Advances in non-mechanical frequency-diverse apertures and reconstruction algorithms have made real-time millimeter-wave data acquisition and volumetric imaging possible. Fast frame rates allow imaging people in motion, which represents a tremendous opportunity to increase security screening throughput over existing solutions where subjects must individually strike and hold a pose. However for non-mechanical systems specularity coupled with limited viewing perspectives diminish coverage for individual images.
To mitigate these issues, a system can leverage relative motion between the aperture and subject for a diversity of perspectives across several images. Such an image set offers overlapping and complementary swaths of subject coverage. By stitching together these images a composite image of the subject can be produced with much better overall coverage.
Of course, people change shape as they move, which significantly complicates the image stitching registration and blending process. A deformable geometric model of a person suitable for real-time stitching is required. Drawing from the field of computer animation, we introduce a deformation model of a person based on Shape Key Deformation (SKD) and Skeletal Subspace Deformation (SSD). SKD blends shapes together, while SSD utilizes a simplified “skeleton” to guide deformation and modulate SKD. Assuming the pose of the skeleton is known, the deformation model is able to map any arbitrary image of a person onto a single rest image for stitching. The model is simple, fast, and robust. We go on to demonstrate image stitching of a simulated person in motion using software that models a massively multistatic MIMO metasurface computational imaging system.
In this paper, a spotlight imaging system integrated with a frequency-diverse aperture is presented for security-screening applications. The spotlight imager consists of holographic metasurface antennas that can dynamically be tuned to radiate spotlight patterns allowing the extraction of high-resolution images from a constrained field-of-view (FOV). The reconfigurable holographic metasurface antennas consist of a metasurface layer used to modulate the guided-mode reference to an aperture field of interest producing the desired radiated wavefronts. The reconfigurable operation is achieved in an all-electronic manner without the need for any mechanical moving apparatus or phase shifting circuits. The spotlight aperture operates at a single frequency, 75 GHz, within the W-band frequency regime (75 – 110 GHz) and is used for the high-resolution identification of threat objects while the frequency-diverse aperture operates at K-band frequencies (17.5 – 26.5 GHz) and is used for low-resolution detection purposes. The scene to be imaged is first interrogated using the K-band aperture at low resolution and the constrained-FOV is imaged using the W-band system to achieve the identification of threat objects.
Computational imaging is a proven strategy for obtaining high-quality images with fast acquisition rates and simpler hardware. Metasurfaces provide exquisite control over electromagnetic fields, enabling the radiated field to be molded into unique patterns. The fusion of these two concepts can bring about revolutionary advances in the design of imaging systems for security screening. In the context of computational imaging, each field pattern serves as a single measurement of a scene; imaging a scene can then be interpreted as estimating the reflectivity distribution of a target from a set of measurements. As with any computational imaging system, the key challenge is to arrive at a minimal set of measurements from which a diffraction-limited image can be resolved. Here, we show that the information content of a frequency-diverse metasurface aperture can be maximized by design, and used to construct a complete millimeter-wave imaging system spanning a 2 m by 2 m area, consisting of 96 metasurfaces, capable of producing diffraction-limited images of human-scale targets. The metasurfacebased frequency-diverse system presented in this work represents an inexpensive, but tremendously flexible alternative to traditional hardware paradigms, offering the possibility of low-cost, real-time, and ubiquitous screening platforms.
We present a novel theoretical approximation for predicting the enhanced optical transmission properties through a periodic array of subwavelength square apertures in perforated metal films. We show that a Fabry-Perot resonance occurs in an effective resonant cavity whose dimensions are determined by the apertures' geometry and the decay lengths of the associated evanescent diffracted modes. This model demonstrates strong agreement to simulated results, and can be used to rapidly and efficiently design aperture arrays with specific transmission properties.
We present an analytical study of resonance properties of square subwavelength apertures at optical and near-IR
frequencies. This approach allows accurate prediction of resonance responses, captures both propagating and
evanescent modes, and can easily be implemented in other analytical techniques. In this approach we avoid
analyzing the detailed behavior of the fields inside the metal walls, but still obtain the effects of the buildup of
charges within those walls. We calculate the dispersion relation and find the cutoff frequency's dependence on
cavity dimensions for a square aperture embedded in a silver film, and support our findings with finite-element
simulations.
In this work we describe how to model the efficiency of solar cells with novel metamaterial coatings optimized
for light harvesting. Full device modeling is implemented using optical and electrical simulations. As a proof
of concept, we simulate the operation of a metamaterial contact on a first generation monocrystalline silicon
solar cell. We compare device characteristics and efficiencies to standard antireflective coatings applied to a grid
contact cell. The effects of the metamaterial contact on silicon solar cell efficiencies is discussed for PN junction
and metal-insulator-semiconductor cell structures. It is found that the metal-insulator-semiconductor solar cell
designed performs better than the PN junction cell.
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