arXiv subjects

Xuchen Wang

Publications and source records attributed to Xuchen Wang.

At least 37 records · Page 2Linked to original sources

arXiv2023

Smith-Purcell radiation from time grating

Smith-Purcell radiation (SPR) occurs when an electron skims above a spatial grating, but the fixed momentum compensation from the static grating imposes limitations on the emission wavelength. It has been discovered that a temporally periodic system can provide energy compensation to generate light emissions in free space. Here, we introduce temporal SPR (t-SPR) emerging from a time grating and propose a generalized t-SPR dispersion equation to predict the relationship between radiation frequency, direction, electron velocity, modulation period, and harmonic orders. Compared to conventional SPR, t-SPR can: 1) Provide a versatile platform for manipulating SPR emission through temporal modulation (e.g., period, amplitude, wave shape). 2) Exhibit strong robustness to the electron-grating separation, alleviating the constraints associated with extreme electron near-field excitation. 3) Introduce additional energy channels through temporal modulation, enhancing and amplifying emission.

physics.optics↗

arXiv2023

Metasurface-Based Realization of Photonic Time Crystals

Photonic time crystals are artificial materials whose electromagnetic properties are uniform in space but periodically vary in time. The synthesis of such materials and experimental observation of their physics remain very challenging due to the stringent requirement for uniform modulation of material properties in volumetric samples. In this work, we extend the concept of photonic time crystals to two-dimensional artificial structures -- metasurfaces. We demonstrate that time-varying metasurfaces not only preserve key physical properties of volumetric photonic time crystals despite their simpler topology but also host common momentum bandgaps shared by both surface and free-space electromagnetic waves. Based on a microwave metasurface design, we experimentally confirmed the exponential wave amplification inside a momentum bandgap as well as the possibility to probe bandgap physics by external (free-space) excitations. The proposed metasurface serves as a straightforward material platform for realizing emerging photonic space-time crystals and as a realistic system for the amplification of surface-wave signals in future wireless communications.

physics.app-ph↗

arXiv2023

Tunable Perfect Anomalous Reflection Using Passive Aperiodic Gratings

Realizing continuous sweeping of perfect anomalous reflection in a wide angular range has become a technical challenge. This challenge cannot be overcome by the conventional aperiodic reflectarrays and periodic metasurfaces or metagratings. In this paper, we investigate means to create scanning reflectarrays for the reflection of plane waves coming from any direction into any other direction without any parasitic scattering. The reflection angle can be continuously adjusted by proper tuning of reactive loads of each array element, while the geometrical period is kept constant. We conceptually study simple canonical two-dimensional arrays formed by impedance strips above a perfectly reflecting plane. This setup allows fully analytical solutions, which we exploit for understanding the physical nature of parasitic scattering and finding means to overcome fundamental limitations of conventional reflectarray antennas. We propose to use subwavelength-spaced arrays and optimize current distribution in $λ/2$-sized supercells. As a result, we demonstrate perfect tunable reflection to any angle. Our work provides an effective approach to design reconfigurable intelligent surfaces with electrically tunable reflection angles.

physics.app-ph↗

arXiv2023

Discrete Impedance Metasurfaces for 6G Wireless Communications in D-Band

Engineering and optimization of wireless propagation channels will be one of the key elements of future communication technologies. Metasurfaces may offer a wide spectrum of functionalities for passive and tunable reflecting devices, overcoming fundamental limits of commonly used conventional phase-gradient reflectarrays and metasurfaces. In this paper, we develop an efficient way for the design and implementation of metasurfaces with high-efficiency anomalous reflector functionalities. The developed numerical method provides accurate, fast, and simple metasurface designs, taking into account non-local near-field interactions between array elements. The design method is validated by manufacturing and experimental testing of highly efficient anomalous reflectors for the millimetre-wave band.

physics.app-ph↗

arXiv2023

Coherent control of wave beams via unidirectional evanescent modes excitation

Conventional coherent absorption occurs only when two incident beams exhibit mirror symmetry with respect to the absorbing surface, i.e., the two beams have the same incident angles, phases, and amplitudes. In this work, we propose a more general metasurface paradigm for coherent perfect absorption, with impinging waves from arbitrary asymmetric directions. By exploiting excitation of unidirectional evanescent waves, the output can be fixed at one reflection direction for any amplitude and phase of the control wave. We show theoretically and confirm experimentally that the relative amplitude of the reflected wave can be tuned continuously from zero to unity by changing the phase difference between the two beams, i.e. switching from coherent perfect absorption to full reflection. We hope that this work will open up promising possibilities for wave manipulation via evanescent waves engineering with applications in optical switches, one-side sensing, and radar cross section control.

physics.app-ph↗

arXiv2022

Controlling surface waves with temporal discontinuities of metasurfaces

In this paper, we investigate the scattering of surface waves on reactive impedance boundaries when the surface impedance undergoes a sudden change in time. We report three exotic wave phenomena. First, it is shown that by switching the value of the surface capacitance of the boundary, the velocity of surface waves can be fully controlled, and the power of reflected and transmitted surface waves are amplified. Second, we show that when a capacitive boundary is switched to an inductive one, the surface wave stops completely, with a "frozen" static magnetic field distribution. The static magnetic fields are "melt" and restore propagating surface waves when the boundary is switched back to a capacitive one. Third, we show that temporal jumps of the boundary impedance couple free-space propagating waves to the surface wave, which is an analog to a spatial prism. These interesting effects enabled by temporal jumps of metasurface properties open up new possibilities for the generation and control of surface waves.

physics.app-ph↗

arXiv2021

Fast and Robust Characterization of Dielectric Slabs Using Rectangular Waveguides

Waveguide characterization of dielectric materials is a convenient and broadband approach for measuring dielectric constant. In conventional microwave measurements, material samples are usually mechanically shaped to fit the waveguide opening and measured in closed waveguides. This method is not practical for millimeter-wave and sub-millimeter-wave measurements where the waveguide openings become tiny, and it is rather difficult to shape the sample to exactly the same dimensions as the waveguide cross-section. In this paper, we present a method that allows one to measure arbitrarily shaped dielectric slabs that extend outside waveguides. In this method, the measured sample is placed between two waveguide flanges, creating a discontinuity. The measurement system is characterized as an equivalent Pi-circuit, and the circuit elements of the Pi-circuit are extracted from the scattering parameters. We have found that the equivalent shunt impedance of the measured sample is only determined by the material permittivity and is rather insensitive to the sample shape, position, sizes, and other structural details of the discontinuity. This feature can be leveraged for accurate measurements of permittivity. The proposed method is very useful for measuring the permittivity of medium-loss and high-loss dielectrics from microwave to sub-terahertz frequencies.

physics.app-ph↗

arXiv2021

Dispersion of Surface Waves above Time-Varying Reactive Boundaries

In this presentation, we analytically derive the dispersion equation for surface waves traveling along reactive boundaries which are periodically modulated in time. In addition, we show numerical results for the dispersion curves and importantly uncover that time-varying boundaries generate band gaps that can be controlled by engineering the modulation spectrum. Furthermore, we also point out an interesting effect of field amplification related to the existence of such band gaps for surface waves. The effect of amplification does not require the synchronization of signal and pumping waves. This unique property is very promising to be applied in surface-wave communications from microwave to optical frequencies.

physics.optics↗

arXiv2021

Time-varying components for enhancing wireless transfer of power and information

Temporal modulation of components of electromagnetic systems provides an exceptional opportunity to engineer the response of those systems in a desired fashion, both in the time and frequency domains. For engineering time-modulated systems, one needs to thoroughly study the basic concepts and understand the salient characteristics of temporal modulation. In this paper, we carefully study physical models of basic bulk circuit elements -- capacitors, inductors, and resistors -- as frequency dispersive and time-varying components and study their effects in the case of periodical time modulations. We develop a solid theory for understanding these elements, and apply it to two important applications: wireless power transfer and antennas. For the first application, we show that by periodically modulating the mutual inductance between the transmitter and receiver, the fundamental limits of classical wireless power transfer systems can be overcome. Regarding the second application, we consider a time-varying source for electrically small dipole antennas and show how time modulation can enhance the antenna performance. The developed theory of electromagnetic systems engineered by temporal modulation is applicable from radio frequencies to optical wavelengths.

physics.app-ph↗

arXiv2021

Space-Time Metasurfaces for Perfect Power Combining of Waves

In passive linear systems, complete combining of powers carried by waves from several input channels into a single output channel is forbidden by the energy conservation law. Here, we demonstrate that complete combination of both coherent and incoherent plane waves can be achieved using metasurfaces with properties varying in space and time. The proposed structure reflects waves of the same frequency but incident at different angles towards a single direction. The frequencies of the output waves are shifted by the metasurface, ensuring perfect incoherent power combining. The proposed concept of power combining is general and can be applied for electromagnetic waves from the microwave to optical domains, as well as for waves of other physical nature.

physics.optics↗

arXiv2020

Nonreciprocity in Bianisotropic Systems with Uniform Time Modulation

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 bianisotropic systems are reciprocal. We theoretically explain the nonreciprocal effect by analyzing wave propagation in an unbounded bianisotropic time-modulated medium. The effect stems from temporal modulation of spatial dispersion effects which to date were not taken into account in previous studies based on the local-permittivity description. We propose a circuit design of a bianisotropic metasurface that can provide phase-insensitive isolation and unidirectional amplification.

physics.app-ph↗

arXiv2020

Theory and Design of Multifunctional Space-Time Metasurfaces

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 plethora of nonreciprocal components such as wave isolators, phase shifters, and circulators, on the same metasurface. This platform allows switching between different functionalities only by modifying the pumping signals (harmonic or non-harmonic), without changing the main body of the metasurface structure. This solution opens the door for future real-time reconfigurable and environment-adaptive nonreciprocal wave controllers.

physics.app-ph↗

arXiv2020

Independent Control of Multiple Channels in Metasurface Devices

In analogy with electromagnetic networks which connect multiple input-output ports, metasurfaces can be considered as multi-port devices capable of providing different functionalities for waves of different polarizations illuminating the surface from different directions. The main challenge in the design of such multichannel metasurfaces is to ensure independent and full control of the electromagnetic response for each channel ensuring the fulilment of the boundary condition at the metasurface. In this work, we demonstrate that by properly engineering the evanescent fields excited at each port (that is, for all possible illumination directions), it is possible to independently control the reflection or transmission for all different illuminations. We develop a fully analytical method to analyze and synthesize general space-modulated impedance metasurfaces, engineering strong spatial dispersion. This method, combined with mathematical optimization, allows us to find a surface impedance profile that simultaneously ensures the desired electromagnetic responses at each port. We validate the technique via the design of phase-controlled multichannel retroreflectors. In addition, we demonstrate that the method is rather powerful in the design of other functional metasurfaces such as multifunctional reflectors and multichannel perfect absorbers.

physics.app-ph↗

arXiv2019

The Software-Defined Metasurfaces Concept and Electromagnetic Aspects

We present the concept and electromagnetic aspects of HyperSurFaces (HSFs), artificial, ultrathin structures with software controlled electromagnetic properties. The HSFs key unit is the metasurface, a plane with designed subwavelength features whose electromagnetic response can be tuned via voltage-controlled continuously-tunable electrical elements that provide local control of the surface impedance and advanced functionalities, such as tunable perfect absorption or wavefront manipulation. A nanonetwork of controllers enables software defined HSFs control related to the emerging Internet of Things paradigm.

physics.app-ph↗

arXiv2019

Electromagnetic Aspects of Practical Approaches to Realization of Intelligent Metasurfaces

We thoroughly investigate the electromagnetic response of intelligent functional metasurfaces. We study two distinct designs operating at different frequency regimes, namely, a switch-fabric-based design for GHz frequencies and a graphene-based approach for THz band, and discuss the respective practical design considerations. The performance for tunable perfect absorption applications is assessed in both cases.

physics.app-ph↗

arXiv2019

Graphene-based Perfect Absorbers: Systematic Design and High Tunability

Experimental realization of efficient graphene-based absorbers is a challenging task due to the low carrier mobility in processed graphene. In this paper, we circumvent this problem by placing uniform graphene sheets on metallic metasurfaces designed for improving the absorption properties of low-mobility graphene. Complete absorption can be achieved for different frequencies with the proper metasurface design. In the THz band, we observe strong tunability of the absorption frequencies and magnitudes when modulating graphene Fermi level.

physics.app-ph↗

arXiv2019

Software-Defined Metasurface Paradigm: Concept, Challenges, Prospects

HyperSurfaces (HSFs) are devices whose electromagnetic (EM) behavior is software-driven, i.e., it can be defined programmatically. The key components of this emerging technology are the metasurfaces, artificial layered materials whose EM properties depend on their internal subwavelength structuring. HSFs merge metasurfaces with a network of miniaturized custom electronic controllers, the nanonetwork, in an integrated scalable hardware platform. The nanonetwork receives external programmatic commands expressing the desired end-functionality and appropriately alters the metasurface configuration thus yielding the respective EM behavior for the HSF. In this work, we will present all the components of the HSF paradigm, as well as highlight the underlying challenges and future prospects.

physics.app-ph↗

arXiv2018

Toward Ultimate Control of Terahertz Wave Absorption in Graphene

It is commonly believed that weak light-matter interactions in low-mobility graphene dramatically limits tunability of graphene-based optoelectronic devices, such as tunable absorbers or switches. In this paper, we develop and use a simple circuit model to understand absorption in graphene sheets. In particular, we show that light interacts weakly also with very high-mobility graphene sheets and propose systematic design means to overcome these problems. The results have allowed us to demonstrate in the terahertz band that perfect absorption with excellent electrical tunability can be achieved within a wide span of mobility values which almost covers the whole range of ever reported room-temperature mobilities. Remarkably, concentrating on the most practical low-mobility graphene devices, we exemplify our theory with two cases: frequency-tunable and switchable absorbers with near 100% modulation efficiencies. Our work provides systematic and instructive insights into the design of highly tunable absorbers, without restrictions on graphene mobility. The design strategy and the developed analytical model can, in principle, be generalized to other wavelength regions from microwave to mid-infrared range, and other two-dimensional materials such as transition metal dichalcogenides (TMDs) and black phosphorus.

physics.optics↗