Water – A Microwave Material for Advanced Wave Control and Sensing

Tunable microwave devices are of great interest as they offer adjustability to their operation, although many of them use rare and expensive materials. In a world with increasing focus on ecological compatibility and recyclability, immense efforts are being made to find bio-friendly alternatives. However, in some cases, one does not have to look far. At microwave frequencies, a high-permittivity dielectric, namely water, is readily available in every household. Recent studies have shown that compact Mie resonators, which are the fundamental blocks in all-dielectric metamaterials and dielectric resonator antennas (DRAs), can be realized with small water inclusions. The temperature-dependent permittivity and liquidity of water enable several ways to reconfigure and tune water-based devices. Moreover, being a polar solvent, water easily dissolves various physiologically important electrolytes, which potentially can be exploited in a sensor design. In this thesis, we review and demonstrate different water-based devices for microwave control and sensing. First, we review the electromagnetic properties of water and its interaction with microwaves. Subsequently, we study the scattering and absorption of microwaves in single inclusions, and examine how these can be used utilized in various microwave devices including DRAs, metasurfaces, absorbers, radio-frequency components and a structure with a so-called bound state in the continuum (BIC). In our work, we demonstrate a metasurface reflectarray, an electrically small DRA, a Huygens DRA, a microwave heating design and a novel BIC structure. The work comprises both numerical and experimental investigations of their dynamic properties and tunabilities. In particular, we present for the first time a practical BIC localized in a single metal-water resonator exhibiting exciting opportunities for sensing applications. Our results showcase the potential of water-based devices to be simple, cheap, bio-friendly and tunable alternatives for many microwave applications.