Timothy Sleasman

Large Metasurface Aperture for Millimeter Wave Computational Imaging at the Human-Scale

We demonstrate a low-profile holographic imaging system at millimeter wavelengths based on an aperture composed of frequency-diverse metasurfaces. Utilizing measurements of spatially-diverse field patterns, diffraction-limited images of human-sized subjects are reconstructed. The system is driven by a single microwave source swept over a band of frequencies (17.5–26.5 GHz) and switched between a collection of transmit and receive metasurface panels. High fidelity image reconstruction requires a precise model for each field pattern generated by the aperture, as well as the manner in which the field scatters from objects in the scene. This constraint makes scaling of computational imaging systems inherently challenging for electrically large, coherent apertures. To meet the demanding requirements, we introduce computational methods and calibration approaches that enable rapid and accurate imaging performance.

Dynamic metamaterial aperture for microwave imaging

We present a dynamic metamaterial aperture for use in computational imaging schemes at microwave frequencies. The aperture consists of an array of complementary, resonant metamaterial elements patterned into the upper conductor of a microstrip line. Each metamaterial element contains two diodes connected to an external control circuit such that the resonance of the metamaterial element can be damped by application of a bias voltage. Through applying different voltages to the control circuit, select subsets of the elements can be switched on to create unique radiation patterns that illuminate the scene. Spatial information of an imaging domain can thus be encoded onto this set of radiation patterns, or measurements, which can be processed to reconstruct the targets in the scene using compressive sensing algorithms. We discuss the design and operation of a metamaterial imaging system and demonstrate reconstructed images with a 10:1 compression ratio. Dynamic metamaterial apertures can potentially be of benefit in microwave or millimeter wave systems such as those used in security screening and through-wall imaging. In addition, feature-specific or adaptive imaging can be facilitated through the use of the dynamic aperture.

Design considerations for a dynamic metamaterial aperture for computational imaging at microwave frequencies

We investigate the imaging capabilities of a one-dimensional, dynamic, metamaterial aperture that operates at the lower part of K-band microwave frequencies (17.5–21.1 GHz). The dynamic aperture consists of a microstrip transmission line with an array of radiating, complementary, subwavelength metamaterial irises patterned into the upper conductor. Diodes integrated into the metamaterial resonators provide voltage-controlled switching of the resonant metamaterial elements between radiating and nonradiating states. Applying a series of on/off patterns to the metamaterial resonators produces a series of distinct radiation patterns that sequentially illuminate a scene. The backscattered signal contains encoded scene information over a set of measurements that can be postprocessed to reconstruct an image. We present a series of design considerations for the dynamic aperture, as well as a series of experimental studies performed using a dynamic aperture prototype. High-fidelity, real-time, diffraction-limited imaging using the prototype is demonstrated. The dynamic aperture suggests a path to fast and reliable imaging with low-cost and versatile hardware, for a variety of applications including security screening, biomedical diagnostics, and through-wall imaging.

Waveguide-Fed Tunable Metamaterial Element for Dynamic Apertures

We present the design of a tunable metamaterial element that can serve as the building block for a dynamically reconfigurable aperture. The element-a complimentary electric-LC (cELC) resonator-is patterned into the upper conductor of a microstrip transmission line, providing both a means of exciting the radiating metamaterial element as well as independent access for biasing circuitry. PIN diodes are connected across the capacitive gaps of the cELC, and a dc bias current is used to switch the junction between conducting and insulating states. The leakage of RF signal through the bias line is mitigated by integration of a radial decoupling stub. The proposed design and operation of the element are demonstrated through full-wave electromagnetic simulations. We discuss the potential application of the cELC element as a building block for metamaterial apertures capable of dynamic beamforming, imaging, or security screening applications.

Application of range migration algorithms to imaging with a dynamic metasurface antenna

Dynamic metasurface antennas are planar structures that exhibit remarkable capabilities in controlling electromagnetic wavefronts, advantages that are particularly attractive for microwave imaging. These antennas exhibit strong frequency dispersion and produce rapidly varying radiation patterns. Such behavior presents unique challenges for integration with conventional imaging algorithms. We adapt the range migration algorithm (RMA) for use with dynamic metasurfaces and propose a preprocessing step that ultimately allows for expression of measurements in the spatial frequency domain, from which the fast Fourier transform can efficiently reconstruct the scene. Numerical studies illustrate imaging performance using conventional methods and the adapted RMA, demonstrating that the RMA can reconstruct images with comparable quality in a fraction of the time. The algorithm can be extended to a broad class of complex antennas for application in synthetic aperture radar and MIMO imaging.

Synthetic aperture radar with dynamic metasurface antennas: a conceptual development

We investigate the application of dynamic metasurface antennas (DMAs) to synthetic aperture radar (SAR) systems. Metasurface antennas can generate a multitude of tailored electromagnetic waveforms from a physical platform that is low-cost, lightweight, and planar; these characteristics are not readily available with traditional SAR technologies, such as phased arrays and mechanically steered systems. We show that electronically tuned DMAs can generate steerable, directive beams for traditional stripmap and spotlight SAR imaging modes. This capability eliminates the need for mechanical gimbals and phase shifters, simplifying the hardware architecture of a SAR system. Additionally, we discuss alternative imaging modalities, including enhanced resolution stripmap and diverse pattern stripmap, which can achieve resolution on par with spotlight, while maintaining a large region-of-interest, as possible with stripmap. Further consideration is given to strategies for integrating metasurfaces with chirped pulse RF sources. DMAs are poised to propel SAR systems forward by offering a vast range of capabilities from a significantly improved physical platform.

Efficient complementary metamaterial element for waveguide-fed metasurface antennas.

We present a metamaterial element designed as an efficient radiator for waveguide-fed metasurface antennas. The metamaterial element is an electrically-small, complimentary electric-LC (cELC) resonator designed to exhibit large radiated power while maintaining low ohmic losses. The shape of the element is tapered to simultaneously achieve broadband operation and suppression of cross polarization radiation. Full-wave numerical studies at the K-band are conducted to examine its performance when etched into a microstrip line. In this configuration, the element shows a radiation efficiency of 90.2% and a fractional bandwidth of 8.7%. To investigate the potential benefits of the proposed element in two-dimensional platforms, the radiative characteristics of the element are calculated when the element is embedded in a dielectric-filled parallel-plate waveguide. This efficient metamaterial element has potential application as a building block for metasurface devices used in imaging, sensing, wireless power transfer, and wireless communication systems.

Analytical modeling of printed metasurface cavities for computational imaging

We derive simple analytical expressions to model the electromagnetic response of an electrically large printed cavity. The analytical model is then used to develop printed cavities for microwave imaging purposes. The proposed cavity is excited by a cylindrical source and has boundaries formed by subwavelength metallic cylinders (vias) placed at subwavelength distances apart. Given their small size, the electric currents induced on the vias are assumed to have no angular dependence. Applying this approximation simplifies the electromagnetic problem to a matrix equation which can be solved to directly compute the electric current induced on each via. Once the induced currents are known, the electromagnetic field inside the cavity can be computed for every location. We verify the analytical model by comparing its prediction to full-wave simulations. To utilize this cavity in imaging settings, we perforate one side of the printed cavity with radiative slots such that they act as the physical layer of a comput...

Single-frequency microwave imaging with dynamic metasurface apertures

Conventional microwave imaging schemes, enabled by the ubiquity of coherent sources and detectors, have traditionally relied on frequency bandwidth to retrieve range information, while using mechanical or electronic beamsteering to obtain cross-range information. This approach has resulted in complex and expensive hardware when extended to large-scale systems with ultrawide bandwidth. Relying on bandwidth can create difficulties in calibration, alignment, and imaging of dispersive objects. We present an alternative approach using electrically large, dynamically reconfigurable, metasurface antennas that generate spatially distinct radiation patterns as a function of tuning state. The metasurface antenna consists of a waveguide feeding an array of metamaterial radiators, each with properties that can be modified by applying a voltage to diodes integrated into the element. By deploying two of these apertures, one as the transmitter and one as the receiver, we realize sufficient spatial diversity to alleviate the dependence on frequency bandwidth and obtain range and cross-range information using measurements at a single frequency. We experimentally demonstrate this proposal by using two 1D dynamic metasurface apertures and reconstructing various 2D scenes (range and cross-range). Furthermore, we modify a conventional reconstruction method—the range migration algorithm—to be compatible with such configurations, resulting in an imaging system that is efficient in software and hardware. The imaging scheme presented in this paper has broad application to radio frequency imaging, including security screening, through-wall imaging, biomedical diagnostics, and synthetic aperture radar.

Near Field Scan Alignment Procedure for Electrically Large Apertures

Computational imaging at microwave frequencies has gained traction due to its potential for obtaining high-quality images with fast acquisition rates. Complex and diverse radiation patterns form the cornerstone of this approach. Electrically large antennas, such as mode-mixing cavities and metamaterial apertures, have proven to be effective platforms for generating such waveforms. Due to the complex nature of these antennas, near field scanning is often required to characterize their radiation patterns. However, accurate knowledge of the produced waveforms’ spatial distribution, with respect to the physical position of the antenna, is imperative. This relies on precise alignment between the antenna and the near field scan stage during the characterization process—a requirement that is especially cumbersome to achieve when operating at high frequencies. We present an effective method to address this problem; by introducing RF markers into the antenna the position of the antenna under test within the near field scanning setup can be obtained directly from the measurements. The proposed method is experimentally verified through comparison with measurements made using optical photogrammetry. The proposed process will find application in the alignment of computational and multistatic imaging systems, commonly used in security screening and threat detection, as well as in tiled electrically large antenna structures.