On the use of absorbing and matching metasurfaces for microwave imaging

Abstract

This thesis aims to investigate the use of electromagnetic metasurfaces in microwave imaging (MWI) to provide advantages for the development of imaging systems. In particular, this thesis is focused on two main contributions: (1) the use of absorbing metasurfaces in MWI, and (2) the use of matching metasurfaces in MWI. Firstly, metallic-backed absorbing metasurfaces are proposed to be used as the casing of MWI systems to not only shield the imaging system from external noise but also reduce the reflections from the casing back to the imaging domain. Metasurfaces are thin and light weight, thus making them suitable for portable MWI systems. In addition, metasurfaces at the microwave frequency range can be fabricated using standard printed circuit board (PCB) technology. Using simulated data in Ansys HFSS, we demonstrate the potential of using this type of enclosure for MWI systems. Finally, we fabricate one metallic-backed absorbing metasurface to evaluate its reflectivity. Secondly, matching metasurfaces are investigated for MWI as impedance transformers to more efficiently couple microwave energy into the target. This is important as sufficient interrogation of the target is essential for successful imaging. This will also alleviate the necessity of using coupling liquids in MWI. Similar to the absorbing metasurfaces, matching metasurfaces are also thin, light weight, and easy to manufacture at the microwave frequency range, thus making them suitable for MWI. Using simulated data in Ansys HFSS and an appropriate calibration technique, we demonstrate the possibility of using matching metasurfaces in MWI. Also, one matching metasurface is fabricated and measured to verify its matching performance. For both cases, the disadvantages of using metasurfaces for MWI are discussed. For the metasurfaces that we used in this research, the main disadvantage is their angular dependency, which affects their performance for complex scattering scenarios. The main future work should be focused on experimental evaluation of these metasurfaces in a real imaging chamber where practical issues such antenna mutual coupling and antenna impedance matching are present.