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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 Future generations of wireless systems are expected to combine the use of high-frequency bands (the terahertz range) with smart interconnected devices (the Internet of Things). To realize this ambitious merging, systems will require antennas that can be mounted on nonplanar objects while generating highly directional beams. Here, we study conformal THz leaky-wave antennas at THz frequencies. We find a rich set of behaviors accessible at THz frequencies dictated by the interplay among the geometrical parameters and the wavelength. We develop simple models to describe the relevant physics, which we verify by an experimental implementation. We also demonstrate data transmission using a conformal THz antenna that can generate multiple high-gain beams with low bit error rates for increased coverage of THz wireless links. 

Abstract Terahertz (THz) technologies have become a focus of research in recent years due to their prominent role in envisioned future communication and sensing systems. One of the key challenges facing the field is the need for tools to enable agile engineering of THz wave fronts. Here, we describe a reconfigurable metasurface based on GaN technology with an array-of-subarrays architecture. This subwavelength-spaced array, under the control of a 1-bit digital coding sequence, can switch between an enormous range of possible configurations, providing facile access to nearly arbitrary wave front control for signals near 0.34 THz. We demonstrate wide-angle beam scanning with 1° of angular precision over 70 GHz of bandwidth, as well as the generation of multi-beam and diffuse wave fronts, with a switching speed up to 100 MHz. This device, offering the ability to rapidly reconfigure a propagating wave front for beam-forming or diffusively scattered wide-angle coverage of a scene, will open new realms of possibilities in sensing, imaging, and networking. 

The coupling of terahertz optical techniques to scattering-type scanning near-field microscopy (s-SNOM) has recently emerged as a valuable new paradigm for probing the properties of semiconductors and other materials on the nanoscale. Researchers have demonstrated a family of related techniques, including terahertz nanoscopy (elastic scattering, based on linear optics), time-resolved methods, and nanoscale terahertz emission spectroscopy. However, as with nearly all examples of s-SNOM since the technique’s inception in the mid-1990s, the wavelength of the optical source coupled to the near-field tip is long, usually at energies of 2.5 eV or less. Challenges in coupling of shorter wavelengths (i.e., blue light) to the nanotip has greatly inhibited the study of nanoscale phenomena in wide bandgap materials such as Si and GaN. Here, we describe the first experimental demonstration of s-SNOM using blue light. With femtosecond pulses at 410 nm, we generate terahertz pulses directly from bulk silicon, spatially resolved with nanoscale resolution, and show that these signals provide spectroscopic information that cannot be obtained using near-infrared excitation. We develop a new theoretical framework to account for this nonlinear interaction, which enables accurate extraction of material parameters. This work establishes a new realm of possibilities for the study of technologically relevant wide-bandgap materials using s-SNOM methods.

 

Considerable recent research interest has focused on the possibility of using metasurfaces for manipulation of terahertz wavefronts. For example, metasurfaces allow a beam to be targeted in any desired direction using strategically placed meta-elements. With rapid prototyping techniques, metasurfaces can be fabricated quickly and at a low cost. These techniques also permit the fabrication of metasurfaces on flexible substrates which can be bent easily. This opens the possibility of employing such devices as conformable arrays on non-flat surfaces. To explore this idea, we experimentally and numerically analyze the performance of a terahertz metasurface printed on paper, as a function of its radius of curvature. We observe that when the metasurface is bent, the direction of the refracted beam is minimally impacted and the performance of the metasurface remains very similar to when it is flat. This conclusion will simplify the design and modeling criteria for conformable metasurfaces.