Mohammadreza F. Imani

Computational imaging using a mode-mixing cavity at microwave frequencies

We present a 3D computational imaging system based on a mode-mixing cavity at microwave frequencies. The core component of this system is an electrically large rectangular cavity with one corner re-shaped to catalyze mode mixing, often called a Sinai Billiard. The front side of the cavity is perforated with a grid of periodic apertures that sample the cavity modes and project them into the imaging scene. The radiated fields are scattered by the scene and are measured by low gain probe antennas. The complex radiation patterns generated by the cavity thus encode the scene information onto a set of frequency modes. Assuming the first Born approximation for scattering dynamics, the received signal is processed using computational methods to reconstruct a 3D image of the scene with resolution determined by the diffraction limit. The proposed mode-mixing cavity is simple to fabricate, exhibits low losses, and can generate highly diverse measurement modes. The imaging system demonstrated in this letter can find application in security screening and medical diagnostic imaging.

Resolution of the Frequency Diverse Metamaterial Aperture Imager

The resolution of a frequency diverse compressive metamaterial aperture imager is investigated. The aperture consists of a parallel plate waveguide, in which an array of complementary, resonant metamaterial elements is patterned into one of the plates. Microwaves injected into the waveguide leak out through the resonant metamaterial elements, forming a spatially diverse waveform at the scene. As the frequency is scanned, the waveforms change, such that scene information can be encoded onto a set of frequency measurements. The compressive nature of the metamaterial imager enables image reconstruction from a significantly reduced number of measurements. We characterize the resolution of this complex aperture by studying the simulated point spread function (PSF) computed using different image reconstruction techniques. We compare the imaging performance of the system with that expected from synthetic aperture radar (SAR) limits.

Comprehensive simulation platform for a metamaterial imaging system

Recently, a frequency-diverse, metamaterial-based aperture has been introduced in the context of microwave and millimeter wave imaging. The generic form of the aperture is that of a parallel plate waveguide, in which complementary metamaterial elements patterned into the upper plate couple energy from the waveguide mode to the scene. To reliably predict the imaging performance of such an aperture prior to fabrication and experiments, it is necessary to have an accurate forward model that predicts radiation from the aperture, a model for scattering from an arbitrary target in the scene, and a set of image reconstruction approaches that allow scene estimation from an arbitrary set of measurements. Here, we introduce a forward model in which the metamaterial elements are approximated as polarizable magnetic dipoles, excited by the fields propagating within the waveguide. The dipoles used in the model can have arbitrarily assigned polarizability characteristics. Alternatively, fields measured from actual metamaterial samples can be decomposed into a set of effective dipole radiators, allowing the performance of actual samples to be quantitatively modeled and compared with simulated apertures. To confirm the validity of our model, we simulate measurements and scene reconstructions with a virtual multiaperture imaging system operating in the K-band spectrum (18-26.5 GHz) and compare its performance with an experimental system.

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.

Near-Field Plates: Metamaterial Surfaces/Arrays for Subwavelength Focusing and Probing

In this paper, we present a brief overview of near-field plates, which are nonperiodic grating-like surfaces/arrays that can focus electromagnetic field to subwavelength resolutions. The general properties of near-field plates are described, and the procedure used to design these devices is outlined. The design of two separate near-field plates is discussed in detail. One of the near-field plates produces a subwavelength line (1-D) focus while the other a spot (2-D) focus. Potential applications of near-field plates are also reviewed.

Near-Field Focusing With a Corrugated Surface

In this letter, we present a theoretical device that can produce electromagnetic near-field patterns with deep subwavelength resolution. The device consists of a single slit in a corrugated metallic surface. The surface exhibits a nonperiodic series of grooves symmetrically positioned about a waveguide-fed slit. A procedure for designing such a device is described in detail. The electromagnetic response of the corrugated surface is shown to be as theoretically predicted, producing a subwavelength focal pattern with a null-to-null beamwidth of lambda/10. In addition, the effect of losses on the performance of the device is studied. Finally, it is shown that the device can be impedance-matched to its waveguide feed. Such devices will find use in noncontact sensing and near-field probing applications.

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.

Generating Evanescent Bessel Beams Using Near-Field Plates

We present a near-field plate that can generate an evanescent Bessel beam. The metallic plate consists of nonperiodic concentric corrugations that surround a coaxially fed aperture. The design procedure for such a device is outlined. The designed plate is simulated using a full-wave electromagnetic solver and is shown to produce an evanescent Bessel beam, thereby verifying its design and operation. The performance of the near-field plate is contrasted against a coaxial probe and a near-field plate designed to produce an Airy focal pattern with the same beamwidth. Such a device, capable of producing evanescent Bessel beams, will find applications in near-field probing/imaging systems, data storage, and biomedical devices.

An Experimental Concentric Near-Field Plate

We present a concentrically corrugated near-field plate that can form a subwavelength near-field focal spot. The experimental plate consists of a coaxial aperture surrounded by nonperiodic concentric corrugations. The measured subwavelength patterns are shown to be significantly narrower than those created by a coaxial probe (without corrugations) of similar dimensions. Close agreement between simulated and measurement results is observed. Further, the subwavelength beam emitted by the corrugated near-field plate is shown to be narrower that of the coaxial probe, confirming the superior electromagnetic confinement achieved by the near-field plate over a focal length. Finally, the near-field plate is used to image two sources separated by subwavelength distances. The images obtained using the near-field plate exhibit significantly higher resolution than those obtained using a coaxial probe. The reported near-field plate will find use in near-field probing and microscopy applications.