Water-based devices for advanced control of electromagnetic waves

Abstract

Tunable devices are of great interest as they offer reconfigurability to their operation, although many of them employ 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, because water, a high-permittivity dielectric at microwave frequencies, is readily available. Recent studies have shown that compact Mie resonators, which are the fundamental blocks in all-dielectric metamaterials and dielectric resonator antennas, can be realized with small water elements. In a variety of applied physics areas, encompassing frequencies from the radio to the optical parts of the spectrum, all-dielectric implementations have received immense attention. When it comes to water, its temperature-dependent permittivity and liquidity enable a multitude of unprecedentedly simple means to reconfigure and tune the resulting devices. Moreover, being a polar solvent, water easily dissolves various physiologically important electrolytes, which potentially can be exploited in a sensor design. Presently, we review water-based devices for advanced microwave control and sensing. We show and discuss the dynamic properties of water and examine the microwave scattering and absorption characteristics of single water elements. We investigate how such water elements can be employed in various microwave designs, including single resonators, metamaterials, metasurfaces, antennas, absorbers, and radio frequency components. The main complications of water are its losses, especially at higher microwave frequencies, and its stability. We discuss how to overcome these and show that even highly loss-sensitive modes, namely, toroidal modes and bound states in the continuum, can be realized with water-based devices. We believe that water-based devices usher the route to meet the UN proclaimed goals on global sustainability and human-friendly environment.

Self-sensing Intelligent Microrobots for Non-Invasive and Wireless Monitoring Systems

“…Solutions with different temperatures were obtained by heating the DI water and the permittivity gradually decreased from ~ 80 at room temperature (25℃) to less than 70 at 60℃. 32,33 The resonance frequency moved from ~ 1.90 GHz to ~ 1.12 GHz when the device was fabricated on the substrate, which was surrounded by the DI water at T = 35℃ as shown in Fig. 5(f).…”

Reconfigurable dielectric resonators with imbedded impedance surfaces—From enhanced and directional to suppressed scattering

“…A range of electromagnetic devices, including antennas and functional material platforms, rely heavily on the utilization of resonant elements. [1][2][3][4][5] In particular, they are exploited in periodic structures such as metamaterials and metasurfaces to enable wave physics otherwise impossible with natural materials and conventional structures. Therefore, tremendous effort is put into studying resonator designs on their own.…”

“…However, liquid water also holds a very high permittivity and modest losses for frequencies below 1 GHz, which can be utilized for antenna and metasurface structures. 5 For example, the permittivity of water is (80 − 𝑗2.9)𝜀 0 at 650 MHz and 20 °C. The losses in water affect the overall scattering response of water-based resonators, and thus only dipole modes can be excited efficiently excited.…”

Boundary-Induced Embedded Eigenstate in a Single Resonator for Advanced Sensing

“…We control the asymmetry in our structure by introducing a tiny water droplet placed atop the metallic resonator, leveraging the high permittivity of water at microwaves to trigger non-negligible perturbations in the local fields. − Different from the case of extended BICs, here the LDOS in the resonator is largely enhanced at the BIC condition, with a controllable Q-factor that enables sensing of deeply subwavelength variations to the water volume, as well as small changes in its refractive index. Remarkably, as we show in the following, we can use this platform to trace in real time the occurrence of chemical reactions, e.g., the dissolution of NaCl in water.…”