Time-modulated Metasurfaces Research Articles

Fully Breaking Entanglement of Multiple Harmonics for Space- and Frequency-Division Multiplexing Wireless Applications via Space-Time-Coding Metasurface.

Harmonic generation and utilization are significant topics in nonlinear science. Although the progress in the microwave region has been expedited by the development of time-modulated metasurfaces, one major issue of these devices is the strong entanglement of multiple harmonics, leading to criticism of their use in frequency-division multiplexing (FDM) applications. Previous studies have attempted to overcome this limitation, but they suffer from designing complexity or insufficient controlling capability. Here a new space-time-coding metasurface (STCM) is proposed to independently and precisely synthesize not only the phases but also the amplitudes of various harmonics. This promising feature is successfully demonstrated in wireless space- and frequency-division multiplexing experiments, where modulated and unmodulated signals are simultaneously transmitted via different harmonics using a shared STCM. To illustrate the advantages, binary frequency shift keying (BFSK) and quadrature phase shift keying (QPSK) modulation schemes are respectively implemented. Behind the intriguing functionality, the mechanism of the space-time coding strategy and the analytical designing method are elaborated, which are validated numerically and experimentally. It is believed that the achievements can potentially propel the time-vary metasurfaces in the next-generation wireless applications.

Multifunctional Space–Time-Modulated Metasurface for Direction of Arrival Estimation and RCS Manipulation in a Single System

Multifunctional Space–Time-Modulated Metasurface for Direction of Arrival Estimation and RCS Manipulation in a Single System

Secure Communication Enabled by Time-Modulated Metasurface Antenna Based on Spread-Spectrum Technique

Secure Communication Enabled by Time-Modulated Metasurface Antenna Based on Spread-Spectrum Technique

Pseudorandom Noise Sequence Time-Modulated Reflective Metasurfaces for Target Recognition

In this article, pseudorandom noise (PN) sequence time-modulated reflective metasurfaces are proposed to provide a low-cost, low-complexity, simple solution for target recognition. By covering the target to be recognized with a time-modulated metasurface modulated by a PN time sequence, the metasurface scatters the interrogating harmonic wave into a binary PN-modulated wave. As a result, the radar detector with the foreknowledge of the modulating PN sequence may recognize the target by applying the properties of strong autocorrelation and weak cross correlation of PN sequences and performing cross correlation between the modulating PN sequence and the picked-up signal after downshifting its frequency without actually decoding and reading the scattered information. A 1-bit reconfigurable metasurface unit cell operating at 2.4 GHz is proposed and full-wave simulated to achieve out-of-phase reflective responses by controlling the ON and OFF states of the p-i-n diode for PN time modulation. The target recognition is demonstrated by experiment in two different scenarios where only one metasurface and multiple metasurfaces are present for recognition. The experimental results show that by providing an appropriate threshold in the cross-correlation detecting results, the proposed method can recognize the targets well in both scenarios.

Jamming of ISAR Imaging With Time-Modulated Metasurface Partially Covered on Targets

The phase information of the echo is essential for inverse synthetic aperture radar (ISAR) imaging. Time-modulated metasurface (TMM) generates time-varying reflection phase that can modulate the phase of echo, thus enabling jamming of ISAR imaging, which has great potential for real applications. In practice, covering the whole target with TMM is difficult to achieve. Consequently, the echo model of a radar target partially covered with a TMM is developed, which extends the model of ISAR jamming with TMM fully covering to a more general and near-realistic case in this letter. Then, the echoes of each component of the target are obtained by electromagnetic simulation, and the time-varying reflection phase of the 1-bit TMM is obtained by experimental testing. Finally, using the abovementioned data, we validate that ISAR image of the component covered metasurface can be blurred using random time modulation sequence. Meanwhile, duplicated components can be produced in the ISAR image using periodic time modulation sequence. Experimental results of range profile jamming confirm the validity of the conclusions in this letter. The partial defocusing or duplicating of the radar target in the ISAR image can reduce the recognized possibility and enhance the jamming performance, effectively.

Full-polarization radar target feature modulation based on active polarization conversion metasurface

Electromagnetic (EM) metasurfaces comprising artificially designed subwavelength unit cells have drawn considerable attention due to the EM properties beyond the limits of natural materials. As one of the representative structures, active polarization conversion metasurface (APCM) is switchable by loading active components. It provides great freedom to manipulate the polarization state of EM waves. However, the current research mainly focused on the application of communication and paid less attention to the radar effect of APCM. APCM redistributes electromagnetic wave energy in multi-polarization channels, so it will have great application potential in polarimetric radar. Herein, based on the fully polarimetric radar one-dimensional high resolution range profile, the radar effect of time-modulated metasurface is studied. For this purpose, a method of target scattering mechanisms manipulation and a polarization-insensitive structure of APCM are proposed. The amplitude-phase joint modulation method is specifically analyzed in detail. The distance transformation and virtual multi-target phenomena are further discovered. Virtual targets along the distance dimension are generated in multi-polarization channels, while the scattering mechanisms of k-order targets are effectively manipulated. The relationship between the target scattering matrix and the modulation parameters is obtained. It may provide an effective method for the application of active metasurface in fully polarimetric radars.

High-Resolution Range Profile Feature Transformation Method Based on Amplitude-Phase Modulation Metasurface

Time-modulated metasurfaces have attracted much attention due to their flexible manipulation of electromagnetic waves. Time-varying response leads to spread-spectrum and reallocation of energy. Thus, an amplitude-phase joint modulation (TAJM) method is proposed based on a time-modulated active polarization conversion metasurface (APCM). Taking radar high resolution range profile (HRRP) as the object, the distance transformation and multi-target phenomena are further discovered. The proposed method achieves a flexible manipulation of target amplitude and phase. The relationship between the modulation parameters and HRRP feature is studied. It is revealed that the absorption depth is the dominant factor for the energy distribution of TAJM. The method can be applied to multi-target generation and target feature manipulation.

Pseudorandom Sequence (Space–) Time-Modulated Metasurfaces: Principles, Operations, and Applications

This article presents a novel class of (space–)time-modulated metasurfaces, namely, (space–)time metasurfaces that are modulated by pseudorandom sequence (PRS) waveforms. In contrast to their harmonically or quasi-harmonically modulated counterparts, these metasurfaces massively alter the temporal spectrum of the waves that they process; as a result, they exhibit distinct properties and offer complementary applications. These metasurfaces are assumed, here, to operate in the “slow-modulation” regime, where the fixed-state time between the state transition is much larger than the transient time associated with the dispersion of the media involved, which allows for the safe separation of the time-variance and frequency-dispersive effects of the system.

Advances in Metasurfaces: Topology, Chirality, Patterning, and Time Modulation

Advances in metasurface design has given rise to the unique ability of manipulating the phase and propagation of electromagnetic (EM) waves. The application of topological protection, chirality, and other concepts borrowed from condensed matter physics have yielded a rich new source of structures with attractive features. Topological metasurfaces show that symmetry can be used to create unique EM modes that are, for instance, immune to many types of scattering. Similar modes can also be created using defects in homogeneous materials, essentially creating chiral waveguides with polarization-locked propagation. Such defects may have topological protection in real space and/or <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">k</i> -space and lead to modes without global changes to the lattice. This review article explores recent results in chiral and topological metasurfaces, primarily consisting of printed metal patterns, but also planar dielectric structures, at microwave, optical, and acoustic frequencies. The exciting potential applications include nonreciprocal structures, self-collimating waves based on diffusive transport, defect-based amorphous structures, time-modulated metasurfaces, and new kinds of chiral waveguides that exhibit unidirectional propagation without requiring a surrounding periodic structure.

Optical nonreciprocity via transmissive time-modulated metasurfaces

Abstract The frequency mixing property of time-modulated metasurfaces, attributed to the well-known phenomenon of temporal photonic transition, has led to several exotic functionalities in the last lustrum. Based on this concept, we demonstrate the possibility of achieving nonreciprocal responses in the near-infrared regime via combining a time-modulated platform and a static high-Q metasurface. In particular, the temporal metasurface is designed to up-convert the incident tone to the first higher-order harmonic, while the static platform is implemented to establish a filtering behavior with respect to the incident frequency. It is shown that while the receiver port acquires the transmitted signal in the forward direction, the amount of received power becomes negligible under the time-reversal scenario, which indicates the presented configuration exhibits different optical responses from opposite directions. In addition, the role of operating wavelength and the modulation frequency on the power isolation level are investigated, and it is demonstrated that by appropriate selection, the isolation level can reach −30 dB. Since this is the first time a nonreciprocal response is obtained in the near-infrared regime via a pure temporal modulation, we believe the idea of this paper can be of utmost importance in various applications, such as tunable optical isolators.

Active Multiple Access Secure Communication Enabled by Graphene-Based Time-Modulated Metasurfaces

This article presents a technique for establishing active secure communication links over single and multiple orthogonal frequency channels via a reflective graphene-based time-modulated metasurface operating in the low terahertz frequency regime. For this purpose, the Fermi energy level of unit cells is modulated via pseudorandom radio frequency biasing signals whose dc offsets are set to steer the reflected beam toward the desired direction by imposing a spatial phase gradient profile across the surface, while their waveforms are engineered to spread the incident wave spectrum into a noise-like spectrum with a near-zero power spectral density via random modulation of the reflection phase of each element between 0 and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\pi $ </tex-math></inline-formula> with respect to its offset phase. This allows for addressing a legitimate user in real-time who can retrieve the signal via synchronous demodulation with the pseudorandom key of the metasurface while camouflaging the signal from the adversary by lowering the probability of detection. The approach is extended to enable multichannel secure communication by dividing the metasurface into interleaved subarrays modulated with orthogonal pseudorandom keys, which provides simultaneous and independent control over multiple beams with nonoverlapping spread spectra which can be retrieved by independent legitimate users while rejecting unwarranted access by eavesdroppers.

Broadband continuous beam-steering with time-modulated metasurfaces in the near-infrared spectral regime

A time-modulated metasurface integrated with indium-tin-oxide (ITO) can dynamically manipulate the scattered light by imparting dispersionless phase discontinuities with 2π span on wavefronts of the generated sidebands. However, maximal frequency conversion efficiency is restricted to a narrow bandwidth at the vicinity of the resonant wavelength due to the dispersive nature of light–matter interactions in such ITO-integrated metasurfaces. Herein, a multiwavelength time-modulated meta-molecule is numerically investigated, which consists of four metal–insulator–metal meta-atoms that are judiciously designed to support four resonant wavelengths at the near-infrared regime. The metasurface platform allows for application of four sets of independent radio-frequency signals to each meta-molecule, which are optimized to excite the output spectra with dominant first-order up-modulated sidebands. Individual modulation of each meta-atom leads to efficient sideband generation at resonant wavelength corresponding to the biased meta-atom. Nevertheless, the conversion efficiency decays rapidly at off-resonant wavelengths. Concurrent modulation of all four meta-atoms within the meta-molecule configuration offers a pathway to simultaneously generate dominant sidebands at all resonant wavelengths while enhancing the frequency conversion efficiency at inter-resonant wavelengths. As the potential application, beam-steering at the up-modulated sideband by both individually and simultaneously biased meta-atoms is demonstrated. It is revealed that simultaneous modulation enables continuous dynamic light deflection over a broad spectrum spanning on 1–1.8 μm, while the steering angle of the wavelengths changes in the range of 34°–85°. Broadband continuous beam-scanning is a non-trivial task that cannot be accomplished by a quasi-static metasurface relying on dispersive resonant phase shift, which hinders simultaneous implementation of linear phase gradients at all incident wavelengths.

Analog signal processing through space-time digital metasurfaces

Abstract In the quest to realize analog signal processing using subwavelength metasurfaces, in this paper, we present the first 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 metasurface is capable of implementing a large class of useful mathematical operators, including 1st- and 2nd-order spatial differentiation, 1st-order spatial integration, and integro-differential equation solving accompanied by frequency conversions. Unlike the recent proposals based on the Green’s function (GF) method, the designed time-modulated signal processor effectively operates for input signals containing wide spatial frequency bandwidths with an acceptable gain level. Proof-of-principle simulations are also reported to demonstrate the successful realization of image processing functions like edge detection. This time-varying wave-based computing system can set the direction for future developments of programmable metasurfaces with highly promising applications in ultrafast equation solving, real-time and continuous signal processing, and imaging.

Spread-Spectrum Selective Camouflaging Based on Time-Modulated Metasurface

This article 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 perfect electric conductor (PEC) mirror and a perfect magnetic conductor (PMC) mirror, using an array of microstrip patches connected to the ground via diode switches, according to a periodic pseudorandom 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.

Time-varying optical vortices enabled by time-modulated metasurfaces

Abstract In this paper, generation of optical vortices with time-varying orbital angular momentum (OAM) and topological charge is theoretically demonstrated based on time-modulated metasurfaces with a linearly azimuthal frequency gradient. The topological charge of such dynamic structured light beams is shown to continuously and periodically change with time evolution while possessing a linear dependence on time and azimuthal frequency offset. The temporal variation of OAM yields a self-torqued beam exhibiting a continuous angular acceleration of light. The phenomenon is attributed to the azimuthal phase gradient in space-time generated by virtue of the spatiotemporal coherent path in the interference between different frequencies. In order to numerically authenticate this newly introduced concept, a reflective dielectric metasurface is modelled consisting of silicon nanodisk heterostructures integrated with indium-tin-oxide and gate dielectric layers on top of a mirror-backed silicon slab which renders an electrically tunable guided mode resonance mirror in near-infrared regime. The metasurface is divided into several azimuthal sections wherein nanodisk heterostructures are interconnected via nanobars serving as biasing lines. Addressing azimuthal sections with radio-frequency biasing signals of different frequencies, the direct dynamic photonic transitions of leaky-guided modes are leveraged for realization of an azimuthal frequency gradient in the optical field. Generation of dynamic twisted light beams with time-varying OAM by the metasurface is verified via performing several numerical simulations. Moreover, the role of modulation waveform and frequency gradient on the temporal evolution and diversity of generated optical vortices is investigated which offer a robust electrical control over the number of dynamic beams and their degree of self-torque. Our results point toward a new class of structured light for time-division multiple access in optical and quantum communication systems as well as unprecedented optomechanical manipulation of objects.

Serrodyne Frequency Translation Using Time-Modulated Metasurfaces

Temporally modulated metamaterials have attracted significant attention recently due to their non-reciprocal and frequency converting properties. Here, a transparent, time-modulated metasurface that functions as a serrodyne frequency translator, is reported at X-band frequencies. With a simple biasing architecture, the metasurface provides electrically-tunable transmission phase that covers $360^\circ$. A sawtooth waveform is used to modulate the metasurface, allowing Doppler-like frequency translation. Two such metasurfaces can be cascaded together to achieve magnetless devices that perform either phase or amplitude non-reciprocity.

Adaptive Multichannel Terahertz Communication by Space-Time Shared Aperture Metasurfaces

We propose a technique for adaptive multichannel communication in the low-terahertz frequency regime through simultaneous and independent multifrequency multibeam scanning via a single time-modulated metasurface consisting of graphene micro-patch antennas whose Fermi energy levels are modulated by radio-frequency biasing signals. For this purpose, we divide the metasurface aperture into interleaved orthogonally modulated sub-array antennas with distinct modulation frequencies, rendering a shared aperture in space-time. The higher-order frequency harmonics generated by the sub-arrays in such a space-time shared-aperture metasurface are mutually orthogonal as they do not yield an observable interference pattern. A distinct constant progressive modulation phase delay can be adopted in each sub-array to independently scan its corresponding higher-order frequency harmonics via dispersionless modulation-induced phase gradient with minimal sidelobe level and full angle-of-view over a wide bandwidth. The number of sub-arrays and distinct channels can be scaled easily without suffering from cross-talk due to orthogonality of the channels. The concept is established theoretically and verified through numerical simulations. We characterize active beam-scanning performance of the space-time shared aperture metasurface in terms of gain and half power beamwidth and their dependence to the array architecture. We consider both one-dimensional and two-dimensional interleaving for scanning the beams along elevation and azimuth angles. We also obtain the upper bounds for the number of independent channels attainable without compromising angle-of-view as 8 and 64 for one-dimensional and two-dimensional interleaving, respectively within the same physical aperture. Our results point toward high-capacity platforms with low size, weight and power for next generations of terahertz communication systems.

Phase-Induced Frequency Conversion and Doppler Effect With Time-Modulated Metasurfaces

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.

Time-Modulated Conducting Oxide Metasurfaces for Adaptive Multiple Access Optical Communication

We demonstrate realization of a time-modulated metasurface in near-infrared frequency regime via integration of conducting oxide layers into plasmonic stripe nanoantennas arranged in a reflectarray configuration. The permittivity of conducting oxide layers is modulated in time through modulation of carrier concentration by applying radio frequency biasing signals which leads to generation of higher order frequency harmonics. We rigorously characterize the electro-optical frequency conversion performance of the metasurface. We then exploit the dispersionless angle-independent phase discontinuities on the wavefront of higher order frequency harmonics to achieve dynamic multi-wavelength multi-beam scanning across S, C, and L telecommunication bands via pixelated control over the modulation phase delay. It is established that such time-modulated metasurfaces can be utilized as reflectarray antennas for active multicasting as well as wavelength and angle multiplexing, thus improving optical communication capacity by providing multiple access and integrating several communication channels into a single antenna platform. The role of modulation waveform in the spectral diversity of harmonics is also explored. Specifically, modulation waveform is optimized to realize unicast and broadcast links. Finally, active beam-scanning performance of the time-modulated conducting oxide metasurface is compared with that of its quasi-static counterpart verifying the enhanced bandwidth as well as suppressed side lobes in the time-modulated platform.

Rigorous space-time coupled-wave analysis for patterned surfaces with temporal permittivity modulation [Invited

Material patterns with temporal modulation of permittivity provide an unprecedented platform for active light manipulation and multifunctional devices. The ongoing discovery of new classes of tunable optical materials combined with various multiphysics paradigms demands an appropriate computational tool to characterize and design the wave manipulation patterns with the additional degree of freedom (time). Here we present, for the first time, a fast and accurate numerical technique to compute light propagation through patterned layers with spatial and temporal modulation of permittivity by extending the well-established rigorous coupled-wave analysis framework. We expand the electric and magnetic field components of an arbitrarily shaped space-time unit cell into a basis of spatio-temporal harmonics and compute their modal field profiles as a set of eigenmodes of the unit cell. The relation between the wave amplitudes of the incident, reflected, and transmitted spatio-temporal harmonics are then computed by enforcing the boundary conditions at each interface of the pattern. The validity of the proposed method is rigorously verified against the results of a time-variant finite-difference time-domain technique. The applicability of the method to study dielectric and plasmonic time-modulated metasurfaces is exemplified by considering two metasurfaces with spatially-variant phase and frequency of temporal modulation, based on electro-optical modulation of materials such as silicon and graphene. Utilizing the developed technique, we demonstrate some recently reported phenomena associated with time-modulated metasurfaces, such as anomalous steering of higher-order frequency harmonics, and dynamic optical beam generation. Even though we restrict ourselves to one-dimensional patterns for simplicity, the developed method can be flexibly employed for the design, characterization and optimization of two-dimensional spatio-temporal modulation patterns of arbitrary composition and shape.