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Abstract Engineering the properties of electromagnetic wavefronts has become essential to imaging, wireless security, sensing, and wireless communication. In particular, wavefronts that exhibit low spatial coherence can enable sensing functionalities with high accuracy and low latency. The typical use of such wavefronts cannot take advantage of these possibilities, as they require the ability to dynamically reconfigure the wavefront in a controllable and repeatable fashion, over a broad spectral bandwidth. Here, we propose a new approach for generating broadband reconfigurable wavefronts which not only exhibit low spatial coherence at a particular frequency, but are also decorrelated with the wavefronts simultaneously generated at other frequencies. We demonstrate that this frequency-domain decorrelation is a key feature that, in combination with dynamic reconfigurability, enables localization measurements with an order-of-magnitude improvement in accuracy compared to the state of the art. 

Abstract A key challenge in millimeter-wave and terahertz wireless networks is blockage of the line-of-sight path between a base station and a user. User and environmental mobility can lead to blockage of highly directional beams by intervening people or objects, yielding link disruptions and poor quality of service. Here, we propose a solution to this problem which leverages the fact that, in such scenarios, users are likely to be located within the electromagnetic near field of the base station, which opens the possibility to engineer wave fronts for link maintenance. We show that curved beams, carrying data at high bit rates, can realize a link by curving around an intervening obstacle. We develop a model to analyze and experimentally evaluate the bandwidth limitations imposed by the use of self accelerating beams. We also demonstrate that such links employ the full aperture of the transmitter, even those portions which have no direct line of sight to the receiver, emphasizing that ray optics fails to capture the behavior of these near-field wave fronts. This approach, which is ideally suited for use at millimeter-wave and terahertz frequencies, opens vast new possibilities for wave front management in directional wireless networks. 

Abstract Wireless systems are facing increasing pressure due to the growing demand for data transmission. One potential solution to this problem is to shift communication frequencies toward the terahertz (THz) spectrum. However, this requires the development of new components that can efficiently process signals at these high frequencies and transmit them via highly directional beams. In this study, a novel approach is proposed to achieving efficient THz signal processing by combining two existing technologies: photonic crystals and leaky‐wave antennas. Incorporating a 2D photonic crystal inside a leaky‐wave waveguide allows to manipulate the wave vector of the guided wave in unique ways, which in turn impacts the far‐field radiation pattern emitted through the leaky‐wave aperture. The device fabrication uses 3D printing of alumina and allows for convenient and scalable manufacturing. Through numerical simulations and experiments, free‐space data transmission at rates of few hundred Mbps at a carrier frequency of 101.2 GHz is demonstrated. The findings illustrate the feasibility of photonic crystal‐based leaky‐wave antennas and lay the groundwork for the development of compact and high‐performance components for THz wireless communication systems. 

Abstract As the need for higher data rates for communication increases, the terahertz (THz) band has drawn considerable attention. This spectral region promises a much wider bandwidth and the transmission of large amounts of data at high speeds. However, there are still challenges that need to be addressed before the THz telecommunications technology hits the consumer market. One of the recurring concerns is that THz radiation is greatly absorbed by atmospheric water-vapor. Although many studies have presented the attenuation of THz signals under different atmospheric conditions, these results analyze specific temperature or humidity values, leaving the need for a more comprehensive analysis over a wider range of climate conditions. In this work, we present the first study of the attenuation of THz radiation over a broad range of temperatures and humidity values. It is worth noticing that all of our measurements have been undertaken at atmospheric pressure unlike many previous studies where the pressure was not kept constant for various temperatures. Furthermore, we extend our analysis beyond the impact of absolute humidity on the bit error rate in THz communications. We also discuss the refractivity of the atmosphere, examining its variations across different temperatures and humidity levels. THz propagation is studied using two different measurement systems, a long-path THz time-domain spectrometer as well as a quasi-optic setup with vector network analyze. We also compare the results with the ITU-R P.676-13 propagation model. We conclude that the attenuation at the absorption peaks increases linearly with water content and has no dependence on the temperature, while the refractive index, away from absorption lines, namely at 300 GHz shows a sub-linear increase with humidity. 

The development of dynamic components for controlling wave fronts in the sub-terahertz region of the electromagnetic spectrum has emerged as a frontier research topic for many applications in sensing and communications. One approach which has attracted much attention involves the use of active metasurfaces, tiled arrays of sub-wavelength elements with properties that can be reconfigured via external actuation. In nearly all cases, these metasurfaces are employed as either transmissive or reflective elements, taking advantage of their strong and tunable interaction with free-space electromagnetic waves. These interactions can be significantly enhanced through the use of surface waves propagating parallel to the metasurface array, although very few studies have exploited this option. Here, we integrate a metasurface into the interior of a parallel-plate waveguide in a configuration explicitly designed to exploit this surface-wave geometry. We show that varying the electrical properties of the active metasurface changes the wave vector of the guided mode, and thereby alters the emission angle of radiation out-coupled through a leaky-wave slot aperture. These results, which are consistent with numerical simulations, represent a new approach to broadband beam steering suitable for the sub-terahertz spectral range. 

We introduce an approach to produce inexpensive linear to circular polarizers for the sub-THz spectral regime with either left or right handedness. Our design consists of a cross-dipole array of slot antennas, fabricated on paper using a fast and inexpensive process. We demonstrate a prototype designed for a frequency of 200 GHz, showing a low axial ratio and acceptable insertion loss. 

Communication in the terahertz (THz) frequency band is a key focus for the next generation of wireless systems, in part because of the possibility for enhanced security in combination with ultra-high data rates. To enable communication links in the presence of blockage of a directional line-of-sight beam, self-accelerating THz beams have recently been proposed. For such beams, the main lobe radiated from an aperture follows some engineered curved trajectory in the electromagnetic near field. Self-accelerating beams enable advantages unlike traditional beams by curving around potential eavesdroppers located in the vicinity of the radiating aperture. Here, we theoretically and experimentally analyze the security implications of such beams by evaluating their secrecy capacity. We also explore the implications of blockage and self-healing on this security metric. 

Privacy-invading biometrics monitoring is becoming a prominent security threat as modern sensing systems move to higher operating frequencies (mmWave, sub-THz), increasing sensing resolution and accuracy. As such, developing systems that can protect or obfuscate biometrics from adversarial intrusion becomes pivotal to preserving user privacy. In this work, we develop and implement MetaHeart, a real-time biometrics misinformation system based on reflective, programmable metasurfaces and dynamic phase-front manipulation of radar inferences. MetaHeart’s key goal is to prevent the leakage of a legitimate user’s heartbeat biometrics by spoofing fake heartbeat signals at a malicious, radar-equipped, heart rate sensing intruder. We experimentally demonstrate MetaHeart’s ability to fake Alice’s presence when she is not there and to fool Trudy’s inferences even when Alice is present, achieving an overall accuracy above 98%. Finally, we conduct a robustness analysis to determine MetaHeart’s required spatial placement within the intruder’s monitoring area that would allow for effective spoofing. 

Metasurfaces enable controllable manipulation of electromagnetic waves and have been shown to be valuable for wireless communications in many diverse ways. In this paper, we explore the notion that these useful components could also provide opportunities for a malicious agent. In particular, we define and experimentally demonstrate for the first time a “MetaSurface-in-the-Middle” (MSITM) attack. In this attack, the adversary Eve places a metasurface in the path of a directive transmission between Alice and Bob and targets to re-direct a portion of the signal towards herself, without being detected. Specifically, we show how Eve can design a metasurface that induces abrupt phase changes at the interface of the metasurface to controllably diffract directional links and establish furtive eavesdropping links. We explore the theoretical foundations of the MSITMattack and demonstrate that an effective metasurface can be prototyped in under 5 min at a minimal cost. We experimentally demonstrate the attack in a THz time-domain system and perform a set of over-the-air experiments. Our results indicate that the MSITM attack yields an acute vulnerability that can significantly reduce empirical secrecy capacity while leaving a minimal energy footprint, making the attack challenging to detect. 

Due to their unique non-diffracting properties, Bessel beams have been studied across microwave to optical frequency regimes. Although several schemes for generating Bessel beams at microwave and terahertz (THz) frequencies have been demonstrated, these works have mainly focused on symmetric Bessel beams with little attention being given to asymmetric forms. In addition, existing methods are often technically involved, bulky, and costly. To address the gap in the generation of asymmetric beams and the identified challenges with contemporary generation schemes, we show that it is possible to produce both asymmetric and symmetric Bessel beams in a relatively simple and cost-effective manner using a planar Archimedean spiral zone plate. We fabricate this device using 3D printing and use it as a Bessel beam launcher operating at 0.2 THz. The zone plate generates an asymmetric Bessel beam close to the aperture through superposition of Bessel modes. As the axial distance is increased, the beam evolves into a symmetric Bessel beam.