This paper reviews some of our recent works on how electromagnetic metasurfaces could be advantageously used for microwave imaging applications. In particular, it discusses some possibilities for augmenting the information content of the data and enhancing the achievable signal-to-noise ratio.

Most existing methods for analyzing reconfigurable intelligent surface (RIS) channels are limited to free-space links and do not account for the mutual coupling of unit cells, or they fail to accurately model several types of realistic scenarios including near-field operation and interaction of RISs with rich multipath environments. Alternatively, some methods that can handle these scenarios are computationally intensive, such as pure full-wave analysis. We present an equivalence principle-based hybrid ray-tracing/full-wave method to model wave propagation in wireless communication channels enabled by RISs. This method uses ray-tracing to determine the incident waves on an RIS, then applies full-wave analysis to model the response of the RIS to these waves. Next, based on the equivalence principle, we introduce equivalent surface electric and magnetic current densities that generate the scattered fields produced by RIS. These equivalent sources are integrated with ray-tracing to derive the site-specific propagation model of the RIS channel. The proposed method readily accounts for multiple incident waves on an RIS, enabling the accurate analysis of propagation channels with RISs, with receivers in both the near and the far region of an RIS. We show that the accuracy of our method is comparable to that of full-wave analysis, through simple examples that are manageable by the finite-element method. Also, we experimentally validate the proposed technique by comparing simulation and measured data for an actual indoor radio environment with an anomalous reflection metasurface.

Microwave imaging (MWI) systems are usually enclosed within casings, e.g., in order to contain the utilized coupling liquid or to help mount the antenna system. On the other hand, inverse scattering algorithms, which are used to process the measured microwave scattering data, often assume that the background medium of the imaging system extends to infinity (i.e., unbounded background medium assumption). Thus, they do not consider the reflections occurring at the system enclosure. For such algorithms to yield successful images, these reflections need to be minimized, e.g., via the use of a lossy coupling liquid. As an alternative to a lossy background medium which also reduces the desired signal level, this paper investigates the use of metallic-backed absorbing metasurfaces as the MWI system enclosure in order to (i) reduce these reflections, and also (ii) to shield the MWI system from external interference. Using simulated data, we then show that standard inverse scattering algorithms, employing the free-space assumption, can successfully process the data collected under the metasurface enclosure and yield acceptable permittivity images. The advantages and disadvantages of absorbing metasurface enclosure, along with the limitations of this study, will also be discussed. Finally, an absorbing metasurface is fabricated and its reflectivity is experimentally evaluated.

Matching fluids used in microwave imaging are often lossy to reduce the reflections from the system enclosure. To enable the use of low-loss matching fluids, we investigate the use of absorbing metasurfaces as the enclosure. This has the potential to enhance the signal-to-noise ratio of the data, thus, enhancing the achievable image accuracy. The presented example has the following limitation: as opposed to design and simulation of the physical structure of the metasurface enclosure, it uses two impedance boundary conditions on top of a metallic-backed substrate. Thus, the angular dependency of a practical absorber is not completely taken into account by this model. At the conference, we’ll also consider a microwave imaging metasurface enclosure via design and simulation of a physical structure.

This paper summarizes, extends, and synthetically evaluates a method for metasurface design which uses electromagnetic inversion. After the inversion algorithm determines a homogenized surface susceptibility model to implement a desired power pattern, the susceptibility distribution is converted to a three-layer admittance sheet topology. Lastly, full-wave commercial software is used to simulate and verify the performance of a metasurface designed and implemented using the proposed procedure.