Dr Hamidreza Taghvaee

Many advances in reflective metasurfaces have been made during the last few years, implementing efficient manipulations of wave fronts, especially for plane waves. Despite numerous solutions that have been developed throughout the years, a practical method to obtain subwavelength focusing without the generation of additional undesired scattering is a challenge to this day. In this paper, we introduce and discuss lossless reflectors for focusing incident waves into a point. The solution is based on the so-called power-flow conformal surfaces that allow theoretically arbitrary shaping of reflected waves. The metamirror shape is adapted to the power flow of the sum of the incident and reflected waves, allowing a local description of the reflector surface based on the surface impedance. In particular, we present a study of two scenarios. First, we study the scenario when the field is emitted by a point source and focused at an image point (in the considered example, with the lambda/20 resolution). Second, we analyze a metasurface capable of focusing the power of an illuminating plane wave. This work provides a feasible strategy for various applications, including detecting biological signals near the skin, sensitive power focusing for cancer therapy, and point-to-point power transfer.

Here we proposed a low profile antenna which is an electrically coupled loop fed by a coaxial cable and operates in UHF band. With acquisition of a square loop, near zone is covered by magnetic field that has no harm to living being. By optimizing the dimensions with genetic algorithm 12% of bandwidth, 2.45 dB realized gain and 33% miniaturization (lambda -> 2/3 lambda) at 500 MHz is obtained. Afterwards, the performance of the antenna is analyzed with a general circuit model and verified using frequency domain solver simulation. In the measurement phase a technique was discovered improving the bandwidth even more, hence 150 MHz bandwidth equal 30% at 500 MHz was achieved. These characteristics together make this antenna very beneficial in mobile terminals that have a very wide area of applications such as ground station targeting UAVs and where antennas need to operate close to the human body. Due to semidirectional radiation pattern it can be used in short and medium range remote wireless bridge networks. (C) 2016 Wiley Periodicals, Inc.

Transmission spectra of microribbon graphene arrays are investigated with a circuit model based on the transmission line method. The accuracy of the proposed method is comparable with full-wave electromagnetic simulation results versus chemical potential, incident angle, dimensions of microribbons and the permittivity of the substrate. This rigorous method takes less than a second to perform, therefore it can be employed to optimize other similar devices instead of numerical methods that involve heavy calculations. Furthermore, we depicted the Kerr effect with the harmonic balance method through calculating the parameters in steady state. The results of this novel approach exhibited an excellent agreement with full-wave simulation results.

The reconfigurable intelligent surface is a promising technology for the manipulation and control of wireless electromagnetic signals. In particular, it has the potential to provide significant performance improvements for wireless networks. However, to do so, a proper reconfiguration of the reflection coefficients of unit cells is required, which often leads to complex and expensive devices. To amortize the cost, one may share the system resources among multiple transmitters and receivers. In this paper, we propose an efficient reconfiguration technique providing control over multiple beams independently. Compared to time-consuming optimization techniques, the proposed strategy utilizes an analytical method to configure the surface for multi-beam radiation. This method is easy to implement, effective and efficient since it only requires phase reconfiguration. We analyze the performance for indoor and outdoor scenarios, given the broadcast mode of operation. The aforesaid scenarios encompass some of the most challenging scenarios that wireless networks encounter. We show that our proposed technique provisions sufficient improvements in the observed channel capacity when the receivers are close to the surface in the indoor office environment scenario. Further, we report a considerable increase in the system throughput given the outdoor environment.

Breaking the so-called diffraction limit on the resolution of optical devices and achieving subwavelength focusing requires tailoring the evanescent spectrum of wave fields. There are several possible approaches, all of which have limitations, such as the generation of strong additional scattering, limited focusing power, issues at the implementation step, and the need for a drain at the focal point. This paper presents a feasible strategy based on the concepts of the perfect lens and power flow-conformal metasurfaces. Desired fields for subwavelength focusing are integrated using double-negative media and then the surface profile of a focusing reflector is designed to be tangential to the desired power flow, so that the metasurface can be modeled as a local impedance boundary, and can be easily implemented using passive and lossless elements. Full-wave simulations demonstrate that an example reactive metasurface is able to break the diffraction limit and provide near-field focusing with subwavelength hotspot size. We expect that the outcome will find applications in antennas, beam-shaping devices, nonradiative wireless power transfer systems, microscopy, and lithography.

The next generation of wireless networks is expected to tap into the terahertz (THz) band (0.1–10 THz) to satisfy the extreme latency and bandwidth density requirements of future applications. However, the development of systems in this band is challenging as THz waves confront severe spreading and penetration losses, as well as molecular absorption, which leads to strong line-of-sight requirements through highly directive antennas. Recently, reconfigurable intelligent surfaces (RISs) have been proposed to address issues derived from non-line-of-sight (non-LoS) propagation, among other impairments, by redirecting the incident wave toward the receiver and implementing virtual-line-of-sight communications. However, the benefits provided by a RIS may be lost if the network operates at multiple bands. In this article, the suitability of the RIS paradigm in indoor THz scenarios for 6G is assessed grounded on the analysis of a tunable graphene-based RIS that can operate in multiple wideband transparency windows. A possible implementation of such a RIS is provided and numerically evaluated at 0.65/0.85/1.05 THz separately, demonstrating that beam steering and other relevant functionalities are realizable with excellent performance. Finally, the challenges associated with the design and fabrication of multiwideband graphene-based RISs are discussed, paving the way to the concurrent control of multiple THz bands in the context of 6G networks.

Reconfigurable Intelligent Surfaces (RIS) are well established as a promising solution to the blockage problem in millimeter-wave (mm-wave) and terahertz (THz) communications, envisioned to serve demanding networking applications, such as 6G and vehicular. HyperSurfaces (HSF) is a revolutionary enabling technology for RIS, complementing Software Defined Metasurfaces (SDM) with an embedded network of controllers to enhance intelligence and autonomous operation in wireless networks. In this work, we consider feedback-based autonomous reconfiguration of the HSF controller states to establish a reliable communication channel between a transmitter and a receiver via programmable reflection on the HSF when Line-of-sight (LoS) between them is absent. The problem is to regulate the angle of reflection on the metasurface such that the power at the receiver is maximized. Extremum Seeking Control (ESC) is employed with the control signals generated mapped into appropriate metasurface coding signals which are communicated to the controllers via the embedded controller network (CN). This information dissemination process incurs delays which can compromise the stability of the feedback system and are thus accounted for in the performance evaluation. Extensive simulation results demonstrate the effectiveness of the proposed method to maximize the power at the receiver within a reasonable time even when the latter is mobile. The spatiotemporal nature of the traffic for different sampling periods is also characterized.