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Space-time modulation systems have garnered significant attention due to their resemblance to moving-matter systems and promising applications. Unlike conventional moving-matter systems, modulation systems do not involve net motion of matter, and are therefore easier to implement and not restricted to subluminal velocities. However, canonical wave-medium interaction aspects, such as scattering and energy-momentum relations, have remained largely unexplored. In this paper, we address the aforementioned issues for three dynamic systems: moving-matter blocs, moving-perturbation interfaces and moving-perturbation periodic structures, and provide corresponding general formulations along with comparisons. Our investigation reveals the significant roles played by the "catch-up" effect between waves and interfaces. Even more interestingly, it reveals different energy and momentum exchanges between moving media and homogenized moving-perturbation structures as a result of conventional and reverse Fresnel-Fizeau drag effects., Comment: 12 pages, 9 figures

We present a general theory for calculating photon transitions in arbitrarily time-varying metamaterials. This theory circumvents the difficulties of conventional approaches in solving such a general problem by exploiting the eigenstates of time-dependent number operators. We demonstrate here the temporal evolution of these operators and the related transition probabilities for the cases of logistic and linear permittivity profiles. The theory is potentially extensible to arbitrary space-time modulations and may hence lead to multiple novel quantum effects and applications., Comment: 3 pages, 1 figure. This work has been submitted to the IEEE for possible publication. Copyright may be transferred without notice, after which this version may no longer be accessible

We present a general analytical solution to the problem of electromagnetic scattering at a one-dimensional arbitrarily accelerated space-time engineered-modulation (ASTEM) interface in the subluminal regime. We show that such an interface fundamentally produces chirping, whose profile can be designed according to specifications. This work represents an important step in the development of ASTEM crystals and holds significant potential for applications in microwave and optical devices reliant on chirp-based functionalities., Comment: 3 pages, 2 figures

We solve the problem of electron scattering at a soft temporal potential step. Given the relativistic nature of the problem, we use the Dirac equation, with its spinor wavefunction. We find solutions in terms of hypergeometric functions, which demonstrate that the observed phenomenon of later forward-wave and backward-wave electron scattering previously reported for a sharp (Heaviside) temporal potential step can be obtained experimentally and applied to new electronic devices., Comment: 2 figures

We solve the problem of electron scattering at a potential temporal step discontinuity. We show that the Schrodinger equation cannot account for scattering in this problem, necessitating resort to the Dirac equation, and that breaking gauge symmetry requires a vector potential, a scalar potential inducing only Aharonov-Bohm type energy transitions. We derive the scattering probabilities, of later forward and backward nature, with the later-backward wave being a relativistic effect, and compare the results with those for the spatial step and classical electromagnetic counterparts of the problem. Given the unrealizability of an infinitely sharp temporal discontinuity - which is of the same nature as its spatial counterpart! - we also provide solutions for a smooth potential step and demonstrate that the same physics as for the infinitely sharp case is obtained when the duration of the potential transition is sufficiently smaller than the de Broglie period of the electron (or deeply sub-period).

Space-time modulation-based metamaterials have recently spurred considerable interest, owing to the fundamental addition of the time dimension to the medium parameters, and resulting novel properties and potential applications. However, the implementation of most related structures -- e.g., involving step, slab or gradient discontinuities -- has been hindered by the impossible requirement of infinitely or prohibitively large device sizes. We provide here a solution to this issue, consisting in a space-time transposition of the conventional Fresnel prism, whereby a copy of the target modulation is periodically re-injected at the input of a Fresnel-reduced finite structure, so as to provide the same anharmonic and nonreciprocal frequency conversion as the target space-time interface in the case of a modulation step. This concept, which may readily extend to slab or gradient modulations, as well as accelerated profiles for space-time chirping operations, may pave the way for the practical development of a wide range of novel microwave and optical space-time systems.

This paper presents a space-time-wise orthogonal analysis of space-time crystals. This analysis provides a solution consisting of a pair of explicit parametric equations that result from a separate application of the Bloch-Floquet theorem in the (orthogonal) directions of space and time. Compared to previous approaches, this solution offers the benefits of greater simplicity, clearer emphasis on space-time duality and deeper physical insight., Comment: letter (4 pages)

We present a generalized FDTD scheme to simulate moving electromagnetic structures with arbitrary space-time configurations. This scheme is a local adaptation and 2+1-dimensional extension of the uniform and 1+1-dimensional scheme recently reported in [1]. The local adaptation, which is allowed by the inherently matched nature of the generalized Yee cell to the conventional Yee cell, extends the range of applicability of the scheme in [1] to moving structures that involve multiple and arbitrary velocity profiles while being fully compatible with conventional absorbing boundary conditions and standard treatments of medium dispersion. We show that a direct application of the conventional FDTD scheme predicts qualitatively correct spectral transitions but quantitatively erroneous scattering amplitudes, we infer from this observation generalized, hybrid-physical and auxiliary (non-physical) - fields that automatically satisfy moving boundary conditions in the laboratory frame, and accordingly establish local update equations based on the related Maxwell's equations and constitutive relations. We subsequently provide a detailed stability analysis with a generalization of the Courant criterion to the dynamic regime. We finally validate and illustrate the proposed method by several representative examples. The proposed scheme fills an important gap in the open literature on computational electromagnetics and offers an unprecedented, direct solution for moving structures in commercial software platforms., Comment: 13 pages, 11 figures

Electromagnetic scattering in moving structures is a fundamental topic in physics and engineering. Yet, no general numerical solution to related problems has been reported to date. We introduce here a generalized FDTD scheme to remedy this deficiency. That scheme is an extension of the FDTD standard Yee cell and stencil that includes not only the usual, physical fields, but also auxiliary, unphysical fields allowing a straightforward application of moving boundary conditions. The proposed scheme is illustrated by four examples -- a moving interface, a moving slab, a moving crystal and a moving gradient -- with systematic validation against exact solutions., Comment: 15 pages, 8 figures

Over the past decade, the issue of gain degradation at broadside in periodic leaky-wave antennas (P-LWAs) has been resolved, using a circuit modeling approach, by introducing proper asymmetry in the unit cell of the antenna structure. This paper provides a more fundamental and insightful perspective of the problem by showing, using a simple coupled-mode analysis, that the optimal level of structural asymmetry corresponds to an exceptional point of the coupling parameter between the two eigenmodes of the P-LWA. This contribution represents a key step towards the development of a full electromagnetic resolution of the broadside issue.

Over the past 30 years, Artificial Magnetic Conductors (AMCs) have generated immense interest in the microwave community. However, their description has been restricted to crude RLC design or subsidiary transmission-line network modelling. This paper introduces an electromagnetic theory of AMCs in terms of bianisotropic susceptibilities.

Space-time varying metamaterials based on uniform-velocity modulation have spurred considerable interest over the past decade. We present here the first extensive investigation of accelerated modulation space-time metamaterials. Using the tools of general relativity, we establish their electrodynamic principles and describe their fundamental phenomena, in comparison with the physics of moving-matter media. We show that an electromagnetic beam propagating in an accelerated modulation metamaterial is bent in its course, which reveals that such a medium curves space-time for light, similarly to gravitation. Finally, we illustrate the vast potential diversity of accelerated modulation metamaterial by demonstrating related Schwarzschild holes.

Cloaking is typically reciprocal. We introduce here the concept of \emph{transmittable nonreciprocal cloaking} whereby the cloaking system operates as a standard omnidirectional cloak for external illumination, but can transmit light from its center outwards at will. We demonstrate a specific implementation of such cloaking that consists in a set of concentric bianisotropic metasurfaces whose innermost element is nonreciprocal and designed to simultaneously block inward waves and pass -- either omnidirectionaly or directionally -- outward waves. Such cloaking represents a fundamental diversification of conventional cloaking and may find applications in areas such as stealth, blockage avoidance, illusion and cooling., Comment: 13 pages, 11 figures

Recent research developments in the area of spacetime metamaterial structures and systems have raised new questions as to how the physics of fundamental phenomena is altered in the presence of spacetime modulation. In this context, we present a generalized and comparative description of the phenomenon of total internal reflection (TIR) at different dynamic interfaces. Such interfaces include, beyond the classical interfaces corresponding to the boundaries of moving bodies (moving interface -- moving matter systems), interfaces formed by a traveling-wave step modulation of an electromagnetic parameter (e.g., refractive index) (moving interface -- stationary matter systems) and fixed interfaces between moving-matter media (stationary interface -- moving matter systems). We first resolve the problem using the evanescence of the transmitted wave as the criterion for TIR and applying the conventional technique of relative frame hopping (between the laboratory and rest frames), which results in closed-form formulas for the relevant critical (incidence, reflection, phase refraction and power refraction) angles. We then introduce the concept of catch-up limit between the dynamic interface and the transmitted wave as an alternative criterion for the critical angle. We use this approach both to analytically verify the critical angle formulas, further validated by full-wave (FDTD) analysis, and to explain the related physics, using Fresnel-Fizeau drag and spacetime frequency transition considerations. These results might find various applications in ultra-fast optics, gravity analogs and quantum processing.

This article presents a global and generalized perspective of electrodynamic meta-materials formed by space and time engineered modulations, which we name Generalized Space-Time Engineered Modulation (GSTEM) Metamaterials, or GSTEMs. In this perspective, it describes metamaterials from a unified spacetime viewpoint and introduces accelerated metamaterials as an extra type of dynamic metamaterials. First, it positions GSTEMs in the even broader context of electrodynamic systems that include (non-modulated) moving sources in vacuum and moving bodies, explains the difference between the moving-matter nature of the latter and the moving-perturbation nature of GSTEMs, and enumerates the different types of GSTEMs considered, namely Space EMs (SEMs), Time EMs (TEMs), Uniform Space-Time EMs (USTEMs) and Accelerated Space-Time EMs (ASTEMs). Next, it establishes the physics of the related interfaces, which includes direct-spacetime scattering and inverse-spacetime transition transformations. Then, it exposes the physics of the GSTEM metamaterials formed by stacking these interfaces and homogenizing the resulting crystals; this includes an original explanation of light deflection by USTEMs as being a spacetime weighted averaging phenomenon and the demonstration of ASTEM light curving and black-hole light attraction. Finally, it discusses some future prospects. Useful complementary information and animations are provided in the Supplementary Material.

This paper addresses the problem of wave scattering at a moving interface formed by a space-time modulation step. Specifically, it derives, using the technique of frame hopping with Lorentz transformation, the formula for the corresponding critical angle beyond which the transmitted field is evanescent. It shows that this angle is smaller (resp. larger) than $\pi/2$ (position of the interface) for a modulation that is codirectional (resp. contradirectional) to the direction of wave propagation, and that the critical angle versus the modulation velocity function monotonically decreases to zero at the velocity where the incident wave cannot catch up any more with the interface. The theory is illustrated and validated by full-wave FDTD simulation., Comment: 3 pages, 2 figures

This paper opens up the field of nonuniform-velocity SpaceTime-Modulated (STM) metamaterials, with the canonical example of an STM metamaterial of constant proper acceleration or, equivalently, hyperbolic acceleration. Combining tools of General Relativity and Classical Electrodynamics, it derives the dispersion relation of this exotic medium and reports its fundamental physics, whose most striking feature is the bending of light in the direction opposite to the direction of the modulation., Comment: 2 pages, 3 figures

We present a (nongyrotropic) metasurface magnetless specular isolator. This device reflects as a mirror a wave incident under a specified angle in one direction and absorbs it in the opposite direction. The metasurface is synthesized in terms of bianisotropic susceptibility tensors, whose nonreciprocity resides in normal components and exhibits a hybrid electric, magneto-electric nature. The metaparticle is implemented in the form of a U-shaped conducting structure loaded by a transistor. The operation principle of the specular isolator is demonstrated by both full-wave simulation and experiment, with isolation levels reaching 41 and 38 dB respectively. This system represents the first realization of a metasurface involving nonreciprocal normal susceptibilities and features a previously unreported type of nonreciprocity.

This Roadmap takes the reader on a journey through the research in electromagnetic wave propagation control via reconfigurable intelligent surfaces. Meta-surface modelling and design methods are reviewed along with physical realisation techniques. Several wireless applications are discussed, including beam-forming, focusing, imaging, localisation, and sensing, some rooted in novel architectures for future mobile communications networks towards 6G., Comment: 55 pages, 22 figures, submitted to Reviews of Electromagnetics

We introduce a metasurface-based system for the microwave hyperthermia treatment of brain tumors, where the metasurface is designed by the time-reversal technique for optimal focusing at the location of the tumor. Compared to previous time-reversal microwave hyperthermia systems, which are based on antenna arrays, the proposed system offers two advantages: 1) it provides superior focal resolution, due to the finer sampling capability of its deeply-subwavelength metasurface particles compared to that of typical array antenna elements, 2) it features much lower complexity and cost, due to its direct illumination principle compared to the heavy feeding network of the array system. The system leverages reflecting metallic walls and a rich scattering structure in combination with the metasurface to achieve high hyperthermia performance with maximal simplicity. The metasurface system is demonstrated by full-wave simulation results, which show that it represents a viable novel noninvasive hyperthermia technology.

We present the concept of a magnetless Reflective Gyrotropic Spatial Isolator (RGSI) metasurface. This is a birefringent metasurface that reflects vertically polarized incident waves into a horizontally polarized waves, and absorbs horizontally polarized incident waves, hence providing isolation between the two orthogonal polarization. We first synthesize the metasurface using surface susceptibility-based Generalized Sheet Transition Conditions~(GSTCs). We then propose a mirror-backed metaparticle implementation of this metasurface, where transistor-loaded resonators provide the desired magnetless nonreciprocal response. Finally, we demonstrate the metasurface by full-wave simulation results. The proposed RGSI metasurface may be used in various electromagnetic applications, and may also serve as a step towards more sophisticated magnetless nonreciprocal metasurface systems.

Optical antennas made of low-loss dielectrics have several advantages over plasmonic antennas, including high radiative quantum efficiency, negligible heating and excellent photostability. However, due to weak spatial confinement, conventional dielectric antennas fail to offer light-matter interaction strengths on par with those of plasmonic antennas. We propose here an all-dielectric antenna configuration that can support strongly confined modes ($V\sim10^{-4}\lambda_{0}^3$) while maintaining unity antenna quantum efficiency. This configuration consists of a high-index pillar structure with a transverse gap that is filled with a low-index material, where the contrast of indices induces a strong enhancement of the electric field perpendicular to the gap. We provide a detailed explanation of the operation principle of such Photonic Gap Antennas (PGAs) based on the dispersion relation of symmetric and asymmetric horizontal slot-waveguides. To discuss the properties of PGAs, we consider silicon pillars with air or CYTOP as the gap-material. We show by full-wave simulations that PGAs with an emitter embedded in the gap can enhance the spontaneous emission rate by a factor of $\sim$1000 for air gaps and $\sim$400 for CYTOP gaps over a spectral bandwidth of $\Delta\lambda\approx300$ nm at $\lambda=1.25$ \textmu m. Furthermore, the PGAs can be designed to provide unidirectional out-of-plane radiation across a substantial portion of their spectral bandwidth. This is achieved by setting the position of the gap at an optimized off-centered position of the pillar so as to properly break the vertical symmetry of the structure. We also demonstrate that, when acting as receivers, PGAs can lead to a near-field intensity enhancement by a factor of $\sim$3000 for air gaps and $\sim$1200 for CYTOP gaps.

Laser cooling of solids keeps attracting attention owing to abroad range of its applications that extends from cm-sized all-optical cryocoolers for airborne and space-based applications to cooling on nanoparticles for biological and mesoscopic physics. Laser cooling of nanoparticles is a challenging task. We propose to use Mie resonances to enhance anti-Stokes fluorescence laser cooling in rare-earth (RE) doped nanoparticles made of low-phonon glasses or crystals. As an example, we consider an Yb3+:YAG nanosphere pumped at the long wavelength tail of the Yb3+ absorption spectrum at 1030 nm. We show that if the radius of the nanosphere is adjusted to the pump wavelength in such a manner that the pump excites some of its Mie resonant modes, the cooling power density generated in the sample is considerably enhanced and the temperature of the sample is consequently considerably (~ 63%) decreased. This concept can be extended to nanoparticles of different shapes and made from different low-phonon RE doped materials suitable for laser cooling by anti-Stokes fluorescence., Comment: 7 pages, 5 figures

We show that a properly designed bianisotropic metasurface placed at the interface between two arbitrary different media, or coating a dielectric medium exposed to the air, provides Brewster (reflectionless) transmission at arbitrary angles and for both the TM and TE polarizations. We present a rigorous derivation of the corresponding surface susceptibility tensors based on the Generalized Sheet Transition Conditions (GSTCs), and demonstrate the system with planar microwave metasurfaces designed for polarization-independent and azimuth-independent operations. Moreover, we reveal that such a system leads to the concept of effective refractive media with engineerable impedance. The proposed bianisotropic metasurfaces provide deeply subwavelength matching solutions for initially mismatched media, and alternatively lead to the possibility of on-demand manipulation of the conventional Fresnel coefficients. The reported generalized Brewster effect represents a fundamental advance in optical technology, where it may both improve the performance of conventional components and enable the development of novel devices.

We propose to use frequency-selective quasi parity-time symmetry to reduce the heat generated by coherent anti-Stokes Raman scattering (CARS) reversed cycles in active Raman media with phase mismatch. This is accomplished using a coupled-waveguide structure, which includes the active Raman waveguide (RW), where the Stokes signal undergoes amplification via stimulated Stokes Raman scattering (SSRS), and a dissipative waveguide (DW), which is tuned to the anti-Stokes wavelength so as to evacuate the corresponding anti-Stokes photons from the RW by coupling. The DW introduces optical loss that partially offsets the growth of the anti-Stokes signal in the RW and hence suppress the reversed CARS cycles that would otherwise result into heat generation in the RW. It is shown that the frequency-selective quasi parity-time symmetry provided by the DW can reduce the heat in active Raman media by a very factor of up to five when the CARS phase mismatch is compensated for by the optimum level of coupling between the RW and the DW., Comment: 8 pages, 9 figures

We present a unified and extended perspective of Bessel beams, irrespective to their orbital angular momentum (OAM) -- zero, integer or noninteger -- and mode -- scalar or vectorial, and LSE/LSM or TE/TM in the latter case. The unification is based on the integral superposition of constituent waves along the angular-spectrum cone of the beam, and allows to describe, compute, relate, and implement all the Bessel beams, and even other types of beams, in a universal fashion. The paper first establishes the integral superposition theory. Then, it demonstrates the previously unreported existence of noninteger-OAM TE/TM Bessel beams, compares the LSE/LSM and TE/TM modes, and establishes useful mathematical relations between them. It also provides an original description of the position of the noninteger-OAM singularity in terms of the initial phase of the constituent waves. Finally, it introduces a general technique to generate Bessel beams by an adequate superposition of properly tuned sources. This global perspective and theoretical extension may open up new avenues in applications such as spectroscopy, microscopy, and optical/quantum force manipulations.

We investigate the possibility to model a metasurface, defined as a zero-thickness sheet of surface polarization currents, by a thin slab, characterized by a subwavelength thickness and usual voluminal medium parameters. First, we elaborate a general equivalence relation between the metasurface and the slab in terms of average electromagnetic fields. Then, we derive exact relations between the metasurface and slab susceptibilities and validate them by full-wave simulations. Finally, we discuss the simple and insightful Average Field Approximation (AFA) formula, illustrate its inappropriate for strong metasurface field transformations, and establish its range of validity. All of these developments are restricted to the simplest case of a uniform isotropic metasurface under normal plane wave incidence. We conclude from the complexity of the equivalence for this case, that a metasurface is generally best modeled in terms of Generalized Sheet Transition Conditions (GSTCs)., Comment: 9 pages, 12 figures

This paper extends current knowledge on electromagnetic wave scattering from bounded moving media in several regards. First, it complements the usual dispersion relation of moving media, $\omega(\theta_\mathbf{k})$ ($\theta_\mathbf{k}$: phase velocity direction, associated with the wave vector, $\mathbf{k}$), with the equally important impedance relation, $\eta(\theta_\mathbf{S})$ ($\theta_\mathbf{S}$: group velocity direction, associated with the Poynting vector, $\mathbf{S}$). Second, it explains the interluminal-regime phenomenon of double-downstream wave transmission across a stationary interface between a regular medium and the moving medium, assuming motion perpendicular to the interface, and shows that the related waves are symmetric in terms of the energy refraction angle, while being asymmetric in terms of the phase refraction angle, with one of the waves subject to negative refraction, and shows that the wave impedances of the two transmitted waves are equal. Third, it generalizes the problem to the case where the medium moves obliquely with respect to the interface. Finally, it highlights the connection between this problem and a spacetime modulated medium., Comment: 14 pages, 10 figures

This paper introduces a general technique for inter-mapping the complex spatial frequency (or propagation constant) $\gamma=\alpha+j\beta$ and the temporal frequency $\omega = \omega_\text{r}+j\omega_\text{i}$ of an arbitrary electromagnetic structure. This technique, based on the analytic property of complex functions describing physical phenomena, invokes the analytic continuity theorem to assert the unicity of the mapping function, and find this function from known data within a restricted domain of its analycity by curve-fitting to a generic polynomial expansion. It is not only applicable to canonical problems admitting an analytical solution, but to \emph{any} problems, from eigen-mode or driven-mode full-wave simulation results. The proposed technique is demonstrated for several systems, namely an unbounded lossy medium, a dielectric-filled rectangular waveguide, a periodically-loaded transmission line, a one-dimensional photonic crystal and a series-fed patch (SFP) leaky-wave antenna (LWA), and it is validated either by analytical results or by full-wave simulated results., Comment: 11 pages, 11 figure

We show that the use of the electromagnetic inverse source framework offers great flexibility in the design of metasurfaces. In particular, this approach is advantageous for antenna design applications where the goal is often to satisfy a set of performance criteria such as half power beamwidths and null directions, rather than satisfying a fully-known complex field. In addition, the inverse source formulation allows the metasurface and the region over which the desired field specifications are provided to be of arbitrary shape. Some of the main challenges in solving this inverse source problem, such as formulating and optimizing a nonlinear cost functional, are addressed. Lastly, some two-dimensional (2D) and three-dimensional (3D) simulated examples are presented to demonstrate the method, followed by a discussion of the method's current limitations., Comment: 13 pages

This paper presents the concept of spread-spectrum selective camouflaging based on time-modulated metasurface. The spectrum spreading is realized by switching the metasurface between the reflective states of a PEC mirror and a PMC mirror, using an array of microstrip patches connected to the ground via diode switches, according to a periodic pseudo-random noise sequence. As the spectrum spreading induces a drastic reduction of the power spectral density of the signal, the level of the scattered wave falls below the noise floor of the interrogating radar, and the object covered by the metasurface is hence perfectly camouflaged to a foe radar. Moreover, the object can be detected by a friend radar possessing the spread-spectrum demodulation key corresponding to the metasurface modulation, and this detection is robust to interfering signals. The proposed system is analyzed theoretically, and demonstrated by both simulation and experimental results.

Conventional surface-relief gratings are inefficient at deflecting normally-incident light by large angles. This constrains their use in many applications and limits the overall efficiency of any optical instrument integrating gratings. Here, we demonstrate a simple approach for the design of diffraction gratings that can be highly efficient for large deflection angles, while also offering additional functionality. The gratings are composed of a unit cell comprising a vertically-oriented asymmetric slot-waveguide. The unit cell shows oscillating unidirectional scattering behavior that can be precisely tuned as a function of the waveguide length. This occurs due to interference between multiple modes excited by the incident light. In contrast to metasurface-based gratings with multiple resonant sub-elements, a periodic arrangement of such non-resonant diffracting elements allows for broadband operation and a strong tolerance for variations in angle of incidence. Full-wave simulations show that our grating designs can exhibit diffraction efficiencies ranging from 94% for a deflection angle of 47$^\circ$ to 80% for deflection angle of 80$^\circ$. To demonstrate the multifunctionality of our grating design technique, we have also proposed a flat polarization beamsplitter, which allows for the separation of the two orthogonal polarizations by 80$^\circ$, with an efficiency of 80%.

This paper presents the authors' vision of the emerging field of spacetime metamaterials in a cohesive and pedagogical perspective. For this purpose, it systematically builds up the physics, modeling and applications of these media upon the foundation of their pure-space and pure-time counterparts., Comment: 28 pages, 19 figures, invited by IEEE Trans. Antennas Propag

Interest for spacetime structures has recently been revived, following developments in metamaterials and ultrafast optics. Such structures essentially consist of successions of space-time interfaces for which the theory is still incomplete, in particular in the regime where the interface velocity lies between the wave velocities in the two media involved. This paper addresses this `interluminal' regime, providing exact scattering solutions for both approaching and receding interfaces. The solutions are verified to be consistent with the transmission matrix perspective and to be continuous at the limits of the subluminal and superluminal regimes., Comment: conference paper with 2 pages, 2 figures

We introduce the concept of nonreciprocal nongyrotropic phase gradient metasurfaces. Such metasurfaces are based on bianisotropic phase shifting unit cells, with the required nonreciprocal and nongyrotropic characteristics. Moreover, we present a transistor-based implementation of a nonreciprocal phase shifting subwavelength unit cell. Finally, we demonstrate the concept with a simulation of a 6-port spatial circulator application.

We derive a general condition for angle-independent bianisotropic nongyrotropic metasurfaces and present two applications corresponding to particular cases: an angle-independent absorber/amplifier and an angle-independent spatial gyrator.

This paper presents a first-principle and global perspective of electromagnetic chirality. It follows for this purpose a bottom-up construction, from the description of chiral particles or metaparticles (microscopic scale), through the electromagnetic theory of chiral media (macroscopic scale), to the establishment advanced properties and design principles of chiral materials and metamaterials. It preliminarily highlights the three fundamental concepts related to chirality -- mirror asymmetry, polarization rotation and magnetodielectric coupling -- and points out the nontrivial interdependencies existing between them. The first part (chiral particles) presents metamaterial as the most promising technology for chirality, compares two representative particles involving magnetoelectric coupling, namely the planar Omega particle and the twisted Omega or helix particle, and shows that only the latter is chiral, and finally links the response of microscopic particles to that of the medium formed by arranging them according to a subwavelength lattice structure. The second part (electromagnetic theory) infers from the previous microscopic study the chiral constitutive relations as a subset of the most general bianisotropic relations, derives parity conditions for the chiral parameters, computes the chiral eigenstates as circularly polarized waves, and finally shows that the circular birefringence of these states leads to polarization rotation. The third part (properties and design) introduces an explicit formulation of chirality based on spatial frequency dispersion or nonlocality, analyzes the temporal frequency dispersion or nonlocality of chiral media, and finally provides guidelines to design a practical chiral metamaterial., Comment: 27 pages, 18 figures

This paper compares Tensor Boundary Conditions (TBCs), which were introduced to model multilayered dielectric structures, with Generalized Sheet Transition Conditions (GSTCs), which have been recently used to model metasurfaces. It shows that TBCs, with their 3 scalar parameters, are equivalent to the direct-isotropic -- cross-antiisotropic, reciprocal and nongyrotropic subset of GSTCs, whose 16 tangential (particular case of zero normal polarizations) or 36 general susceptibility parameters can handle the most general bianisotropic sheet structures. It further shows that extending that TBCs scalar parameters to tensors and allowing a doubly-occurring parameter to take different values leads to a TBCs formulation that is equivalent to the tangential GSTCs, but without reflecting the polarization physics of sheet media, such as metasurfaces and two-dimensional material allotropes., Comment: 6 pages, 1 figure

We present a novel type of Suppressed-Order Diffraction Grating(SODG). An SODG is a diffraction grating whose diffraction orders have all been suppressed except one that is selected to provide electromagnetic deflection. The proposed SODG is a transmissive grating that exhibits high-efficiency refraction-like deflection for all angles, including large angles that are generally challenging to achieve, while featuring a deeply subwavelength thickness, as required in the microwave regime. We first present the design rationale and guidelines, and next demonstrate such a 10.5 GHz SODG that reaches an efficiency of 90% at 70 degrees.

We demonstrate a novel grating coupler design based on double asymmetric and vertically oriented waveguide scatterers to efficiently couple normally incident light to a fundamental mode silicon waveguide laying on a buried oxide layer., Comment: 4 pages, 1 figure

This paper presents a pulse compansion, i.e. compression or expansion, technique based on co-propagating space-time modulation. An engineered asymmetric space-time modulated medium, co-propagating with a pulse compands the pulse continuously and at a constant rate. The space-time medium locally modifies the velocity of different sections of the pulse in order to shape the pulse as it propagates. There is no theoretical limit on the compansion factor with the proposed system. Moreover, it can be designed to transform the pulse shape and its modulation linearly, without any distortion. Therefore the proposed technique can be used for up or down-conversion of modulated pulses, with extreme conversion ratios. The presented compansion technique is linear with respect to the input wave and therefore can be used to perform compansion or frequency conversion on multiple pulses simultaneously.

This paper presents a technique, combining the integral equations (IE) and the Generalized Sheet Transition Conditions (GSTCs) with bianisotropic susceptibility tensors, to compute electromagnetic wave scattering by cylindrical metasurfaces -- forming two-dimensional porous cavities -- of arbitrary cross sections. Moreover, it applies this technique to two problems -- cloaking with circular and rhombic shapes and illusion optics with an elliptic shape -- that both validate it, from comparison with specifications used in an exact synthesis of the metasurfaces, and reveal interesting capabilities of such metasurface structures. Particularly, active cylindrical metasurfaces can perfectly cloak and hence eliminate the extinction cross section of various cylindrical shapes, and simple purely passive versions of them, practically more accessible, still perform quite good cloaking and provide remakable extinction cross section reduction., Comment: 11 pages, 13 figures

A microwave Hilbert transformer is introduced as a new component for Real-time Analog Processing (RAP). In contrast to its optical counterpart, that resort to optical fiber gratings, this Hilbert transformer is based on the combination of a branch-line coupler and a loop resonator. The transfer function of the transformer is derived using signal flow graphs, and two figures of merits are introduced to effectively characterize the device: the rotated phase and the transition bandwidth. Moreover, a detailed physical explanation of its physical operation is given, using both a steady-state regime perspective and a transient regime perspective. The microwave RAP Hilbert transformer is demonstrated experimentally, and demonstrated in three applications: edge detection, peak suppression and single sideband modulation.

This paper presents space-time varying (STV) metasurfaces for simultaneously controlling the spatial and temporal spectra of electromagnetic waves. These metasurfaces transform incident electromagnetic waves into specified reflected and transmitted waves, with arbitrary temporal and spatial frequencies. They are synthesized in terms of time-domain generalized sheet transition conditions (GSTCs). Moreover, they are characterized using an analytical method and the unstaggered finite-difference time-domain (FDTD) technique adapted to space-time metasurfaces. STV metasurfaces performing pulse shaping, time reversal and differentiation are demonstrated as examples., Comment: IEEE Transactions on Antennas and Propagation (2019)

We perform a comprehensive analysis of uniform-velocity bilayer spacetime crystals, combining concepts of conventional photonic crystallography and special relativity. Given that a spacetime crystal consists of a sequence of spacetime discontinuities, we do this by solving the following sequence of problems: 1) the spacetime interface, 2) the double spacetime interface, or spacetime slab, 3) the unbounded crystal, and 4) the truncated crystal. For these problems, we present the following respective new results: 1) an extension of the Stokes principle to spacetime interfaces, 2) an interference-based analysis of the interference phenomenology, 3) a quick linear approximation of the dispersion diagrams, a description of simultaneous wavenumber and frequency bandgaps, and 4) the explanation of the effects of different types of spacetime crystal truncations, and the corresponding scattering coefficients. This work may constitute the foundation for a virtually unlimited number of novel canonical spacetime media and metamaterial problems.

We introduce the concept of a Rotman-lens spectrum decomposer (RLSD) real-time spectrum-sniffer (RTSS) for cognitive radio. Compared to a previously existing RTSS, the RLSD-RTSS offers the advantages of being 1) based an a simpler and lower-cost purely passive structure, 2) easier to design and easily amenable to tunability, 3) of much broader bandwidth, and 4) of accommodating more channels. The electrical size of the device is electrically larger, but perfectly acceptable in the millimeter-wave frequency range. The proposed RLSD-RTSS is demonstrated theoretically and experimentally, and been shown to support tunability in terms of both bandwidth-resolution and operation band. Given its unique features, this device may find wide applications in 5G UHD and 3D video systems.

We introduce the concept of Metasurface Absorbing-Boundary Condition (MS-ABC). This represents the first application of metasurface technology to computational electromagnetics. MS-ABCs have similar performance as previously reported surface ABCs, but they provide a richer description of absorption, that is independent from the incident wave, and deeper insight into the physics, through bianisotropic susceptibilities. The MS-ABC susceptibilities are synthesized using GSTC synthesis, from which the metasurface is found to be mono-cross-anisotropic, i.e. having only electric-to-magnetic and magnetic-to-electric coupling susceptibility tensors being non-zero, which is physically impossible, but numerically perfectly appropriate. Moreover, it is found that the metasurface is asymmetric, being amplifying in the opposite direction of its basic utilization as an ABC. The MS-ABC is implemented in a GSTC-FDTD scheme, which, for the first time, replaces the virtual grid concept by straightforward surface polarization currents residing on the regular Yee grid., Comment: not be completed

This paper presents an overview of Unidirectional Loop Metamaterial (ULM) structures and applications. Mimicking electron spin precession in ferrites using loops with unidirectional loads (typically transistors), the ULM exhibits all the fundamental properties of ferrite materials, and represents the only existing magnetless ferrimagnetic medium. We present here an extended explanation of ULM physics and unified description of its component and system applications.

This paper is the second part of a two-part paper on \emph{Electromagnetic (EM) Nonreciprocity (NR)}. Part~I has defined NR, pointed out that linear NR is a stronger form of NR than nonlinear (NL) NR, explained EM Time-Reversal (TR) Symmetry (TRS) Breaking (TRS-B), described linear Time-Invariant (TI) NR media, generalized the Lorentz reciprocity theorem for NR, and provided a physical interpretation of the resulting Onsager-Casimir relations~\cite{Caloz_AWPL_NR_I_2018}. This part first explains the TR specificity of lossy and open systems. Next, it proposes an extended version of the S-parameters for \emph{all NR} systems. Then, it presents the fundamentals of linear-TI (LTI) NR, linear Time-Variant (LTV) Space-Time (ST) modulated NR and NL NR systems. Finally, it addresses confusions between with systems.