Mixer-Duplexer-Antenna Leaky-Wave System Based on Periodic Space-Time Modulation

We present a mixer-duplexer-antenna leaky-wave system based on periodic space-time modulation. This system operates as a full transceiver, where the upconversion and downconversion mixing operations are accomplished via space-time transitions, the duplexing operation is induced by the nonreciprocal nature of the structure, and the radiation operation is provided by the leaky-wave nature of the wave. A rigorous electromagnetic solution is derived for the dispersion relation and field distributions. The system is implemented in the form of a spatio-temporally modulated microstrip leaky-wave structure incorporating an array of sub-wavelengthly spaced varactors modulated by a harmonic wave. In addition to the overall mixer-duplexer-antenna operation, frequency beam scanning at fixed input frequency is demonstrated as one of the interesting features of the system. A prototype is realized and demonstrated by full-wave and experimental results

Dynamic Modulation Yields One-Way Beam Splitting

This article demonstrates the realization of an extraordinary beam splitter based on nonreciprocal and synchronized photonic transitions in obliquely illuminated space-time-modulated (STM) slabs which impart the coherent temporal frequency and spatial frequency shifts. As a consequence of such unusual photonic transitions, a one-way beam splitting and amplification is exhibited by the STM slab. Beam splitting is a vital operation for various optical and photonic systems, ranging from quantum computation to fluorescence spectroscopy and microscopy. Despite the beam splitting is conceptually a simple operation, the performance characteristics of beam splitters significantly influence the repeatability and accuracy of the entire optical system. As of today, there has been no approach exhibiting a nonreciprocal beam splitting accompanied with transmission gain and an arbitrary splitting angle. Here, we show that oblique illumination of a periodic and semi-coherent dynamically-modulated slab results in coherent photonic transitions between the incident light beam and its counterpart space-time harmonic (STH). Such photonic transitions introduce a unidirectional synchronization and momentum exchange between two STHs with same temporal frequencies, but opposite spatial frequencies. Such a beam splitting technique offers high isolation, transmission gain and zero beam tilting, and is expected to drastically decrease the resource and isolation requirements in optical and photonic systems. In addition to the analytical solution, we provide a closed-form solution for the electromagnetic fields in STM structures, and accordingly, investigate the properties of the wave isolation and amplification in subluminal, superluminal and luminal ST modulations

Space-Time Medium Functions as a Perfect Antenna-Mixer-Amplifier Transceiver

We show that a space-time-varying medium can function as a front-end transceiver, i.e., an antenna-mixer-amplifier. Such a unique functionality is endowed by space-time surface waves associated with complex space-time wave vectors in a subluminal space-time medium. The proposed structure introduces pure frequency up- and down-conversions and with very weak undesired time harmonics. In contrast to other recently proposed space-time mixers, a large frequency up-/down conversion ratio, associated with gain is achievable. Furthermore, as the structure does not operate based on progressive energy transition between the space-time modulation and the incident wave, it possesses a subwavelength thickness (metasurface). Such a multi-functional, highly efficient and compact medium is expected to find various applications in modern wireless telecommunication systems

Nonreciprocal Electromagnetic Scattering from a Periodically Space-Time Modulated Slab and Application to a Quasisonic Isolator

Scattering of obliquely incident electromagnetic waves from periodically space-time modulated slabs is investigated. It is shown that such structures operate as nonreciprocal harmonic generators and spatial-frequency filters. For oblique incidences, low-frequency harmonics are filtered out in the form of surface waves, while high-frequency harmonics are transmitted as space waves. In the quasisonic regime, where the velocity of the space-time modulation is close to the velocity of the electromagnetic waves in the background medium, the incident wave is strongly coupled to space-time harmonics in the forward direction, while in the backward direction it exhibits low coupling to other harmonics. This nonreciprocity is leveraged for the realization of an electromagnetic isolator in the quasisonic regime and is experimentally demonstrated at microwave frequencies

Enhanced Bandwidth and Diversity in Real-Time Analog Signal Processing (R-ASP) using Nonuniform C-section Phasers

We show that a continuously nonuniform coupled line C-section phaser, as the limiting case of the step discontinuous coupled-line multisection commensurate and non-commensurate phasers, provides enhanced bandwidth and diversity in real-time analog signal processing (R-ASP). The phenomenology of the component is explained in comparison with the step-discontinuous using multiple-reflection theory and a simple synthesis procedure is provided. The bandwidth enhancement results from the suppression of spurious group delay harmonics or quasi-harmonics, while the diversity enhancement results from the greater level of freedom provided by the continuous nature of the nonuniform profile of the phaser. These statements are supported by theoretical and experimental results

Programmable Nonreciprocal Metaprism

Optical prisms are made of glass and map temporal frequencies into spatial frequencies by decomposing incident white light into its constituent colors and refract them into different directions. Conventional prisms suffer from their volumetric bulky and heavy structure and their material parameters are dictated by the Lorentz reciprocity theorem. Considering various applications of prisms in wave engineering and their growing applications in the invisible spectrum and antenna applications, there is a demand for compact apparatuses that are capable of providing prism functionality in a reconfigurable manner, with a nonreciprocal/reciprocal response. Here, we propose a nonreciprocal metasurface-based prism constituted of an array of phase- and amplitude-gradient frequency-dependent spatially variant radiating super-cells. In conventional optical prisms, nonreciprocal devices and metamaterials, the spatial decomposition and nonreciprocity functions are fixed and noneditable. Here, we present a programmable metasurface integrated with amplifiers to realize controllable nonreciprocal spatial decomposition, where each frequency component of the incident polychromatic wave can be transmitted under an arbitrary and programmable angle of transmission with a desired transmission gain. Such a polychromatic metasurface prism is constituted of frequency-dependent spatially variant transistor-based phase shifters and amplifiers for the spatial decomposition of the wave components. Interesting features include three-dimensional prism functionality with programmable angles of refraction, power amplification, and directive and diverse radiation beams. Furthermore, the metasurface prism can be digitally controlled via a field-programmable gate array (FPGA), making the metasurface a suitable solution for radars, holography applications, and wireless telecommunication systems

Pure and Linear Frequency Converter Temporal Metasurface

Metasurfaces are ultrathin structures which are constituted by an array of subwavelength scatterers with designable scattering responses. They have opened up unprecedented exciting opportunities for extraordinary wave engineering processes. On the other hand, frequency converters have drawn wide attention due to their vital applications in telecommunication systems, health care devices, radio astronomy, military radars and biological sensing systems. Here, we show that a spurious-free and linear frequency converter metasurface can be realized by leveraging unique properties of engineered transmissive temporal supercells. Such a metasurface is formed by time-modulated supercells; themselves are composed of temporal and static patch resonators and phase shifters. This represents the first frequency converter metasurface possessing large frequency conversion ratio with controllable frequency bands and transmission magnitude. In contrast to conventional nonlinear mixers, the proposed temporal frequency converter offers a linear response. In addition, by taking advantage of the proposed surface-interconnector-phaser-surface (SIPS) architecture, a spurious-free and linear frequency conversion is achievable, where all undesired mixing products are strongly suppressed. The proposed metasurface may be digitally controlled and programmed through a field programmable gate array. This makes the spurious-free and linear frequency converter metasurface a prominent solution for wireless and satellite telecommunication systems, as well as invisibility cloaks and radars. This study opens a way to realize more complicated and enhanced-efficiency spectrum-changing metasurface

Application of Space-and Time-Modulated Dispersion Engineered Metamaterials to Signal Processing and Magnetless NONRECIPROCITY

RÉSUMÉ Les métamatériaux sont des structures conçues pour intéragir avec les composantes électriques et magnétiques de la lumière d’une manière particulière qui n’est pas possible avec des matériaux naturels. Elles sont composées de méta-atomes, qui sont faits d’un ensemble d’éléments de taille plus petite que la longueur d’onde, réalisés à partir de matériaux composites tels que des métaux ou des diélectriques. Les métamatériaux acquièrent leurs propriétés de leur structure macroscopique plutôt que des propriétés microscopiques des élements qui les composent. Le mot « méta » provient du Grec dont la signification est au-delà, indiquant le concept d’une abstraction au-delà d’un autre concept. Les métamatériaux statiques conventionnels tirent profit de l’ingénierie spatiale de la dispersion pour présenter des propriétés exotiques non observées dans les matériaux usuels, tel qu’un indice de réfraction négatif. Un type plus sophistiqué de métamatériaux statiques, basé sur une structure dispersive modulée spatialement, peut être employé pour former un manteau d’invisibilité. Au cours de la dernière décennie, les métamatériaux dynamiques ont été présentés comme une nouvelle génération de systèmes électromagnétiques versatiles et ont rapidement acquis un grand intérêt de la part de la communauté scientifique. Les milieux modulés dans “l’espacetemps”, dont les paramètres constitutifs varient périodiquement dans l’espace et le temps, représentent une classe avancée de métamatériaux dynamiques non-réciproques. De tels milieux sont dotés de propriétés particulières telles que la capacité à générer des fréquences de façon non-réciproque. Contrairement aux métamatériaux périodiques statiques tels que les cristaux photoniques, les milieux modulés dans l’espace-temps présentent une dispersion asymétrique. D’ailleurs, par analogie avec les milieux en mouvement, où la vitesse du milieu est limitée à la vitesse de la lumière, les milieux modulés dans l’espace-temps peuvent acquérir des vitesses subluminales et superluminales. En conséquence, un éventail varié de bandes d’énergie orientées horizontalement, obliquement et verticalement sont accessibles dans les milieux modulés dans l’espace-temps, alors que dans les métamatériaux conventionnels ou les réseaux de Bragg, les bandes d’énergie sont seulement orientées horizontalement. Ces bandes d’énergie obliques et verticales apportent des degrés de liberté additionnels qui peuvent être utilisés pour la conception de différents systèmes électromagnétiques. La non-réciprocité basée sur la modulation dans l’espace-temps offre un chemin viable vers la conception de systèmes électromagnétiques non-réciproques intégrés.----------ABSTRACT Metamaterials are engineered structures which interact with the electric and magnetic components of light in a peculiar way that natural materials do not. These so-called meta-atoms are made of assemblies of subwavelengthly spaced elements fashioned from composite materials such as metals or dielectrics. Metamaterials acquire their properties from their macroscopic structure rather than the microscopic material of which they are made of. The word “Meta” originates from Greek whose meaning is beyond, indicating a concept as an abstraction beyond another concept. Conventional static metamaterials take advantage of spatial dispersion engineering to exhibit exotic properties not observed in bulk materials, such as for instance negative refractive index. A more sophisticated type of conventional static metamaterials based on a space-modulated (gradient-index) spatially dispersive structure, was used to form an invisibility cloak. Over the past decade, dynamic metamaterials, as a new generation of versatile electromagnetic systems, have been introduced and soon acquired a surge of scientific interest. “Spacetime modulated” media, whose constitutive parameters are periodically varying in space and time, represent an advanced class of nonreciprocal dynamic metamaterials. Such media are endowed with peculiar properties such as nonreciprocal frequency generation. In contrast to static periodic metamaterials such as photonic crystals, periodic space-time modulated media exhibit asymmetric dispersion. Moreover, by analogy with the moving media, where the velocity of the medium is limited to the speed of light, space-time modulated medium may acquire both subluminal and superluminal velocities. As a result, a diverse range of horizontally-, obliquely- and vertically-oriented electromagnetic band-gaps are accessible in space-time modulated media, compared to horizontal bandgaps in conventional metamaterials and Bragg structures. These oblique and vertical electromagnetic band-gaps offer extra degrees of freedom which may be leveraged for the design of different electromagnetic systems. Nonreciprocity based on space-time modulation grants a viable path towards integrated nonreciprocal electromagnetic systems. This addresses issues of conventional nonreciprocity techniques, such as for instance bulkiness and incompatibility with integrated circuit technology in magnet-based nonreciprocity, and signal power restrictions in nonlinear-based nonreciprocity. Space and time modulation combined with spatial and temporal dispersion engineering techniques offers a variety of unique electromagnetic properties to be discovered

Four-Dimensional Wave Transformations By Space-Time Metasurfaces

Static metasurfaces have shown to be prominent compact structures for reciprocal and frequency-invariant transformation of electromagnetic waves in space. However, incorporating temporal variation to static metasurfaces would result in dynamic apparatuses which are capable of four-dimensional tailoring of both the spatial and temporal characteristics of electromagnetic waves, leading to functionalities that are far beyond the capabilities of conventional static metasurfaces. This includes nonreciprocal full-duplex wave transmission, pure frequency conversion, parametric wave amplification, spatiotemporal decomposition, and space-time wave diffraction. This paper reviews recent progress and opportunities offered by space-time metasurfaces to break reciprocity, revealing their potential for low-energy, compact, integrated non-reciprocal devices and sub-systems, and discusses the future of this exciting research front.Comment: arXiv admin note: text overlap with arXiv:1902.0988

Full-Duplex Nonreciprocal-Beam-Steering Metasurfaces Comprising Time-Modulated Twin Meta-Atoms

We present the concept, theoretical model and experimental implementation of a full-duplex nonreciprocal-beam-steering transmissive phase-gradient metasurface. Such a metasurface is realized by exploiting the unique properties of the frequency-phase transition in coupled time-modulated twin meta-atoms. The metasurface may be placed on top of a source antenna to transform the radiation pattern of the source antenna, and introduce different radiation patterns for the transmit and receive states. In contrast to the recently proposed applications of time modulation, here the incident and transmitted waves share the same frequency. The metasurface is endowed with directive, diverse and asymmetric transmission and reception radiation beams, and tunable beam shapes. Furthermore, these beams can be steered by simply changing the modulation phase. The proposed coupled meta-atoms inherently suppress undesired time harmonics, leading to a high conversion efficiency which is of paramount importance for practical applications such as point to point full-duplex communications