Jonah N. Gollub

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.

Characterization of complementary electric field coupled resonant surfaces

We present angle-resolved free-space transmission and reflection measurements of a surface composed of complementary electric inductive-capacitive (CELC) resonators. By measuring the reflection and transmission coefficients of a CELC surface with different polarizations and particle orientations, we show that the CELC only responds to in-plane magnetic fields. This confirms the Babinet particle duality between the CELC and its complement, the electric field coupled LC resonator. Characterization of the CELC structure serves to expand the current library of resonant elements metamaterial designers can draw upon to make unique materials and surfaces.

An efficient broadband metamaterial wave retarder

Metamaterials with anisotropic electromagnetic properties have the capability to manipulate the polarization states of electromagnetic waves. We describe a method to design a broadband, low-loss wave retarder with graded constitutive parameter distributions based on non-resonant metamaterial elements. A structured metamaterial half-wave retarder that converts one linear polarization to its cross polarization is designed and its performance is characterized experimentally.

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.

Isotropic frequency selective surfaces made of cubic resonators

Isotropic frequency selective surface (FSS) made of cubic arrangements of split ring resonators (SRRs) is proposed and analyzed. For this purpose, a suitable isotropic modification of the SRR was used in the design of a cubic unit element invariant under the tetrahedral point symmetry group. It was experimentally demonstrated that the transmission through such a FSS is angle and polarization independent. For comparison, another FSS, whose unit elements do not satisfy necessary symmetries, was measured, showing clearly anisotropic behavior. We feel then that symmetries play an important role. Potential device applications are envisioned for antenna technology at microwave and terahertz frequencies.

Spatially resolving antenna arrays using frequency diversity

Radio imaging devices and synthetic aperture radar typically use either mechanical scanning or phased arrays to illuminate a target with spatially varying radiation patterns. Mechanical scanning is unsuitable for many high-speed imaging applications, and phased arrays contain many active components and are technologically and cost prohibitive at millimeter and terahertz frequencies. We show that antennas deliberately designed to produce many different radiation patterns as the frequency is varied can reduce the number of active components necessary while still capturing high-quality images. This approach, called frequency-diversity imaging, can capture an entire two-dimensional image using only a single transmit and receive antenna with broadband illumination. We provide simple principles that ascertain whether a design is likely to achieve particular resolution specifications, and illustrate these principles with simulations.

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.

Printed Aperiodic Cavity for Computational and Microwave Imaging

We demonstrate a frequency-diverse aperture for microwave imaging based on a planar cavity at K-band frequencies (18-26.5 GHz). The structure consists of an array of radiating circular irises patterned into the front surface of a double-sided printed circuit board. The irises are distributed in a Fibonacci pattern to maximize spatial diversity at the scene. The printed cavity is a phase-diverse system and encodes imaged scene information onto a set of frequencies that span the K-band. Similar to recently reported metamaterial apertures, the printed cavity imager does not require any mechanically moving parts or complex phase shifting networks. Imaging of a number of targets is shown; these reconstructed images demonstrate the ability of the system to perform imaging at the diffraction limit. The proposed printed cavity imager possesses a relatively large quality factor that can be traded off to achieve higher radiation efficiency. The general mode characteristics of the printed cavity suggest advantages when used in computational imaging scenarios.