Laura Pulido-Mancera

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.

Discrete Dipole Approximation Applied to Highly Directive Slotted Waveguide Antennas

We present an analysis of a slotted waveguide antenna (SWA) whose directivity has been enhanced by using metamaterial parasitic elements. We apply an adapted form of the discrete dipole approximation (DDA) as a modeling tool and verify the accuracy and versatility of this method for different configurations, including matched and shorted SWAs, and with and without parasitic elements. The results presented in this letter demonstrate the capabilities of the DDA for the fast and accurate simulation of aperture antennas composed of small radiators, and its further application for the design of complex metamaterial structures.

Alternative synthetic aperture radar (SAR) modalities using a 1D dynamic metasurface antenna

Synthetic aperture radar (SAR) systems conventionally rely on mechanically-actuated reflector dishes or large phased arrays for generating steerable directive beams. While these systems have yielded high-resolution images, the hardware suffers from considerable weight, high cost, substantial power consumption, and moving parts. Since these disadvantages are particularly relevant in airborne and spaceborne systems, a flat, lightweight, and low-cost solution is a sought-after goal. Dynamic metasurface antennas have emerged as a recent technology for generating waveforms with desired characteristics. Metasurface antennas consist of an electrically-large waveguide loaded with numerous subwavelength radiators which selectively leak energy from a guided wave into free space to form various radiation patterns. By tuning each radiating element, we can modulate the aperture’s overall radiation pattern to generate steered directive beams, without moving parts or phase shifters. Furthermore, by using established manufacturing methods, these apertures can be made to be lightweight, low-cost, and planar, while maintaining high performance. In addition to their hardware benefits, dynamic metasurfaces can leverage their dexterity and high switching speeds to enable alternative SAR modalities for improved performance. In this work, we briefly discuss how dynamic metasurfaces can conduct existing SAR modalities with similar performance as conventional systems from a significantly simpler hardware platform. We will also describe two additional modalities which may achieve improved performance as compared to traditional modalities. These modalities, enhanced resolution stripmap and diverse pattern stripmap, offer the ability to circumvent the trade-off between resolution and region-of-interest size that exists within stripmap and spotlight. Imaging results with a simulated dynamic metasurface verify the benefits of these modalities and a discussion of implementation considerations and noise effects is also included. Ultimately, the hardware gains coupled with the additional modalities well-suited to dynamic metasurface antennas has poised them to propel the SAR field forward and open the door to exciting opportunities.

Aperture synthesis with a monochromatic metasurface imaging system for 3D near-field imaging

Microwave imaging systems have become increasingly prevalent owing to their ability to obtain 3D images while penetrating optically-opaque materials. These capabilities have motivated the development of various microwave imaging systems for applications ranging from security screening to biomedical imaging. Recent demonstrations have evidenced the idea that metasurface apertures can improve the hardware characteristics of microwave imaging systems due to their lightweight, low-cost, and planar nature. While metasurfaces can improve the antenna hardware, the large spectral bandwidth required for microwave imaging still incurs complex, costly, and performance-limiting RF components. To address the drawbacks inherent to using a large bandwidth, recent works have suggested that near-field microwave imaging can be performed at a single frequency point. In this work, monochromatic imaging is demonstrated by deploying two metasurface apertures to form a near-field microwave imaging system. By leveraging the unique radiation patterns emitted by metasurfaces, a pair of metasurface antennas, one acting as a transmitter and the other as a receiver, can acquire range and cross range information with measurements taken at a single frequency. We will show that this operation can then be supplemented by introducing aperture synthesis in the height direction to obtain fully 3D images. To account for the unusual illumination strategy, a reconstruction algorithm based on the range migration algorithm is formulated and implemented to enable efficient reconstruction of 3D images. Ultimately, the metasurface hardware, aperture synthesis, and monochromatic operation are combined to form an imaging system with high performance capabilities, without requiring complex and costly hardware.

Near-field SAR imaging with dynamic metasurface antennas using an adapted range migration algorithm

Synthetic aperture radar (SAR) is a well-established approach for retrieving images with high resolution. How- ever, common hardware used for SAR systems is usually complex and costly, and can suffer from lengthy signal acquisition. In near-field imaging, such as through-wall-sensing and security screening, simpler and faster hardware can be found in the form of dynamic metasurface antennas (DMAs). These antennas consist of a waveguide-fed array of tunable metamaterial elements whose overall radiation patterns can be altered by DC signals. By sweeping through a set of tuning states, near-field imaging can be accomplished by multiplexing scene information into a collection of measurements, which are post-processed to retrieve scene information. While DMAs simplify hardware, the post-processing can become cumbersome, especially when DMAs are moving in a fashion similar to SAR. In this presentation, we address this problem by modifying the range migration algorithm (RMA) to be compatible with DMAs. To accommodate complex patterns generated by DMAs in the RMA, a pre-processing step is introduced to transform the measurements into an equivalent set corresponding to an effective multistatic configuration, for which specific forms of the algorithm have been derived. As we are operating in the near field of the antennas, some approximations made in the classical formulation of RMA may not be valid. In this paper, we examine the effect of one such approximation: the discarding of amplitude terms in the signal-target Fourier relationship. We demonstrate the adaptation of the RMA to near field imaging using a DMA as central hardware of a SAR system, and discuss the effects of this approximation on the resulting image quality.

Single-frequency 3D synthetic aperture imaging with dynamic metasurface antennas

Through aperture synthesis, an electrically small antenna can be used to form a high-resolution imaging system capable of reconstructing three-dimensional (3D) scenes. However, the large spectral bandwidth typically required in synthetic aperture radar systems to resolve objects in range often requires costly and complex RF components. We present here an alternative approach based on a hybrid imaging system that combines a dynamically reconfigurable aperture with synthetic aperture techniques, demonstrating the capability to resolve objects in three dimensions (3D), with measurements taken at a single frequency. At the core of our imaging system are two metasurface apertures, both of which consist of a linear array of metamaterial irises that couple to a common waveguide feed. Each metamaterial iris has integrated within it a diode that can be biased so as to switch the element on (radiating) or off (non-radiating), such that the metasurface antenna can produce distinct radiation profiles corresponding to different on/off patterns of the metamaterial element array. The electrically large size of the metasurface apertures enables resolution in range and one cross-range dimension, while aperture synthesis provides resolution in the other cross-range dimension. The demonstrated imaging capabilities of this system represent a step forward in the development of low-cost, high-performance 3D microwave imaging systems.

Discrete dipole approximation for simulation of unusually tapered leaky wave antennas

In this paper, Discrete Dipole Approximation (DDA) is presented as a simulation tool for predicting the electromagnetic properties of one dimensional metasurface antennas and slotted waveguide antennas. The proposed method is verified by demonstrating excellent agreement between DDA predictions and that of a full-wave electromagnetic solver. This technique is especially attractive since it allows us to simulate and design metasurface antennas with unusual tapering and irises with unconventional geometries (e.g. metamaterial elements), in order to achieve the desired radiation characteristics, such as beamwidth and sidelobe level, while maintaining steering capabilities.

Reconfigurable metasurface aperture for security screening and microwave imaging

Microwave imaging systems have seen growing interest in recent decades for applications ranging from security screening to space/earth observation. However, hardware architectures commonly used for this purpose have not seen drastic changes. With the advent of metamaterials a wealth of opportunities have emerged for honing metasurface apertures for microwave imaging systems. Recent thrusts have introduced dynamic reconfigurability directly into the aperture layer, providing powerful capabilities from a physical layer with considerable simplicity. The waveforms generated from such dynamic metasurfaces make them suitable for application in synthetic aperture radar (SAR) and, more generally, computational imaging. In this paper, we investigate a dynamic metasurface aperture capable of performing microwave imaging in the K-band (17.5–26.5 GHz). The proposed aperture is planar and promises an inexpensive fabrication process via printed circuit board techniques. These traits are further augmented by the tunability of dynamic metasurfaces, which provides the dexterity necessary to generate field patterns ranging from a sequence of steered beams to a series of uncorrelated radiation patterns. Imaging is experimentally demonstrated with a voltage-tunable metasurface aperture. We also demonstrate the aperture’s utility in real-time measurements and perform volumetric SAR imaging. The capabilities of a prototype are detailed and the future prospects of general dynamic metasurface apertures are discussed.

Polarizability extraction of complementary metamaterial elements in waveguides for aperture modeling

We consider the design and modeling of metasurfaces that couple energy from guided waves to propagating wavefronts. This is a first step towards a comprehensive, multiscale modeling platform for metasurface antennas—large arrays of metamaterial elements embedded in a waveguide structure that radiates intro free-space—in which the detailed electromagnetic responses of metamaterial elements are replaced by polarizable dipoles. We present two methods to extract the effective polarizability of a metamaterial element embedded in a oneor two-dimensional waveguide. The first method invokes surface equivalence principles, averaging over the effective surface currents and charges within an element to obtain the effective dipole moments; the second method is based on computing the coefficients of the scattered waves within the waveguide, from which the effective polarizability can be inferred. We demonstrate these methods on several variants of waveguidefed metasurface elements, finding excellent agreement between the two, as well as with analytical expressions derived for irises with simpler geometries. Extending the polarizability extraction technique to higher order multipoles, we confirm the validity of the dipole approximation for common metamaterial elements. With the effective polarizabilities of the metamaterial elements accurately determined, the radiated fields generated by a metasurface antenna (inside and outside the antenna) can be found self-consistently by including the interactions between polarizable dipoles. The dipole description provides an alternative language and computational framework for engineering metasurface antennas, holograms, lenses, beam-forming arrays, and other electrically large, waveguide-fed metasurface structures.