Broadband Microwave and Millimeter-wave Metasurfaces

Metasurfaces have shown unprecedented control over electromagnetic waves. Utilizing structures with dimensions much smaller than the wavelength, they eliminate the need for bulky metamaterials and provide novel functionalities not shown in conventional diffractive devices. Among metasurface architectures demonstrated to date, thin patterned metallic layers modeled as surface impedances have robust design methods, enabling arbitrary wave-front control in a deeply sub-wavelength thickness. However, these metasurfaces are often limited to very narrow-band operation. In metasurfaces, resonant particles made from patterned metallic layers provide control over the wavefront by spatial variation of their physical geometry. In transmissive operation, three layers of metallic particles provide complete control of electric and magnetic responses by which the reflected wave can be annihilated. The spectral response of each resonance is generally dispersive, therefore a rigorous method to engineer the dispersion is necessary to achieve the wavefront manipulation in a broader bandwidth. This thesis provide a systematic study on the bandwidth of metasurfaces by first investigating their performance limits and then proposing a synthesis method based on engineering of surface impedances. Broadband achromatic and dispersive metasurfaces are designed and realized in microwave and millimeter wave regimes, and their aberrations are systematically analyzed. In Chapter 1, a brief introduction to the history and basic concepts of metasurfaces is given. In Chapter 2, to understand physical limits on the achievable operating bandwidth of achromatic metasurfaces, we apply Foster's reactance theorem to the surface impedances of the metasurfaces and derive general limits relating the bandwidth and total size of the metasurface. Having explored the general limits of achromatic metasurfaces, in Chapter 3, we develop a more realistic limit using the time bandwidth product of a single resonance, revealing the critical role of the substrate electrical thickness on the magnetic response. In Chapter 4, a rigorous design methodology is proposed to obtain a transmissive metasurface in which the Huygens' condition is maintained over a broad bandwidth and range of phase tuning. In Chapter 5, the performance of a broadband focusing metasurface operating at oblique incidence is systematically characterized. Chapter 6 summarizes the thesis, discusses the outlook and proposes possible follow-up projects for broadband metasurfaces

Method for Extracting the Equivalent Admittance from Time-Varying Metasurfaces and Its Application to Self-Tuned Spatiotemporal Wave Manipulation

With their self-tuned time-varying responses, waveform-selective metasurfaces embedded with nonlinear electronics have shown fascinating applications, including distinguishing different electromagnetic waves depending on the pulse width. However, thus far they have only been realized with a spatially homogeneous scattering profile. Here, by modeling a metasurface as time-varying admittance sheets, we provide an analytical calculation method to predict the metasurface time-domain responses. This allows derivation of design specifications in the form of equivalent sheet admittance, which is useful in synthesizing a metasurface with spatiotemporal control, such as to realize a metasurface with prescribed time-dependent diffraction characteristics. As an example, based on the proposed equivalent admittance sheet modeling, we synthesize a waveform-selective Fresnel zone plate with variable focal length depending on the incoming pulse width. The proposed synthesis method of pulse-width-dependent metasurfaces may be extended to designing metasurfaces with more complex spatiotemporal wave manipulation, benefiting applications such as sensing, wireless communications and signal processing

Frequency-Hopping Wave Engineering with Metasurfaces

Wave phenomena can be artificially engineered by scattering from metasurfaces, which aids in the design of radio-frequency and optical devices for wireless communication, sensing, imaging, wireless power transfer and bio/medical applications. Scattering responses vary with changing frequency; conversely, they remain unchanged at a constant frequency, which has been a long-standing limitation in the design of devices leveraging wave scattering phenomena. Here, we present metasurfaces that can scatter incident waves according to two variables - the frequency and pulse width - in multiple bands. Significantly, these scattering profiles are characterized by how the frequencies are used in different time windows due to transient circuits. In particular, with coupled transient circuits, we demonstrate variable scattering profiles in response to unique frequency sequences, which can markedly increase the available frequency channels in accordance with a factorial function. Our proposed concept, which is analogous to frequency hopping in wireless communication, advances wave engineering in electromagnetics and related fields.Comment: 62 pages, 25 figure

Design guidelines for the SPICE parameters of waveform-selective metasurfaces varying with the incident pulse width at a constant oscillation frequency

In this study, we numerically demonstrate how the response of recently reported circuit-based metasurfaces is characterized by their circuit parameters. These metasurfaces, which include a set of four diodes as a full wave rectifier, are capable of sensing different waves even at the same frequency in response to the incident waveform, or more specifically the pulse width. This study reveals the relationship between the electromagnetic response of such waveform-selective metasurfaces and the SPICE parameters of the diodes used. First, we show that reducing a parasitic capacitive component of the diodes is important for realization of waveform-selective metasurfaces in a higher frequency regime. Second, we report that the operating power level is closely related to the saturation current and the breakdown voltage of the diodes. Moreover, the operating power range is found to be broadened by introducing an additional resistor into the inside of the diode bridge. Our study is expected to provide design guidelines for circuit-based waveform-selective metasurfaces to select/fabricate optimal diodes and enhance the waveform-selective performance at the target frequency and power level.Comment: 9 pages, 9 figure

Pulse-Driven Self-Reconfigurable Meta-Antennas

Wireless communications and sensing have notably advanced thanks to the recent developments in both software and hardware. Although various modulation schemes have been proposed to efficiently use the limited frequency resources by exploiting several degrees of freedom, antenna performance is essentially governed by frequency only. Here, we present a new antenna design concept based on metasurfaces to manipulate antenna performances in response to the time width of electromagnetic pulses. We numerically and experimentally show that by using a proper set of spatially arranged metasurfaces loaded with lumped circuits, ordinary omnidirectional antennas can be reconfigured by the incident pulse width to exhibit directional characteristics varying over hundreds of milliseconds or billions of cycles, far beyond conventional performance. We demonstrate that the proposed concept can be applied for sensing, selective reception under simultaneous incidence and mutual communications as the first step to expand existing frequency resources based on pulse width.Comment: 61 pages, 6 figures, 26 supplementary figure

Metasurface‐inspired maintenance‐free Internet of things tags characterised in both frequency and time domains

Abstract The authors present metasurface‐inspired maintenance‐free Internet of things (IoT) tags that can be characterised not only by frequency‐domain profiles but also by time‐domain profiles. In particular, time‐domain characterisation is made possible by implementing the waveform‐selective mechanisms of recently developed circuit‐based metasurfaces that behave differently, even at the same frequency, in accordance with the pulse duration of the incident wave. The proposed designs are numerically and experimentally validated and potentially contribute to accommodating an increasing number of IoT tags within a single wireless network while reducing maintenance effort

Improved Numerical Estimation Method for Surface Wave Attenuation on Metasurfaces

Comprehensive numerical tools to investigate and analyze the surface wave attenuation on a metasurface waveguide that consists of periodic patches deposited on a dielectric substrate are presented. The surface wave attenuation is evaluated in both waveguide and open-space environments at high frequencies around the 28-GHz band. Two approaches including a driven modal approach and an integrated eigenmode approach for investigating the attenuation behaviors in a metasurface waveguide operating in transverse electromagnetic (TEM) mode are employed. These results show different attenuation values depending on the waveguide height due to an unavoidable coupling issue despite the same metasurface design. Thus, an alternative method for more accurately evaluating the frequency-dependent attenuation characteristics through field distribution in open space with a customized horn antenna is proposed. A relationship between the surface wave attenuation and the propagation distance by analyzing the analytical electric field distribution at different points on the propagation path in the open space is established. Thus, this study provides an improved method for assessing surface wave attenuation characteristics, which contributes to designing surface wave control for wireless communications, wireless power transfer, signal processing, and electromagnetic compatibility

Inkjet printed intelligent reflecting surface for indoor applications

Abstract A passive, low‐cost, paper‐based intelligent reflecting surface (IRS) is designed to reflect a signal in a desired direction to overcome non‐line‐of‐sight scenarios in indoor environments. The IRS is fabricated using conductive silver ink printed on paper with a specific nanoparticle arrangement, yielding a cost‐effective paper‐based IRS that can easily be mass‐produced. Full‐wave numerical simulation results were consistent with measurement results, demonstrating the IRS's ability to reflect incident waves into a desired nonspecular direction based on the inkjet‐printed design and materials