Metasurfaces consisting of electrically thin and densely packed planar arrays of subwavelength elements enable an unprecedented control of the impinging electromagnetic fields. Spatially modulated metasurfaces can efficiently tailor the spatial distribution of these fields with great flexibility. Similarly, time modulated metasurfaces can be successfully used to manipulate the frequency content and time variations of the impinging field. In this paper, we present time-modulated reflective metasurfaces that cause a frequency shift to the impinging radiation, thus realizing an artificial Doppler effect in a non-moving electrically thin structure. Starting from the theoretical analysis, we analytically derive the required time modulation of the surface admittance to achieve this effect, and present a realistic time-varying structure, based on a properly designed and dynamically tuned high-impedance surface. It is analytically and numerically demonstrated that the field emerging from the metasurface is up-,down-converted in frequency according to the modulation profile of the metasurface. The proposed metasurface concept, enabling a frequency modulation of the electromagnetic field on-the-fly, may find application in telecommunication, radar, and sensing scenarios.
This work theoretically and experimentally studies metasurfaces with spatially-discrete, traveling-wave modulation (SD-TWM). A representative metasurface is considered consisting of columns of time-modulated subwavelength unit cells, referred to as stixels. SD-TWM is achieved by enforcing a time delay between temporal waveforms applied to adjacent columns. In contrast to the continuous traveling-wave modulation commonly assumed in studies of space-time metasurfaces, here the modulation is spatially discretized. In order to account for the discretized spatial modulation, a modified Floquet anal…
This paper presents an end-to-end design method for the synthesis of dielectric metasurfaces with modulated thickness that are able to realize desired radiation patterns in the far-field with high precision. The method relies on integral equations and the Method of Moments to calculate the induced surface and volumetric current densities throughout the metasurface upon a given incident illumination. By avoiding homogenization of the dielectric layer and calculating directly the scattered fields through the Method of Moments, the near-field and far-field response of the metasurface can be accur…
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 transmitt…
In this paper, we present the design procedure of modulated metasurface leaky-wave antennas (MMLWAs) to generate coaxial superposition of vortex beams with several orbital angular momentum (OAM) states. Based on the Flat Optics (FO) technique and aperture field estimation (AFE) method, an analytical framework is proposed to facilitate the implementation of MMLWAs generating multiple topological charges in the OAM regimen. Furthermore, using the spectral analysis which has been derived from the proposed model, we have shown that the symmetry of aperture shape can affect the purity of the mode. …
Emerging photonic functionalities are mostly governed by the fundamental principle of Lorentz reciprocity. Lifting the constraints imposed by this principle could circumvent deleterious effects that limit the performance of photonic systems. A variety of approaches have recently been explored to break reciprocity, yet most efforts have been limited to confined photonic systems. Here, we propose and experimentally demonstrate a spatio-temporally modulated metasurface capable of extreme breakdown of Lorentz reciprocity. Through tailoring the momentum and frequency harmonic contents of the scatte…
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 metasurfa…
In recent years a significant progress has been made in the development of magnet-less nonreciprocity using space-time modulation, both in electromagnetics and acoustics. This approach has so far resulted in a plethora of non-reciprocal devices, such as isolators and circulators, over different parts of the spectrum, for guided waves. On the other hand, very little work has been performed on non-reciprocal devices for waves propagating in free space, which can also have many practical applications. For example, it was shown theoretically that non-reciprocal scattering by a metasurface can be o…
Modulation of metasurfaces in time gives rise to several exotic space-time scattering phenomena by violating the reciprocity and generation of higher-order frequency harmonics. We introduce a new design paradigm for time-modulated metasurfaces, offering electrically tunable engineering of the generated frequency harmonics and their emerging wavefronts by controlling the phase delay in modulation. It is demonstrated that the light acquires a dispersionless phase shift regardless of incident angle and polarization, upon undergoing frequency conversion in a time-modulated metasurface which is lin…
A circularly polarized leaky-wave antenna capable of frequency scanning is proposed in this paper. The main objective is to achieve high gain and polarization purity without the need for a complex feed network. The antenna consists of two independent modules: (1) Anisotropic modulated metasurface antenna (MoMetA) for generating vertically polarized radiation with an operational scanning range of 19 to 47 degrees in elevation; (2) Wide-band polarization converter with high angular stability and capable of converting vertically linear polarization into circular polarization (CP). Aperture field …
Known coherent metasurfaces control interference of waves of a given frequency with other coherent waves at the same frequency, either illuminating from a different direction or created as intermodulation products. In this paper, we introduce a class of metasurfaces that are modulated in time coherently with the illuminating radiation. Importantly, such modulation opens a possibility to control reflection, absorption, and transmission at multiple frequencies, including illuminations by two or more incoherent waves. In particular, we study dynamic resistive layers and show how to use them to de…
We conceptualized three-array scalable metasurfaces comprising only three thin strip grids and numerically demonstrated their characteristics in the mid-infrared spectral regime for switchable polarization manipulation and related asymmetric diodelike transmission (AT). A few or all of the grids were taken to be made of VO$_2$, a phase-change material. For each proposed metasurface, two effective structures and, therefore, two different functionalities exist, corresponding to the metallic and insulating phases of VO$_2$. The achieved scenarios of functionality switching that depend on the VO$_…
Integrating multiple functionalities into a single metasurface is becoming of great interest for future intelligent communication systems. While such devices have been extensively explored for reciprocal functionalities, in this work, we integrate a wide variety of nonreciprocal applications into a single platform. The proposed structure is based on spatiotemporally modulated impedance sheets supported by a grounded dielectric substrate. We show that, by engineering the excitation of evanescent modes, nonreciprocal interactions with impinging waves can be configured at will. We demonstrate a p…
In the quest to realize analog signal processing using sub-wavelength metasurfaces, in this paper, we demonstrate the first experimental demonstration of programmable time-modulated metasurface processors based on the key properties of spatial Fourier transformation. Exploiting space-time coding strategy enables local, independent, and real-time engineering of not only amplitude but also phase profile of the contributing reflective digital meta-atoms at both central and harmonic frequencies. Several illustrative examples are demonstrated to show that the proposed multifunctional calculus metas…
In this letter, we propose a magnet-less non-reciprocal isolating system based on time-varying metasurfaces. Two parallel time-varying metasurfaces, one for frequency up-conversion and one for down-conversion by the same amount, are used for realizing a region of space where incident waves from opposite directions experience an opposite Doppler frequency shift. As a result, any device within this region becomes sensitive to the illumination direction, exhibiting a different scattering response from opposite directions and thus breaking reciprocity. Very importantly, thanks to the opposite freq…
Traditional sound diffusers are quasi-random phase gratings attached to reflecting surfaces whose purpose is to augment the spatiotemporal incoherence of the acoustic field scattered from reflective surfaces. This configuration allows one to cover a large reflecting surface by periodically tiling the diffuser unit cells to cover a large area while reducing undesirable specular reflection for incident plane waves. However, the periodic arrangement of the unit cells leads to coherent constructive and destructive interference in the scattered field in some directions which is undesirable for achi…
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 trans…
Space-time modulated metasurfaces have attracted significant attention due to the additional degree of freedom in manipulating the electromagnetic (EM) waves in both space and time domains. However, the existing techniques have limited wave control capabilities, leading to just a few feasible schemes like regulation of only one specific harmonic. Here, we propose to realize independent manipulations of arbitrarily dual harmonics and their wave behaviors using a space-time-coding (STC) digital metasurface. By employing different STC sequences to the reflection phase of the metasurface, independ…
Vortex beams carrying orbital angular momentum (OAM) have been widely applied in various electromagnetic, optical, and quantum systems. A tailored OAM spectrum composed of several specific modes as expected holds a promise for expanding the degrees of freedom of the systems. However, such a broadband high-purity tailored spectrum is difficult to be achieved by the present devices, where the broadband amplitude manipulation has not been explored yet. In this work, inspired by the envelope-modulation theory, an elegant and universal way to manipulate the OAM spectrum in wide bandwidth is propose…
This work continues the development of the raytracing method of [1] for computing the scattered fields from metasurfaces characterized by locally periodic reflection and transmission coefficients. In this work, instead of describing the metasurface in terms of scattering coefficients that depend on the incidence direction, its scattering behavior is characterized by the surface susceptibility tensors that appear in the generalized sheet transition conditions (GSTCs). As the latter quantities are constitutive parameters, they do not depend on the incident field and thus enable a more compact an…
For neutral atom qubits, the residual thermal motion of the cold atoms constitutes a major challenge that limits the accessible two-qubit gate fidelity. Recently, an interesting type of two-qubit controlled-PHASE quantum gate protocol has been introduced for neutral atom qubit platform, which relies upon off-resonant modulated driving and Rydberg blockade effect. Building upon this progress, we have further developed an upgrade in the form of dual-pulse off-resonant modulated driving. Besides the inherent advantages of avoiding shelving population in Rydberg levels, not necessarily requiring i…
In this paper, the aperture field estimation technique and generalized adiabatic Floquet-wave expansion method is developed for two-dimensional temporally modulated anisotropic holograms. The determinant equations for propagation eigenstates in the presence of space-time modulation are obtained, and the behaviors of the corresponding dispersion curves are investigated. Injecting temporal modulation causes asymmetrical displacement of the dispersion curve, which is studied in detail in this paper. The consequence of temporal modulation is the Doppler-shift effect and nonreciprocal response obse…
Rapidly developing augmented reality (AR) and 3D holographic display technologies require spatial light modulators (SLM) with high resolution and viewing angle to be able to satisfy increasing customer demands. Currently available SLMs, as well as their performance, are limited by their large pixel sizes of the order of several micrometres. Further pixel size miniaturization has been stagnant due to the persistent challenge to reduce the inter-pixel crosstalk associated with the liquid crystal (LC) cell thickness, which has to be large enough to accumulate the required 2π phase difference. Her…
A simple method of superimposing multiple near field scans using a single horn antenna in different configurations to characterize a planar electromagnetic metasurface is proposed and numerically demonstrated. It can be used to construct incident fields for which the metasurface is originally designed for, which may otherwise be difficult or not possible to achieve in practice. While this method involves additional effort by requiring multiple scans, it also provides flexibility for the incident field to be generated, simply by changing the objective of a numerical optimization which is used t…
Time-Controlled Resonances in 2-D Metasurfaces via Equivalent Circuits
Universidad de Málaga（马拉加大学）；Telecommunication Research Institute (TELMA)（电信研究所（TELMA））；E.T.S. Ingeniería de Telecomunicación（电信工程学院）；Universidad Autónoma de Madrid（马德里自治大学）；RFCAS Research Group（RFCAS研究组）；Department of Electronic and Communication Technology（电子与通信技术系）；University of Seville（塞维利亚大学）；Faculty of Physics（物理学院）
Spatio-temporally modulated impedance surfaces can be good candidates for generation of radiating waves with arbitrary eigenstates by breaking momentum and energy conservations. Here, we present a theoretical framework based on the holographic technique and generalized Floquet-wave expansion to analyze spatio-temporally modulated impedance surfaces. The holographic technique estimates the required impedance distribution to achieve the desired momentum. Injecting temporal modulation deviates the eigenvalues and changes the radiation frequency. Using the proposed analytical model, the eigenvalue…
The intrinsic limitation of the material nonlinearity inevitably results in the poor mode purity, conversion efficiency and real-time reconfigurability of the generated harmonic waves, both in optics and acoustics. Rotational Doppler effect provides us an intuitive paradigm to shifting the frequency in a linear system, which needs to be facilitated by a spiraling phase change upon the wave propagation. Here we numerically and experimentally present a rotating linear vortex metasurface and achieve close-to-unity mode purity (above 95%) and conversion efficiency (above 65%) in audible sound freq…
Over the last couple of years, the digital coding acoustic metasurfaces have been developed rapidly as a highly active research area for their unique and flexible manipulation of acoustic wavefronts. Nevertheless, all recent attentions in the acoustic community have been mainly concentrated on space-encoded architectures, leaving the room free for benefiting from the unique features of spatiotemporally modulated metasurfaces. By entering the world of time, here, we propose a space-time-coding acoustic digital metasurface with exotic ability to dynamically transfer the energy of the carrier aco…
Index modulation (IM) is one of the candidate technologies for the upcoming sixth generation (6G) wireless communications networks. In this paper, we propose a space-time-modulated reconfigurable intelligent metasurface (RI-MTS) that is configured to implement various frequency-domain IM techniques in a multiple-input multiple-output (MIMO) array configuration. Unlike prior works which mostly analyze signal theory of general RI-MTS IM, we present novel electromagnetics-compliant designs of specific IMs such as sub-carrier index modulation (SIM) and MIMO orthogonal frequency-domain modulation I…
We present the design of a phased, modulated, power distribution network for metasurface array antennas. The specific metasurface array design comprises a series of waveguides, each feeding a sub-array of dynamically tunable metamaterial elements that radiate at microwave (X-band) frequencies. To remain a one-port device, the composite array requires a power divider that can excite each branch waveguide with equal power. Further, the power divider must also apply a prescribed phase shift to each branch waveguide to mitigate metasurface-specific grating lobes. The presented design features holl…
Plasmonic metasurfaces have spawned the field of flat optics using nanostructured planar metallic or dielectric surfaces that can replace bulky optical elements and enhance the capabilities of traditional far-field optics. Furthermore, the potential of flat optics can go far beyond far-field modulation, and can be exploited for functionality in the near-field itself. Here, we design metasurfaces based on aperiodic arrays of plasmonic Au nanostructures for tailoring the optical near-field in the visible and near-infrared spectral range. The basic element of the arrays is a rhomboid that is modu…
We investigate how Rayleigh waves interact with modulated resonators located on the free surface of a semi-infinite elastic medium. We begin by studying the dynamics of a single resonator with time-modulated stiffness. In particular, we evaluate the accuracy of an analytical approximation of the resonator response and identify the parameter ranges in which its behavior remains stable. Then, we develop an analytical model to describe the interaction between surface waves and an array of resonators with spatio-temporally modulated stiffness. By combining our analytical models with full-scale num…
Physical systems with material properties modulated in time provide versatile routes for designing magnetless nonreciprocal devices. Traditionally, nonreciprocity in such systems is achieved exploiting both temporal and spatial modulations, which inevitably requires a series of time-modulated elements distributed in space. In this paper, we introduce a concept of bianisotropic time-modulated systems capable of nonreciprocal wave propagation at the fundamental frequency and based on uniform, solely temporal material modulations. In the absence of temporal modulations, the considered bianisotrop…
Active metasurfaces, whose optical properties can be modulated post-fabrication, have emerged as an intensively explored field in recent years. The efforts to date, however, still face major performance limitations in tuning range, optical quality, and efficiency especially for non mechanical actuation mechanisms. In this paper, we introduce an active metasurface platform combining phase tuning covering the full 2$π$ range and diffraction-limited performance using an all-dielectric, low-loss architecture based on optical phase change materials (O-PCMs). We present a generic design principle en…
Manipulation of wave-matter interactions in systems with loss and gain have opened new mechanisms to control wave propagation at will. Metamaterials and metasurfaces having spatially inhomogeneous loss and gain have been studied in the past few years by exploiting parity-time (PT) symmetry concepts inspired from quantum-physics. In this work we theoretically study the control of light-matter interactions in spatially unbounded metamaterials having a time-modulated permittivity whose imaginary part is temporally modulated to induce loss and gain, while the real part stays unchanged. We show bot…
Inspired by the ever-increasing demand for higher data transmission rates and the tremendous attention toward all-optical signal processing based on miniaturized nanophotonics, in this paper, for the first time, we investigate the integrable design of coherent ultrashort light pulse code-division multiple-access (CDMA) technique, also known as femtosecond CDMA, using all-dielectric metasurfaces (MSs). In this technique, the data bits are firstly modulated using ultrashort femtosecond optical pulses generated by mode-locked lasers, and then by employing a unique phase metamask for each data str…
