Passive Array Correlation-Based Imaging in a Random Waveguide

We consider reflector imaging in a weakly random waveguide. We address the situation in which the source is farther from the reflector to be imaged than the energy equipartition distance, but the receiver array is closer to the reflector to be imaged than the energy equipartition distance. As a consequence, the reflector is illuminated by a partially coherent field and the signals recorded by the receiver array are noisy. This paper shows that migration of the recorded signals cannot give a good image, but an appropriate migration of the cross correlations of the recorded signals can give a very good image. The resolution and stability analysis of this original functional shows that the reflector can be localized with an accuracy of the order of the wavelength even when the receiver array has small aperture, and that broadband sources are necessary to ensure statistical stability, whatever the aperture of the array.Comment: 27 pages; second version with minor revisions; an extended explanation on stability of the imaging method

Computational polarimetric microwave imaging

We propose a polarimetric microwave imaging technique that exploits recent advances in computational imaging. We utilize a frequency-diverse cavity-backed metasurface, allowing us to demonstrate high-resolution polarimetric imaging using a single transceiver and frequency sweep over the operational microwave bandwidth. The frequency-diverse metasurface imager greatly simplifies the system architecture compared with active arrays and other conventional microwave imaging approaches. We further develop the theoretical framework for computational polarimetric imaging and validate the approach experimentally using a multi-modal leaky cavity. The scalar approximation for the interaction between the radiated waves and the target---often applied in microwave computational imaging schemes---is thus extended to retrieve the susceptibility tensors, and hence providing additional information about the targets. Computational polarimetry has relevance for existing systems in the field that extract polarimetric imagery, and particular for ground observation. A growing number of short-range microwave imaging applications can also notably benefit from computational polarimetry, particularly for imaging objects that are difficult to reconstruct when assuming scalar estimations.Comment: 17 pages, 15 figure

Phaseless computational imaging with a radiating metasurface

Computational imaging modalities support a simplification of the active architectures required in an imaging system and these approaches have been validated across the electromagnetic spectrum. Recent implementations have utilized pseudo-orthogonal radiation patterns to illuminate an object of interest---notably, frequency-diverse metasurfaces have been exploited as fast and low-cost alternative to conventional coherent imaging systems. However, accurately measuring the complex-valued signals in the frequency domain can be burdensome, particularly for sub-centimeter wavelengths. Here, computational imaging is studied under the relaxed constraint of intensity-only measurements. A novel 3D imaging system is conceived based on 'phaseless' and compressed measurements, with benefits from recent advances in the field of phase retrieval. In this paper, the methodology associated with this novel principle is described, studied, and experimentally demonstrated in the microwave range. A comparison of the estimated images from both complex valued and phaseless measurements are presented, verifying the fidelity of phaseless computational imaging.Comment: 18 pages, 18 figures, articl

Long-range forces in controlled systems

This thesis investigates new phenomena due to long-range forces and their effects on different multi-DOFs systems. In particular the systems considered are metamaterials, i.e. materials with long-range connections. The long-range connections characterizing metamaterials are part of the more general framework of non-local elasticity. In the theory of non-local elasticity, the connections between non-adjacent particles can assume different configurations, namely one-to-all, all-to-all, all-to-all-limited, random-sparse and all-to-all-twin. In this study three aspects of the long-range interactions are investigated, and two models of non-local elasticity are considered: all-to-all and random-sparse. The first topic considers an all-to-all connections topology and formalizes the mathematical models to study wave propagation in long-range 1D metamaterials. Closed forms of the dispersion equation are disclosed, and a propagation map synthesizes the properties of these materials which unveil wave-stopping, negative group velocity, instability and non-local effects. This investigation defines how long-range interactions in elastic metamaterials can produce a variety of new effects in wave propagation. The second one considers an all-to-all connections topology and aims to define an optimal design of the long-range actions in terms of spatial and intensity distribution to obtain a passive control of the propagation behavior which may produces exotic effects. A phenomenon of frequency filtering in a confined region of a 1D metamaterial is obtained and the optimization process guarantees this is the best obtainable result for a specific set of control parameters. The third one considers a random-sparse connections topology and provides a new definition of long-range force, based on the concept of small-world network. The small-world model, born in the field of social networks, is suitably applied to a regular lattice by the introduction of additional, randomly selected, elastic connections between different points. These connections modify the waves propagation within the structure and the system exhibits a much higher propagation speed and synchronization. This result is one of the remarkable characteristics of the defined long-range connections topology that can be applied to metamaterials as well as other multi-DOFs systems. Qualitative experimental results are presented, and a preliminary set-up is illustrated. To summarize, this thesis highlights non-local elastic structures which display unusual propagation behaviors; moreover, it proposes a control approach that produces a frequency filtering material and shows the fast propagation of energy within a random-sparse connected material