Claire M. Watts

Metamaterial Electromagnetic Wave Absorbers

The advent of negative index materials has spawned extensive research into metamaterials over the past decade. Metamaterials are attractive not only for their exotic electromagnetic properties, but also their promise for applications. A particular branch-the metamaterial perfect absorber (MPA)-has garnered interest due to the fact that it can achieve unity absorptivity of electromagnetic waves. Since its first experimental demonstration in 2008, the MPA has progressed significantly with designs shown across the electromagnetic spectrum, from microwave to optical. In this Progress Report we give an overview of the field and discuss a selection of examples and related applications. The ability of the MPA to exhibit extreme performance flexibility will be discussed and the theory underlying their operation and limitations will be established. Insight is given into what we can expect from this rapidly expanding field and future challenges will be addressed.

Terahertz single pixel imaging with an optically controlled dynamic spatial light modulator

We present a single pixel terahertz (THz) imaging technique using optical photoexcitation of semiconductors to dynamically and spatially control the electromagnetic properties of a semiconductor mask to collectively form a THz spatial light modulator (SLM). By co-propagating a THz and collimated optical laser beam through a high-resistivity silicon wafer, we are able to modify the THz transmission in real-time. By further encoding a spatial pattern on the optical beam with a digital micro-mirror device (DMD), we may write masks for THz radiation. We use masks of varying complexities ranging from 63 to 1023 pixels and are able to acquire images at speeds up to 1/2 Hz. Our results demonstrate the viability of obtaining real-time and high-fidelity THz images using an optically controlled SLM with a single pixel detector.

Bi-layer metamaterials as fully functional near-perfect infrared absorbers

In this letter, we discuss the design, fabrication, and experimental characterization of a bi-layer fully functional near-perfect metamaterial absorber (MMA) in the long-wavelength infrared (LWIR), which is broadband and generally insensitive to polarization up to a 60° incidence angle. A spectral absorptance of ≥99% was attained simultaneously at multiple LWIR wavelengths, with a bandwidth of 2 μm where the absorptance is ≥90%. This remarkable behavior is attributed to the strong mixing of coupling modes between the two resonators and the ground plane in the presence of a lossy dielectric, in which single layer structures do not exhibit. Furthermore, we show, by comparing two different MMA structures, how the absorption can be tailored by design within and across several IR subdivisions through a slight change in geometrical parameters. The bi-layer MMA has the immediate application of a functionally versatile, low-profile thermal sensor or emitter.

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.

Metamaterial-based imaging for potential security applications

In this paper, we present two different types of THz spatial light modulators (SLMs) that use dynamic metamaterials (MMs) to enable multiplex imaging. One imaging setup consists of a doped semiconducting MM as the SLM, with multi-color super-pixels composed of arrays of electronically controlled metamaterial absorbers (MMAs). Our device is capable of modulation of THz radiation at frequencies up to 12 MHz with maximum modulation depths over 50%. We have also implemented a different system enabling high resolution, high-fidelity, multiplex single pixel THz imaging. We use optical photoexcitation of semiconductors to dynamically tune the electromagnetic properties of MMs. By copropagating a THz and collimated optical laser beam through a high-resistivity silicon (Si) wafer with a MM patterned on the surface, we may modify the THz transmission in real-time by modifying the optical power. By further encoding a spatial pattern on the optical beam, with a digital micro-mirror device (DMD), we may write masks for THz radiation.

Partitioned inverse image reconstruction for millimeter-wave SAR imaging

Synthetic aperture radar (SAR) images are representations of the microwave or millimeter-wave reflectivity of the observed scenes. SAR image reconstruction is an inverse problem, which can be solved via an approximation, e.g. matched filter (MF), or the explicit inverse using a large amount of measurement data. However, the approximation limits the resolution while the explicit inverse is computationally complex and mostly ill-conditioned. This paper proposes a partitioned inverse (PI) approach based on the Moore-Penrose pseudo inverse using truncated singular value decomposition for regularization, which is robust to noise. It is shown that PI has an improved resolution of 24% over MF even at 0 dB SNR and is three orders of magnitude faster than the explicit inverse. A measurement based proof of concept experiment using a laboratory K-Band (15–26.5 GHz) ultra-wideband SAR system is shown to validate the proposed approach.

Enhanced Resolution Stripmap Mode Using Dynamic Metasurface Antennas

To maintain sufficient signal-to-noise ratio (SNR) for image reconstruction and image interpretation, conventional synthetic aperture radar (SAR) systems must trade off resolution and scene size. This paper proposes a new SAR mode of operation, which improves resolution while maintaining good SNR and a large scene size. It leverages the unique properties of dynamic metasurface antennas (MSAs) to subsample a large virtual beamwidth utilizing multiple small distinct antenna beams. Due to this parallelization in scene sampling, the constraints on the azimuth sampling rate can be relaxed while maintaining an aliasing-free cross range. Due to the versatile properties of MSAs and their cost effective manufacturing process, this paper proposes SAR systems, which can obtain high resolution images over a wide scene size with lower cost and complexity than competing approaches. Point-spread functions and proof-of-concept SAR simulations are shown to verify this approach. In addition, laboratory experiments using a commercial prototype MSA are presented, which show an improvement of 62% in cross-range resolution of the proposed approach, compared with the cross-range resolution of stripmap mode SAR with the same aperture.

Coded and compressive THz imaging with metamaterials

Imaging in long wavelength regimes holds huge potential in many fields, from security to skin cancer detection. However, it is often difficult to image at these frequencies – the so called ‘THz gap1’ is no exception. Current techniques generally involve mechanically raster scanning a single detector to gain spatial information2, or utilization of a THz focal plane array (FPA)3. However, raster scanning results in slow image acquisition times and FPAs are relatively insensitive to THz radiation, requiring the use of high powered sources. In a different approach, a single pixel detector can be used in which radiation from an object is spatially modulated with a coded aperture to gain spatial information. This multiplexing technique has not fully taken off in the THz regime due to the lack of efficient coded apertures, or spatial light modulators (SLMs), that operate in this regime. Here we present the implementation of a single pixel THz camera using an active SLM. We use metamaterials to create an electronically controllable SLM, permitting the acquisition of high-fidelity THz images. We gain a signal-to-noise advantage over raster scanning schemes through a multiplexing technique4. We also use a source that is orders of magnitude lower in power than most THz FPA implementations3,5. We are able to utilize compressive sensing algorithms to reduce the number of measurements needed to reconstruct an image, and hence increase our frame rate to 1 Hz. This first generation device represents a significant step towards the realization of a single pixel THz camera.

GPU accelerated partitioned reconstruction algorithm for millimeter-wave 3D synthetic aperture radar (SAR) images

3D reconstruction using synthetic aperture radar (SAR) imaging is a computationally complex process due to the large amount of data involved. This paper proposes a partitioned reconstruction method for 3D SAR imaging, which leads to computationally efficient algorithms. The proposed method allows for parallel processing, e.g. using a general purpose graphic processing unit (GPU). Experimental results using a laboratory K-Band (15–26.5 GHz) ultra-wideband SAR system are presented. It is shown that 3D SAR reconstruction with GPU acceleration using the proposed algorithms is 300 times faster than the conventional matched filter approach.

2D and 3D millimeter-wave synthetic aperture radar imaging on a PR2 platform

Optical depth cameras, including both time-of-flight and structured-light sensors, have led to dramatic improvements in robot sensing and perception. We propose the use of millimeter-wave (mmW) radar as an important complement to optical sensors. While the millimeter wavelengths of radar sensors do not support as high resolution as the nanometer wavelength of optical sensors, the ability of mmW signals to penetrate smoky and foggy environments as well as see through many opaque objects makes them a compelling sensor for navigation as well as manipulation in challenging environments. We present a series of 2D and 3D mmW images made with a hand-held antenna grasped by a PR2 robot. The radar image sensor uses a mechanical “painting” motion to acquire multiple views of the target object over the 15 - 26.5 GHz K-band. A GPU-based reconstruction algorithm synthesizes 2D and 3D images of the target object. We demonstrate a ground range resolution of 13.6 mm and a cross-range resolution of 7.1 mm for objects up to 0.5 m away from the robot. We further demonstrate imaging objects through fog, as well as through opaque paper.