KEYWORDS: Antennas, Reflectivity, Synthetic aperture radar, K band, V band, 3D image processing, Radar imaging, Image resolution, Metals, Imaging systems
We demonstrate active millimeter wave (mmWave) 3D synthetic aperture radar (SAR) imaging through common building and construction materials. Human-safe mmWaves penetrate many non-metallic construction materials to a surprising degree, allowing for non-destructive structural and utility integrity analysis. We demonstrate that mmWave imaging can be used to locate pipes, electrical wiring, wood and metal studs, rebar, and other structural elements and utility infrastructure embedded in building walls. Millimeter wave imaging can also be used to find contraband items hidden inside walls. In addition, liquid water strongly reflects mmWave energy, allowing the localization of potential leaks. We present K-Band (15 - 26:5 GHz) and V-Band (50 - 65 GHz) 3D SAR images of brick, drywall, tile, wood, and concrete wall test structures. Imaging was performed using a 2D raster scan of a bistatic horn antenna pair, yielding a 3D SAR image with a voxel resolution of approximately 1.5 cm (K-band) and 5 mm (V-band).
Single pixel cameras are useful imaging devices where it is difficult or infeasible to fashion focal plan arrays. For example in the Far Infrared (FIR) it is difficult to perform imaging by conventional detector arrays, owing to the cost and size of such an array. The typical single pixel camera uses a spatial light modulator (SLM) - placed in the conjugate image plane – and is used to sample various portions of the image. The spatially modulated light emerging from the SLM is then sent to a single detector where the light is condensed with suitable optics for detection. Conventional SLMs are either based on liquid crystals or digital mirror devices. As such these devices are limited in modulation speeds of order 30 kHz. Further there is little control over the type of light that is modulated.
We present metamaterial based spatial light modulators which provide the ability to digitally encode images – with various measurement matrix coefficients – thus permitting high speed and fidelity imaging capability. In particular we use the Hadamard matrix and related S-matrix to encode images for single pixel imaging. Metamaterials thus permit imaging in regimes of the electromagnetic spectrum where conventional SLMs are not available. Additionally, metamaterials offer several salient features that are not available with commercial SLMs. For example, metamaterials may be used to enable hyperspectral, polarimetric, and phase sensitive imaging. We present the theory and experimental results of single pixel imaging with digital metamaterials in the far infrared and highlight the future of this exciting field.
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.
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.
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