Sergei A. Tretyakov’s research while affiliated with Aalto University and other places

This paper discusses the applicability conditions of the Physical Optics (PO) approximation and investigates its predictive power for far-field diffraction patterns of Reconfigurable Intelligent Surfaces (RISs) based on binary metasurfaces (BMSs). We compare the PO model with the approximation of so-called Reflection Locality (RL). Although these two approximations are conceptually different, in the case of BMSs, RL turns out to be a prerequisite of the applicability of the PO. If RL holds for such RISs, PO adequately predicts the diffraction pattern only in the case of large periods, which restricts the applicability of the PO to rather small deflection angles. Recently, it was shown that the RL approximation and the popular approximation of so-called angular stability for periodically non-uniform reflecting metasurfaces are equivalent. Therefore, we conclude that BMSs with angular stability can be successfully designed using the PO approximation if the required deflection angles are restricted. In the absence of angular stability, the accuracy of PO is very poor. This finding highlights the importance of considering both the angular stability and the electromagnetic dimensions in RISs designs.

In this work, we consider instantaneous transitions of an infinitely extended uniaxial dielectric into a wire medium (WM) of continuous infinitely long conducting wires. Due to the strong spatial dispersion in the WM the known (Morgenthaler's) theory of temporal discontinuities is not applicable. We solve this problem analytically in time domain. We show that a transverse electromagnetic (TM) plane wave transforms into four waves: a pair of TM waves and a pair of transverse electromagnetic waves. This way, the power flow splits into two different directions, with one of them along the wires. Such a transition can possibly be achieved by nanoplasma discharges in the gaps of the split wires, initiated by an external voltage source applied to the wire and transforming the split wires forming the uniaxial dielectric into continuous ones.

In this work, we theoretically study temporal interfaces between media with strong spatial dispersion and dielectrics. In particular, we consider a temporal discontinuity that transforms a wire medium sample, a metamaterial with resonant spatial dispersion, into a uniaxial dielectric. We show that this transition results in a transformation of the deeply subwavelength spatial spectrum of TEM waves propagating in the wire medium at a certain frequency into a spectrum of plane waves at new frequencies that are all higher than the initial one. The waves at different frequencies propagate in different directions. Their complex amplitudes and propagation directions are uniquely related to the amplitudes of the spatial harmonics of the fields which existed before the transition. We explain how to implement this transition. The revealed effect may result in a promising method of subwavelength imaging.

In this article, we propose a new coil-energizing method for multitransmitter (Tx) wireless power transfer (WPT) systems that enable complete freedom of receiver (Rx) positioning in a large area. The principle is based on noncoherent power combining, where multiple Tx coils are supplied in a predefined pattern by mixing two slightly different frequencies and four different phase angles. The powers delivered from Tx coils at two frequencies are then combined incoherently at the Rx side, resulting in full positional and rotational freedom for the Rx charging. The power transfer blind spots in conventional WPT systems are completely eliminated with the proposed noncoherent power combining method due to its natural robustness against cancelations. The experimental setup shows constant dc–dc power transfer efficiency around 90% for arbitrary Rx movements without any further requirements on dynamic controls of Txs or Rxs other than the simplest activation/deactivation.

Developing intelligent radio environments with the ability to engineer and optimize propagation channels has been made possible by metasurfaces that perform anomalous reflection and control the direction of wave reflection at will. Several design methods for creation of anomalous reflectors have been developed, from the simplest locally periodic designs of individual unit cells to advanced global optimizations. In this study we design, fabricate, and experimentally characterize anomalous reflectors that have the same size and perform the same function but are designed using two different, most commonly used methods. Experimental characterization of the reflector performance over a broad frequency range and for a variety of illumination angles allows comparison of the two methods and helps to select the most suitable design approach for specific application requirements.

Roadmap on Electromagnetic Metamaterials and Metasurfaces

In this study, we elaborate on the concept of scalable anomalous reflector (AR) to analyze the angular response, frequency response, and spatial scalability of a designed AR across a broad range of angles and frequencies. We utilize theoretical models and ray tracing simulations to investigate the communication performance of two different-sized scalable finite ARs, one smaller configuration with 48 x 48 array of unit cells and the other constructed by combining four smaller ARs to form a larger array with 96 x 96 unit cells. To validate the developed theoretical approach, we conducted measurements in an auditorium to evaluate the received power through an AR link at different angles and frequencies. In addition, models of scalable deflectors are implemented in the MATLAB ray tracer to simulate the measurement scenario. The results from theoretical calculations and ray tracing simulations achieve good agreement with measurement results.