Metasurfaces for Far-Field Wireless Power Transfer and Energy Harvesting
Authors: Ramahi Omar 
Published: July 2019

The concept of electromagnetic energy harvesting and wireless power transfer had been proposed more than half a century ago. In all published works related to electromagnetic energy collection, classical antennas have been used while the focus shifted to the rectification circuitry. Recent works showed that the weakest link in the traditional energy harvesting chain is the antenna itself. Here, we show that metasurfaces provide a viable alternative to classical antennas yielding efficiencies approaching unity. This presentation will be confined to the microwaves frequency regime.Far-field wireless power transfer (WPT) is a relatively old concept where antennas are used to transfer power between two distant points. WPT perhaps dates back to Tesla but it was Brown in the 1950s that demonstrated its viability [1]. In recent years, WPT has been reconsidered as practical means to transfer power from outer space where satellites collect solar power with high efficiency using photovoltaic technology and then convert the power to microwaves for beaming to antenna farms at specific locations on earth. This would facilitate availability of plenty of power everyday throughout the year.Conventional antennas have been the traditional microwaves transducers used for WPT applications. Conventional antenna types include log-periodic, patch, dipole or any of the varieties of antennas that were conceived in the past 100 years. Almost all antennas that were considered for WPT applications were designed in the first place for communication applications where traditional parameters such as gain, directivity and efficiency were considered the most critical. For WPT applications, at the transmission stage, the gain, directivity and efficiency play an important role in the design. At the receiving stage, however, the primary concern is to collect as much power as possible per footprint and based on specific polarization and incident angle. In applications of classical communication antennas, the real estate or footprint of the antenna has partial relevance. For instance, a patch printed antenna occupies some space on copper but its antenna parameters assume the antenna is present in a larger empty sphere. In other words, its functional space extends beyond its size.Metamaterials are made of a three-dimensional ensemble of electrically-small resonators. Metasurfaces are considered as a two-dimensional version of metamaterials. The interesting and desired properties of metamaterials or metasurfaces are achieved when all elements of the ensemble operate at their resonance frequency (for simplicity, we assume all elements are identical). The resonance of each particle of a metamaterial or metasurface is fundamentally indicative of its ability to store energy. Metamaterials, therefore, can be effective energy collectors. This does not come as a surprise since metamaterials have been shown to be highly effective absorbers. However, in the case of absorption, the absorbed energy is mostly dissipated in the dielectric host. For the effective use of metamaterial as energy harvesters or collectors, not only the energy absorption is of high importance but also channeling the absorbed energy into energy collection channels is critical.In this paper we demonstrate that metasurfaces can indeed be effective electromagnetic energy harvesters and can provide energy harvesting efficiency appreciably higher than what classical antennas can achieve. The effectiveness of the metasurface for energy harvesting arises from the close proximity between the electrically-small resonators that constitute the metasurface. While the spacing between electrically-small resonators is critical to achieve homogenization for classical metamaterial applications, in energy harvesting as the case in our work, the spacing between elements allow for careful input impedance tuning of all elements, thus enabling highly efficient energy transduction [2].In this talk, we present metasurfaces composed of different types of resonators including split-ring resonators, electric-inductive-capacitive resonators, complementary split-ring resonators and dielectric resonators. We show that it is possible to achieve energy absorption with approximately 100% efficiency. In fact, using the concept of stacking of metasurfaces (which does not necessarily lead to metamaterials), it is possible to achieve efficiencies significantly higher than 100% as based on the efficiency definition provided in [3–4]. Simulation and experimental results will be provided to fully validate the feasibility and practicality of electromagnetic energy harvesting using metasurfaces.References:[1] Brown, W. C., “The History of Power Transmission by Radio Waves,” IEEE Transactions on Microwave Theory and Techniques, Vol. 32, No. 9, pp. 1230–1242, 1984.[2] Almoneef, T. and O. M. Ramahi, “Split-Ring Resonator Arrays for Electromagnetic Energy Harvesting,” Progress in Electromagnetic Research B, in press.[3] Ramahi, O.M., T. Almoneef and M. AlShareef, “Metamaterial Particles for Electromagnetic Energy Harvesting,” Applied Physics Letters, Vol. 101, No. 17, pp. 173903–173908, 2012.[4] Almoneef, T and O. M. Ramahi, “A 3-Dimensional Stacked Metamaterial Array for Electromagnetic Energy Harvesting,” Progress in Electromagnetic Research B, Vol. 146, pp. 109–115, 2014.