The motion of a rigid body interacting with a wave leaves spectral signatures, with the Doppler shift as the dominant contribution. Since any motion can be decomposed into translational and rotational components, rotational Doppler provides additional information about the object's dynamics. In the electromagnetic domain, rotating objects generally produce rotational Doppler, or micro-Doppler, signals determined by the symmetry and spatial structure of the scattering process. For objects that are subwavelength or comparable in size to the wavelength, the response is typically dominated by the lowest dipolar scattering channel, so the leading spectral component commonly appears at twice the angular frequency. Here, we introduce the concept of artificially engineered rotational micro-Doppler by designing a compact, strongly scattering structure that operates through a high-order multipolar cascade of resonances, thereby producing a giant enhancement. Magneto-electric arrays composed of strongly coupled electric and magnetic resonators are optimized in the GHz range using a covariance matrix adaptation genetic algorithm to maximize the micro-Doppler frequency. Unlike conventional higher-order multipole designs used in superscatterers for a specific angle of incidence and polarization, our approach jointly optimizes excitation and scattering under radar-relevant conditions for a rotating blade. The resulting arrays exhibit a giant rotational meta-micro-Doppler response, exceeding the dipolar limit by two orders of magnitude and mapping rotations of tens of hertz into the kilohertz range. Beyond its fundamental significance, this mapping has practical value because it shifts rotor micro-Doppler signatures well above slow-moving radar clutter, thereby improving the detectability of slow motion.

Laws of electrodynamics constrain scattering cross-sections of resonant objects. Nevertheless, a fundamental bound that expresses how larger that scattering cross-section can be is yet to be found. Approaches based on cascading multiple resonances permitted to push the scattering responses of subwavelength structures and to exceed existing estimators, for which the Chu-Harrington criterion is, potentially, the most commonly considered one. The superradiant empirical limit, addressing scattering performances of near-field coupled resonator arrays, was subsequently developed to tighten existing estimates, setting a new bound that prompted efforts to find structures that exceed it. Here, we demonstrate that genetically designed superscattering structures, encompassing arrays of constructively interfering electric and magnetic dipoles, can build enormously high scatting cross-sections exceeding those imposed by existing criteria in electromagnetic theory including the superradiant empirical limit. After undergoing thousands of evolutionary generations, iterating sizes, mutual orientations, and locations of resonators, the structures approach their heuristically maximized performance, which is unlikely to be obtained by a random distribution given more than a billion trials. As an additional practically valuable parameter, the scattering bandwidth also underwent optimization. We demonstrate that flat wavelength-comparable structures can have significant backscattering alongside more than 40% fractional bandwidth. The result demonstrates the fundamental capability to untighten scattering cross-section from bandwidth limitations. New capabilities of genetic optimization algorithms, equipped with fast computational tools and constrained by experimentally obtainable electromagnetic parameters, allow chasing well-accepted traditional bounds, demonstrating ever-seen electromagnetic performances.

Radiofrequency identification (RFID) empowers numerous modern applications, enabling efficient accurate tracking and management of products, assets, and individuals in retail, logistics, and access control. Considering those and other perspective applications, there is a need to reduce the overall footprint of a passive tag while maintaining its reading range on at least a meter scale. Employing dielectric resonant antennas as a core of RFID tag design presents an appealing strategy, as it facilitates size minimization while counterbalancing this reduction by increasing the refractive index. Here we investigate the fundamental and practical constraints of this type of miniaturization, primarily focusing on the bandwidth limitations and temperature stabilities associated with high-index ceramic elements. Specifically, tags with relative dielectric permittivities ranging from 100 to 1250 were explored, demonstrating that a permittivity of 500 is optimal for footprint miniaturization and temperature monitoring, with a sensitivity of 3 MHz/°C. In contrast, a permittivity of 100 is ideal for thermostable tags. Further size reduction using permittivities above 500 decreases the communication channel bandwidth below the thresholds required by standard UHF RFID protocols, which thus serves as a practical limitation for further footprint reduction. RFID tags with cubic millimeter-scale footprints, capable of being accessed from distances well over a meter, and ideally integrated with sensor functionalities, have the potential to revolutionize the Internet of Small Things where compact and resource-limited items can become active participants in a global network.

The rapid growth in drone air traffic calls for enhanced radar surveillance systems to ensure reliable detection in challenging conditions. Increasing radar scattering cross-section can greatly improve detection reliability in civilian applications. Here, we introduce a concept of evolutionarily designed metamaterials in the form of multilayer stacks of arrays, featuring strongly coupled electric and magnetic resonators. These structures demonstrate a broadband end-fire scattering cross-section exceeding 1 m² at 10 GHz and, despite their compact footprint, achieve over 10

Near-field communication is considered to have a high level of hardware security protection owing to its natural short-range wireless operation, which makes a man-in-the-middle attack impossible. Here we question this statement by demonstrating a several-meter range NFC communication channel, supported by resonance-tuned large-area distributed coils. Typical NFC antenna architectures encompass multi-turn wires, forming flat resonant coils. Being several centimeters across, those devices cannot provide reliable communication between items, situated more than a fraction of a meter apart. An appealing approach to the range extension is to enlarge the coil area, thus spreading the magnetic field over larger distances. However, in this case, the overall length of folded conducting wires becomes wavelength comparable, nevertheless, the overall size of the coil remains electrically small, considering the 13.56MHz operation frequency. Here we demonstrate several coil designs and establish a reliable NFC channel over several-meter distances. Adaptive impedance matching is implemented to maintain an energy power transfer between resonant coils, thus further extending the communication channel and making it robust to clutter. The ability for long-range NFC communication raises security concerns in sensitive contactless operations like wireless payments, prompting the need for enhanced countermeasures due to potential hardware vulnerabilities.

Radio frequency identification is a wireless technology that allows contactless readout of data from a passive device through time-modulated backscattering. High epsilon ceramic resonators, capable of shrinking interaction volumes by orders of magnitude, open new possibilities for developing long-range compact tags. Here we demonstrate that this technology also allows for accurate tracking after environmental temperature changes without an additional sensing circuit. Several material platforms, demonstrating significant permittivity temperature dependence, were explored and demonstrated less than a 1 ○ C sensitivity. Specifically, the passive tag, based on BaTiO3 with ε=500 and interrogated from a 4-meter distance, was demonstrated. Equipping high-index ceramic RFID tags with additional capabilities apart from their long reading ranges further promotes this technology in application to the Internet of Small Things, where small low-cost low-resource entities are foreseen to perform multiple functions.

With the rapid development of automated systems, it became necessary to use wireless communication systems to monitor a variety of different parameters. Wireless sensors are actively used in many modern intelligent systems to track and control various characteristics of the system and its environment. The task of developing compact sensors with high sensitivity and measurement accuracy is extremely urgent in this field. In this paper, two microwave sensor designs based on dielectric resonators, whose parameters are optimized for quasi-BIC mode maintenance, have been proposed. Furthermore, the application of these sensors extends to the realm of temperature tracking, water temperature control, and impurity detection, showcasing their potential in manufacturing processes, wireless power transfer, and various other industrial applications.

NFC and RFID technologies have seen significant advancements, with expanding applications necessitating the design of novel antenna structures that enhance range capabilities. This paper presents a study on the design and impedance matching of long-range NFC coils, focusing on optimizing antenna performance over distances exceeding one meter. Through numerical analyses of various coil geometries, including single-turn and multi-wire configurations, we explore the effects of coil size, wire separation, and current distribution on magnetic field generation. Additionally, an adaptive impedance matching approach is proposed to maintain efficient power transfer, significantly improving field strength and system performance. The proposed designs demonstrate superior interrogation distances compared to existing configurations, highlighting the potential for enhanced long-range NFC applications.

The rapidly growing volume of drone air traffic demands improved radar surveillance systems and increased detection reliability in challenging conditions. The scattering cross-section, which characterizes a target's radar visibility, is a key element in detection schemes and thus becomes a primary objective in civilian applications. Here, we introduce a concept of genetically designed metamaterials, specifically engineered to enhance scattering for end-fire incidence scenarios. Multi-layer stacks of arrays, encompassing strongly coupled electric and magnetic resonators, demonstrated above 1 m^2 broadband scattering at 10 GHz, despite having an end-fire physical cross-section smaller than one squared wavelength. Those performances, crucial for effective civil radar air traffic monitoring, facilitate exploring highly scattering structures as labels for small airborne targets. This objective has been demonstrated with a set of outdoor experiments with the DJI Mini 2 drone. Lightweight, conformal add-ons with significantly high scattering cross-sections can serve as auxiliary tools to empower passive monitoring systems, thereby providing an additional layer of security in urban airspace.

We study feasible physical mechanisms for tuning the electromagnetic topological states in one-dimensional arrays of coupled dielectric resonators. We demonstrate two approaches for varying the topological properties: (i) mechanical, based on the mutual orientation of meta-atoms with a broken mirror symmetry, and (ii) thermal, based on temperature variation.

Creeping waves traveling around a volumetric electromagnetic scatterer provide a significant contribution to its radar cross-section. While quite a few efforts were devoted to suppressing creeping waves as a part of radar countermeasures, here we utilize specially engineered creeping waves to our advantage to create broadband, all-angle, and polarization scatterers. Metalized spherical surfaces, patterned with corona virus-like spikes are designed to provide a broadband constructive interference between the specular reflection and creeping waves, elevating the scattering cross-section. The demonstrated miniature corona scatterers utilize both resonant cascading phenomena and traveling wave interference to tailor electromagnetic interactions, outperforming a resonant dipole in terms of amplitude and bandwidth quite significantly. Our experimental samples are fabricated with an additive manufacturing technique, where a 3D-printed plastic skeleton is subsequently metalized. Micron-thick layers allow governing electromagnetic interactions as if the entire object was made of solid metal. Lightweight, all-angle, all-polarization, and broadband compact scatterers such as these, reported here, have numerous applications, including radar deception, electromagnetic beckoning, and many others.

We introduce a detunable wire metasurface that can electromagnetically couple to an external transmit coil to increase local magnetic resonance imaging (MRI) sensitivity while being almost transparent during excitation. The structure is implemented as an array of wires printed on a dielectric substrate inserted into high-permittivity dielectric blocks at both ends. Detuning is achieved by changing the effective electrical dimensions of the metasurface by inserting switchable nonmagnetic crossed diodes into each wire. We experimentally demonstrate that the metasurface is effectively passively switched off during the transmission by a high current flowing within the wires. During the reception, the currents are much lower; hence the metasurface remains switched on, increasing the local signal-to-noise ratio (SNR). Detuning the metasurface guarantees the patient’s safety and allows standard clinical calibration techniques. This nonlinear mechanism expands the functionalities of metasurfaces and is an essential step toward direct integration in MRI systems.

Implementation of tunable and switchable topological systems is the next step towards realistic applications of topological photonics. Here, we study topological zigzag arrays of dielectric resonators and demonstrate a novel method to control their topological edge states by applying local heating. Numerical and experimental studies confirm that the properties of the topological edge states in such systems can be tuned and controlled by temperature.

Radio-frequency identification (RFID) is a widely used technology for wireless data transfer between tags and readers. Passive ultrahigh-frequency (uhf) RFID architecture is a compromise between cost and performance in numerous retail applications, in which multiple goods must be labeled and simultaneously interrogated from a distance. Furthermore, for robust operation, passive tags must be visible from any direction and for any polarization to compensate for their accidental misalignments with respect to the reader's antenna. Obtaining long-range omnidirectional operation with miniaturized tags remains a challenge, which limits the scope of emerging applications, including the Internet of small things. Here we develop the concept of resonance cascading and demonstrate a new architecture based on a high-index ceramic resonator. Taking advantage of frequency hopping between communication channels, we design several mutually orthogonal spectrally separated dipolar resonances to enable omnidirectional operation inside an RFID frequency band, instead of using traditional single-band quasi-isotropic antennas. As a result, we experimentally demonstrate a compact 28.5 \ifmmode\times\else\texttimes\fi{} 27.5 \ifmmode\times\else\texttimes\fi{} 27 mm$^{3}$ device, which can be omnidirectionally interrogated from a distance of over 10 m, which is further than has been previously achieved in the field of long-range omnidirectional uhf RFID tags. The concept of resonance cascading and spectral sharing can be further employed in a variety of wireless communication applications.

The emerging need for green technologies motivates the development of new approaches to manufacture electronic consumables. In case of low-cost mass-production sensors, the problem becomes even more severe due to the generation of environmental waste. Here we demonstrate an RFID-type sensor based on a caramel substrate with a micron-scale conductive layer. The device, being primarily made of sugar, attracts insects, which consume it almost completely. As an application, we demonstrate a tag that can be applied for remote pest monitoring. In the experiment, a long-range UHF RFID communication channel is established and monitored over time. An RFID-on-caramel tag consumed by insects loses its connection with a reader, indicating the presence of pests. We show the new caramel-based devices to communicate with a reader over a 10-meter distance, paving the way to remote crop monitoring. Such low-cost biodegradable sensors are highly promising for smart agriculture, warehouse management, and stock monitoring approaches.

Capabilities to monitor the purity and mixture composition of liquids with the aid of low-cost portable devices can grant essential advantages in maintaining personal health safety. The overwhelming majority of consumer wireless devices operate at relatively small operational bandwidth, thus not allowing for retrieving material composition via dispersion characteristics. To mitigate the bandwidth limitations, resonant methods, granting precision in a small frequency window, might be of use. Here, we demonstrate a liquid sensor able to provide 90.5 kHz/RIU sensitivities owing to a resonator, supporting high-quality factor quasi-bound states in the continuum. The sensor's architecture encompasses a high-permittivity ceramic resonator and a capillary wrapped around it. The volumetric design increases the overlap between the electromagnetic mode and the liquid under test while maintaining resonant conditions within a relatively narrow frequency band. To demonstrate the capabilities of the proposed method, the UHF RFID band was considered, and temperature dependence of the distilled water permittivity was retrieved. Interfacing standalone low-cost electromagnetic sensors with widely available consumer-level wireless devices offers promising opportunities that contribute to the paradigm shift toward IoT.

The ability to obtain dynamic control over an antenna radiation pattern is one of the main functions, desired in a vast range of applications, including wireless communications, radars, and many others. Widely used approaches include mechanical scanning with antenna apertures and phase switching in arrays. Both of those realizations have severe limitations, related to scanning speeds and implementation costs. Here we demonstrate a solution, where the antenna pattern is switched with optical signals. The system encompasses an active element, surrounded by a set of cylindrically arranged passive dipolar directors, functionalized with tunable impedances. The control circuit is realized as a bipolar transistor, driven by a photodiode. Light illumination in this case serves as a trigger, capable of either closing or opening the transistor, switching the impedance between two values. Following this approach, a compact half-a-wavelength footprint antenna, capable of switching between 6 dBi directional patterns within a few milliseconds' latency was demonstrated. The developed light activation approach allows constructing devices with multiple almost non-interacting degrees of freedom, as a branched feeding network is not required. The capability of flexible switching between multiple electromagnetic degrees of freedom opens pathways to new wireless applications, where fast beam steering and beamforming performances are required.

Electromagnetic scattering bounds on subwavelength structures play an important role in estimating performances of antennas, RFID tags, and other wireless communication devices. An appealing approach to increase a scattering cross-section is accommodating several spectrally overlapping resonances within a structure. However, numerous fundamental and practical restrictions have been found and led to the formulation of Chu-Harrington, Geyi, and other limits, which provide an upper bound to scattering efficiencies. Here we introduce a 2D array of near-field coupled split-ring resonators and optimize its scattering performances with the aid of a genetic algorithm, operating in 19th-dimensional space. Experimental realization of the device is demonstrated to surpass the theoretical single-channel limit by a factor of >2, motivating the development of tighter bounds of scattering performances. A super-radiant criterion is suggested to compare maximal scattering cross-sections versus the single-channel dipolar limit multiplied by the number of elements within the array. This new empirical criterion, which aims on addressing performances of subwavelength arrays formed by near-field coupled elements, was found to be rather accurate in application to the superscatterer, reported here. Furthermore, the super-radiant bound was empirically verified with a Monte-Carlo simulation, collecting statistics on scattering cross sections of a large set of randomly distributed dipoles. The demonstrated flat superscatterer can find use as a passive electromagnetic beacon, making miniature airborne and terrestrial targets to be radar visible.

Radio frequency identification (RFID) is a widely used approach for a short-range contactless data exchange, i.e., employed in billing systems. During the last years, unauthorized access has become an issue, as it has been proven on numerous unpleasant occasions. A typical theft scheme is based on approaching a victim with a card reader. However, a straightforward adaptation of the reader's antenna elements allows performing the attack from a distance, opening a severe security loophole. Here, we propose and demonstrate hardware-based protection capable of preventing a far-field attack of this kind. Our solution is based on an RFID chip shielding with an opaque metal. The Faraday type of the enclosure has a small aperture, which suppresses electromagnetic field leakage from the device. This property affects both up and down interrogation links, virtually making the far-field attack impossible. Activation of the card is done by holding it in hand-this way, it is made accessible to an authorized readout, which still remains wireless. The physical principle of the operation is placing a high-index dielectric structure next to a subwavelength aperture, making it electromagnetically larger and, as a result, supporting the field leakage. Our experimental prototype, validated by several different users, shows the capability to diminish far-field attacks on ultrahigh frequency (UHF) RFID tags. This hardware security solution can find usage in numerous applications, such as biometric passports, credit cards, and many others, where unauthorized access to sensitive data is highly undesirable.

Radio frequency identification (RFID) allows to perform a wireless communication with special tags, attached to the objects. Being sensitive to a surrounding environment, RFID tags are usually designed for specific applications. Here, we propose an RFID tag, accommodating three essential functions simultaneously, namely small footprint, long reading range, and a capability of on-metal labeling. Tag’s design is based on a compact dielectric resonator and an inductively coupled metal ring functionalized with an RFID chip. We demonstrate experimentally a $16.5 \times 16.5 \times 12$ footprint tag, which can be successfully interrogated from 22 m while being placed on a $40 \times 40$ metal sheet. Multifunctional miniature long-range ceramic tags can be extremely useful in numerous practical applications, including the Internet of Things and many others.