalism based on the formulas for the E–B–A– $\phi $ formulas with different gauge conditions. The finite-difference time-domain (FDTD) method is employed to discretize these formulas. In addition, the convolutional perfectly matched layer (CPML) technique is successfully applied to absorb outgoing scattered waves described by the proposed formulas. The gauge invariance of EM fields in inhomogeneous environment is demonstrated by numerical examples. Moreover, the proposed EM framework integrated with the Schrodinger equation is introduced to investigate the mesoscopic phenomenon for light–matter interaction, which is useful to design laser pulses for controlling discrete quantum states. The work offers a simple and general numerical EM framework, which is essential to bridge the classical EM and quantum mechanical systems.

The understanding of the Casimir force foretells applications in micro/nanoelectromechanical sensors and quantum friction. We review the history of quantum theory and Casimir force modeling and, using the homomorphism between electromagnetic oscillations and a linear pendulum with simple harmonic oscillation, we explore various concepts needed for Casimir force calculations. Through the argument principle, we simplify the derivation of Casimir energy and, hence, the Casimir force. Then, we discuss the connection between the Casimir force and computational electromagnetics (CEM), focusing on the method of moments (MOM). Finally, we consider the marriage of these concepts with recently developed broadband fast algorithms. The resulting approaches illustrate that the Casimir force calculation is for various highly complex structures.

In terahertz (THz) technologies, generation and manipulation of THz waves are two key processes usually implemented by different device modules. Integrating THz generation and manipulation into a single compact device will advance the applications of THz technologies in various fields. Here, we demonstrate a hybrid nonlinear plasmonic metasurface incorporating an epsilon-near-zero (ENZ) indium tin oxide (ITO) layer to seamlessly combine efficient generation and manipulation of THz waves across a wide frequency band. The coupling between the plasmonic resonance of the metasurface and the ENZ mode of the ITO thin film enhances the THz conversion efficiency by more than 4 orders of magnitude. Meanwhile, such a hybrid device is capable of shaping the polarization and wavefront of the emitted THz beam via the engineered nonlinear Pancharatnam-Berry (PB) phases of the plasmonic meta-atoms. The presented hybrid nonlinear metasurface opens a new avenue toward miniaturized integrated THz devices and systems with advanced functionalities.

The calculation method for light emission efficiency splits external quantum efficiency (EQE) into internal quantum efficiency (IQE) and light extraction efficiency (LEE) independently. Consequently, the IQE connected to Purcell factor and the LEE are calculated separately. This traditional method ignores the interplays between the Purcell factor and transmittance coefficient in spectral domain, which all strongly depend on emitting directions. In this work, we propose a new figure of merit to describe the light emission process accurately by using the direction-dependent Purcell factor and transmittance coefficient simultaneously. We use a specific LED structure as a numerical example to illustrate the calculation method and optimization procedure.

Topological valley photonics provides a unique way to manipulate the flow of light. In general, valley edge states that exhibit unidirectional propagation and are immune to defects and disorders could be realized at the interface between two valley photonic crystals with opposite valley Chern numbers. Herein, by merging the physics of valley edge states and bound states in the continuum, we propose and numerically demonstrate a novel, to the best of our knowledge, concept of edge states termed bound valley edge states in the continuum, which enjoys the topological features of valley edge states, such as, unidirectional propagation and immunity to disorders, but are formed at the interface between air and a single valley photonic crystal. Our results not only provide an effective way to reduce the size of valley photonic structures but also facilitate new applications where the proposed concept of bound valley edge states in the continuum could be exploited for optical sensing and unidirectional waveguiding.

In this article, a novel miniaturized concept named strong-coupled frequency selective surface (SC-FSS) with excellent angular stability is presented. First, a conceptual design of SC-FSS is introduced, where the resonant frequencies are insensitive to almost all incident angles. A corresponding equivalent circuit model is developed to interpret the operating principle and formulate relevant design equations with an error of 0.2%. Furthermore, an ultraminiaturized and ultrathin band-stop structure with a unit cell size of λ 0/35 and thickness of 0.26 mm ( λ0 /578) is designed and fabricated to verify this concept. Analysis and experiment show that the proposed band-stop SC-FSS structure can work stably at 2 GHz for both TE and TM modes, even when the incident angle varies from 0° to 84°. The measurement results are consistent with those obtained by full-wave simulations and the equivalent circuit model, which fully demonstrates that the concept of SC-FSS can be widely used in designing antenna reflectors, electromagnetic interference shielding, and angle-insensitive absorbers.

Active metamaterials, i.e., coupled systems consist of metamaterials and active or gain media. Modelling the interaction between unconventional electromagnetic response in metamolecules and quantum dynamics of active media in active metamaterials requires the development of a unified and efficient semiclassical model. Here, we propose a semiclassical model based on Langevin Maxwell-Bloch equations and simplified rate equations for a multi-level quantum system. Using the semiclassical model and the finite-difference time-domain (FDTD) algorithm, a methodology for the self-consistent descriptions of both the coherent lasing process and the spatio-temporal spontaneous noise dynamics in active metamaterials is developed. To validate the proposed method, we numerically study the luminescence of QDs hybridized with plasmonic and all-dielectric metasurfaces and compare with the experimental results. The proposed semiclassical method reveals a new approach for developing new types of nanodevices based on active metasurfaces.

In this letter, a 2 × 2 wideband bidirectional dual circularly polarized (CP) orbital angular momentum (OAM) antenna array at millimeter-wave frequency is presented, which can be used to generate dual mode simultaneously by the sequential rotation feeding method. The proposed antenna array is composed of four bidirectional radiating dual CP elements that rotate 90° sequentially to each other in the clockwise direction. The antenna element structure consists of two metasurface patch layers that are loaded on the bidirectional cavity-back slot antenna based on substrate integrated waveguide technology. Results indicate that the left-hand circular polarization (LHCP) with a mode l OAM = −1 in the + z -direction and right-hand circular polarization (RHCP) with a mode l OAM = +1 in the − z -direction, over a wide impedance ( S 11 < 10 dB) and 3 dB axial-ratio relative bandwidth of 32.3% from 26 to 36 GHz. An acceptable agreement has been achieved with the comparison between the measurement and simulation results. The proposed antenna provides a method of generating multimode OAM beams for wideband millimeter-wave wireless communication applications.

To evaluate the interaction between the orbital angular momentum (OAM) beams and the electrically large standard objects, the backward scattering characteristics of the perfect electrical conductor (PEC) sphere and PEC cone are studied. First, the incident vortex electromagnetic field carrying OAM is generated by Hertizian dipole arrays. Subsequently, the backward-scattered field is calculated by the physical optics algorithm. The phase profiles and OAM spectra are analyzed in detail, which indicates that the scattered field for axial symmetric objects still keeps the main characters of the vortex field due to the angular momentum conservation. Finally, the radar cross section for PEC sphere and cone are calculated, respectively. The backward scattering features show significant differences when OAM beams change their topological charges. Consequently, given a specific propagation direction, compared to plane waves, more information will be offered by the OAM beams for object detection and recognition.

This letter proposes a novel deep learning (DL) approach to resolve the electromagnetic inverse scattering (EMIS) problems. The conventional approaches of resolving EMIS problems encounter assorted difficulties, such as high contrast, high computational cost, inevitable intrinsic nonlinearity, and strong ill-posedness. To surmount these difficulties, a novel DL approach is proposed based on a novel complex-valued deep fully convolutional neural network structure. The proposed complex-valued deep learning model for solving EMIS problems composes of an encoder network and its corresponding decoder network, followed by a final pixel-wise regression layer. The complex-valued encoder network extracts feature fragments from received scattered field data, while the role of the complex-valued decoder network is mapping the feature fragments to retrieve the final contrasts (permittivities) of dielectric scatterers. Hence, the proposed deep learning model functions as an “heterogeneous” transformation process, where measured scattering field data is converted into the corresponding contrasts of scatterers. As a consequence, the EMIS problem could be resolved accurately even for extremely high-contrast targets. Numerical benchmarks have illustrated the feasibility and accuracy of our proposed approach. The proposed approach opens a novel path for the deep learning-based real-time quantitative microwave imaging for high-contrast scatterers.