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In this paper, the design of a new flexible ultra-thin curvature metasurface energy harvester is presented. The use of cylindrical metasurface electromagnetic (EM) harvester would be desirable for the ambient EM energy harvesting since it can absorb the EM energy with maximum efficiency. The harvester is made by an 11×11 unit-cell metasurface with a flexible substrate to demonstrate the 2D-isotropic harvesting; as a small slice of the cylinder. We have proposed a sub-wavelength (∼0.13λ0) complementary quad split ring resonator (CQSRR) unit-cell which is loaded with a lump resistor mounted on the metal-backed substrate. The full-wave simulation shows that the efficiency of the flat metasurface energy harvester with thickness of 0.004λ0 at 5.33 GHz (WiFi) is up to 0.86 for normal radiation. It is 0.72 and 0.62 for 70° oblique angle of incidence from H- and E-plane. In addition to this, the experimental results show an agreement with the results of full-wave simulations. Due to the non-uniform mutual coupling between the cells in the finite array of the fabricated energy harvester and efficiency definition for the central cell as a metric of evaluating the device performance, an effective area for the central cell has been obtained experimentally which is 4.3 times greater than the physical area of a single unit-cell in an infinite array.

The current study aims to present the physical perception of a meta-surface energy harvester’s (MEH’s) design based on space-time physics of a traveling wave. Regarding the relation between the wave-velocity and field-impedance, the balance condition in Huygens’s meta-atoms is provided. Accordingly, it was demonstrated that MEH behaves as a transformer at far-field. It was observed that the location of the metallic-via is mimicked by the number of loop coils in the secondary of the transformer in the unit-cell. In addition, the impedance matching between the wave impedance in a lossless medium and MEH’s load was to be tuned by adjusting the size parameters of the unit-cell at a desired resonance frequency. For this purpose, the present study developed a simple design framework to achieve the resonance frequency at a more optimum pace based on surrogate modeling. The theoretical analyses are validated by the results of full-wave and circuit simulations. Finally, a recently developed flexible MEH was further extended to a multi-polarization structure using more compact cells. The fabricated flexible MEH has 10 × 10 number of deep subwavelength thick cells (≈0.004λ0), while traditional MEH was basically designed only to fit on the planar surface. The new design paves the way for the multi-polarized MEH to wrap around the cylindrical surface as a 2D-isotropic MEH. The results of the data analyses show that the simulation and experimental results enjoy an acceptable agreement.

An ultra-thin double-functional metasurface patch antenna (MPA) was proposed, where it can operate not only in the antenna mode but also can simultaneously act as perfect absorber for normal incident waves, suitable for RFID applications in the 868 MHz band. The MPA structure consists of a typical coaxially-fed patch antenna merged, for the first time, with a metasurface absorber acting as artificial ground. A methodology for the unit-cell design of the metasurface is proposed followed by an equivalent circuit model analysis, which makes it possible to transform a low-loss ($$tan\delta =0.0015$$ t a n δ = 0.0015 ) unit-cell with highly-reflective characteristics to a perfect absorber for normal incident waves. It is based on modifying the critical external coupling by properly introducing slits on the unit-cell, allowing to design an ultra-thin ($$\lambda _0/225$$ λ 0 / 225 at 868 MHz) and a very compact structure in comparison to previously developed designs. For validation purposes, the MPA was fabricated and its performances in both functional modes were characterized numerically and experimentally. It is demonstrated that merging the absorber with the patch not only allows obtaining a well-matched ($$|S_{11}|<-30$$ | S 11 | < - 30  dB) antenna with an enhanced gain (by 175.6% compared to a typical patch) at the desired frequency but also leads to an overall thickness of only 2.5 mm ($$\lambda _0/138.1$$ λ 0 / 138.1 at 868 MHz). With an absorber size limited to the MPA dimensions, a reasonable 1.3 dB reduction in powers reflected by the MPA was achieved compared to a similar size metallic sheet. Whilst having the lowest profile among the so far reported RFID readers, the proposed MPA can be conveniently fitted for example within the required volume of smart shelf RFID readers or used in portable RFID readers while being capable of mitigating multipath reflection issues and incorrect reading of RFID.

In this paper, we design and simulate a new dual-band ultrathin metasurface energy harvester (EH) in X- and V-bands. One of the significant advantages of the proposed EH is the efficiency, which is more than 70% over 170° of the TM-polarized oblique incident angles at f = 10 GHz. The full-wave simulation results show that the maximum harvesting efficiencies of 93.4% (in 10.1 GHz) and 84.3% (in 42.86 GHz) are obtained by the TM 75°-polarized and the normal incident waves, respectively. Besides, half-power bandwidths (ratios) are about 1.12 GHz (11.08%) and 2.36 GHz (5.5%), respectively. The designed unit-cell is miniaturized using a C-shaped slot with the dimensions of 0.1λ0 × 0.1λ0 × 0.008λ0 at the lowest resonance frequency (λ0 = 30.12 mm). To analyze the physical mechanism of energy harvesting, the full-wave simulation results of the current and electric field distributions at two resonance frequencies are presented. We demonstrate how the rotation of the load around the metallic via can affect the resonance frequency and the other characteristics of the EH at the X-band. We validate the load’s rotation effects using the waveguide measurement method as well. Finally, we show that the location of the metallic via has a greater impact compared to the load’s rotation using the full-wave numerical simulation at the X-band.