We demonstrate the effectiveness of frequency selective surface filters in wireless communications at low terahertz (THz) frequencies. Full-wave simulations of pass-band filters designed at 270 GHz and 330 GHz are compared with measurements over 220–360 GHz, showing remarkable agreement. The filter spectral response is used to analytically model a THz filter-based wireless channel for modulated signals. In particular, numerical results and measurements for an OOK modulated signal are in good agreement for both free-space and filtered transmission at 14 Gb/s. In both cases, bit error rates (BER) as low as 10-10 are measured. This result demonstrates that the filters marginally affect the BER with respect to free-space, interference-free transmission, whereas interfering signals are strongly rejected. This result is demonstrated through a systematic evaluation of the BER in presence of an interfering signal with different carriers and amplitudes. Results confirm a strong filter rejection to interference carriers close to the filter central frequency. Conversely, without the filters the BER performance is fully compromised. Finally, we demonstrate numerically and experimentally that the constellation diagram for 104 Gb/s QAM-16 communication is not significantly affected by the filter. The investigated filters may provide a robust approach towards efficient spectrum management for future 6G wireless applications.

Metasurfaces with a spatially varying phase profile enable the design of planar and compact devices for manipulating the radiation pattern of electromagnetic fields. Aiming to achieve tunable beam steering at terahertz frequencies, we numerically investigate metasurfaces consisting of one dimensional arrays of metal-insulator-metal (MIM) cavities infiltrated with liquid crystals (LCs). The spatial phase profile is defined by a periodic voltage pattern applied on properly selected supercells of the MIM-cavity array. By means of the electro-optic effect, the voltage controls the orientation of LC molecules and, thus, the resulting effective LC refractive index. Using this approach, the spatial phase profiles can be dynamically switched among a flat, binary, and gradient profile, where the corresponding metasurfaces function as mirrors, beam splitters or blazed gratings, respectively. Tunable beam steering is achieved by changing the diffraction angle of the first diffraction order, through the reconfiguration of the metasurface period via the proper adjustment of the applied voltage pattern. ? 1995-2012 IEEE.

We demonstrate graphene on flexible, low-loss, cyclo-olefin polymer films as transparent electrodes for terahertz electro-optic devices and applications. Graphene was grown by chemical vapor deposition and transferred to cyclo-olefin polymer substrates by the thermal release tape method as layers on an approximate area of 4 cm(2). The structural and electromagnetic properties of the graphene samples as well as their spatial variation were systematically mapped by means of mu Raman, terahertz time-domain and mid-infrared spectroscopy. Thanks to the small thickness and very low intrinsic absorption of the employed substrates, both high transmittance and conductivity were recorded, demonstrating the suitability of the technique for the fabrication of a new class of transparent and flexible electrodes working in the terahertz spectrum.

In this work, a novel wavelength selective switch is proposed and demonstrated. This new configuration avoid the use of grating and lenses, as a result, a very compact and lightweight ultra-selective wavelength switch can be obtained. For this, a novel tunable ultra-selective mirror is used. The tunability is achieved by using LC, the ultra-high Q-factor is obtained by means of dielectric metasurfaces based on hollow core nanocuboids. The metasurfaces are placed in the optical path with a specific angle. Each mirror can select any specific wavelength by applying an AC voltage. For 10°, a 44 nm of tunable range is possible. Absorption is negligible for low applied voltages. The bandwidth of the reflection resonance at 10° , ranges from 0.02 nm to 0.035 nm, corresponding to a 2.5 GHz and 4.5 GHz resolution, respectively. This is far beyond the required bandwidth for future Ultra-DWDM systems.

The complex mode spectra of a grounded anisotropic dielectric slab (GADS) excited by a horizontal magnetic dipole are presented in the low THz range. A liquid crystal mixture with high birefringence, low dispersive behavior and moderate losses at THz frequencies is selected as anisotropic layer. The dispersion curves of the complex modes (in both the surface-wave and leaky-wave regime) supported by the GADS are compared to those supported by the equivalent isotropic grounded dielectric slab GDS with the ordinary and the extraordinary components of the liquid crystal permittivity tensor. The modal analysis of the GADS is then performed for different values of the bias voltage to show the potential dynamic control of the phase and attenuation constant. The proposed study constitutes an approximate yet solid background for the design of THz leaky-wave antennas and suggests the feasibility of THz planar antennas with a tunable pointing angle.

In this work, a design flow for leaky-wave antennas based on metasurfaces is proposed. In particular, the possibility to extract the surface impedance of partially reflecting surfaces (PRS) for which homogenization formulas are not yet available in the literature has been exploited by means of numerical tools. The knowledge of the surface impedance in a PRS-based Fabry-Perot cavity leaky-wave antenna (FPC-LWA) allows for accurately evaluating the radiating performance by means of simple analytical formulas. A fishnet-like metasurface is designed to validate the proposed approach and its implementation as a partially reflective sheet for a highly-directive FPC-LWA working at terahertz frequencies is discussed. ? 2019 European Association on Antennas and Propagation.

We discuss arrays of metal-semiconductor-metal cavities as electrically tunable terahertz metasurfaces. The operation of the considered device is based on reverse biasing the Schottky junction formed between top metal strips and the n-type semiconductor buried beneath. The effective Drude permittivity of the cavity array is tuned by changing the depletion layer thickness via a gate bias applied between the strips and a back metal reflector. Combining Maxwell equations for terahertz waves with a drift-diffusion model for the semiconductor carriers into a multiphysics framework, we show that the proposed modulation concept is promising for a large part of the terahertz spectrum. ? 2019 IEEE.

In this work, a novel method to obtain all-dielectric toroidal response metasurfaces in the W-band and THz range is demonstrated. Two designs are proposed, a symmetric and asymmetric disk metasurface. The first design is intended to corroborate the theoretical analysis, demonstrating the excitation of a strong toroidal mode resonance at 93.2 GHz. Then, the second design is used to demonstrate that symmetry-breaking variations in the disk dimensions, could lead to birefringent metasurfaces, affecting the polarization of the impinging light. Two structures are designed, a polarization beam splitter and a polarization converter. Such devices are difficult to obtain at the target frequency range with low absorption, so they could be of particular interest for the next generation of 5G communications and THz devices. ? 2019 SPIE.

We theoretically propose the concept of loading microwave waveguides with periodic arrays of dielectric elements that emulate the electromagnetic response of two-dimensional metasurfaces. We focus on the case study of a metasurface composed of dielectric cuboids, which is designed to exhibit a strong toroidal dipole resonance in the microwave X-band. Standard waveguides are then loaded with one-dimensional arrays of the cuboids, such that they reproduce the resonant characteristics of the reference infinite metasurface. It is demonstrated that parallel-plate and rectangular waveguides retain almost the same properties on resonance, while the main attributes are also preserved in standard impedance-matched microstrip lines. ? 2019 IEEE.

A novel free-standing dielectric metasurface that shows a strong toroidal resonance in the sub- THz range is proposed and demonstrated. Theoretical and experimental studies confirm the basic operation principle and electromagnetic response of the metasurface. This first demonstration of a single-layer silicon metasurface opens new venues of research for the investigation of toroidal multipoles and the engineering of ultra-high Q factor devices in the sub-THz spectral range, where high Q factors are very difficult to obtain. ? 2019 IEEE.

This work proposes the use of the refractive index sensitivity of non-radiating anapole modes of high-refractive-index nanoparticles arranged in planar metasurfaces as a novel sensing principle. The spectral position of anapole modes excited in hollow silicon nanocuboids is first investigated as a function of the nanocuboid geometry. Then, nanostructured metasurfaces of periodic arrays of nanocuboids on a glass substrate are designed. The metasurface parameters are properly selected such that a resonance with ultrahigh Q-factor, above one million, is excited at the target infrared wavelength of 1.55 ?m. The anapole-induced resonant wavelength depends on the refractive index of the analyte superstratum, exhibiting a sensitivity of up to 180 nm/RIU. Such values, combined with the ultrahigh Q-factor, allow for refractometric sensing with very low detection limits in a broad range of refractive indices. Besides the sensing applications, the proposed device can also open new venues in other research fields, such as non-linear optics, optical switches, and optical communications. ? 2018 by the authors. Licensee MDPI, Basel, Switzerland.

In this work, a dielectric metasurface consisting of hollow dielectric nanocuboids, with ultrahigh quality factor, is theoretically proposed and demonstrated. The variation of the hole size of the cuboid allows for the tuning of the resonant anapole mode in the nanoparticles. The metasurface is designed to operate in two complementary modes, namely electromagnetically induced transparency and narrowband selective reflection. Thanks to the non-radiative nature of the anapole resonances, the minimal absorption losses of the dielectric materials, and the near-field coupling among the metasurface nanoparticles, a very high quality factor of Q = 2.5 ? 10 6 is achieved. The resonators are characterized by a simple bulk geometry and the subwavelength dimensions of the metasurface permit operation in the non-diffractive regime. The high quality factors and strong energy confinement of the proposed devices open new avenues of research on light-matter interactions, which may find direct applications, e.g., in non-linear devices, biological sensors, laser cavities, and optical communications. ? 2019 Optical Society of America.

Ever since the advent of microwave technology, tunable devices, such as phase shifters, resonators, and antennas, have been indispensable in applications requiring radiofrequency signal filtering, beam shaping, or steering. This necessity becomes even more relevant in view of a new era in high-bandwidth wireless communications in 5G networks, satellite links, and advanced radars for sensing and safety. As the operating frequency is pushed toward the millimeter-wave range or beyond, the performance of traditional microwave tunable elements, such as PIN diodes, micro-electromechanical system switches, ferrites, or ferroelectric films, degrades, while their fabrication complexity and cost increases. In this respect, liquid crystals present a promising solution, as they provide continuous tuning, low dielectric constants, moderate losses, low dispersion, and potentially low cost. Here, a comprehensive overview of the available microwave liquid crystal materials is provided, including their key application-relevant properties, and the techniques employed for their characterization, focusing on the spectrum above 1 GHz. Their performance metrics in terms of dielectric constant tunability, response speed, insertion losses are discussed and guidelines for their selection in different microwave applications are provided. Moreover, the differences observed in the experimental data regarding their characterization are highlighted and possible sources of the observed discrepancies are discussed. ? 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

We propose metal-semiconductor-metal cavity arrays as active elements of electrically tunable metasurfaces operating in the terahertz spectrum. Their function is based on reverse biasing the Schottky junction formed between top metal strips and the n-type semiconductor buried beneath. A gate bias between the strips and a back metal reflector controls the electron depletion layer thickness thus tuning the Drude permittivity of the cavity array. Using a rigorous multiphysics framework which combines Maxwell equations for terahertz waves and the drift-diffusion model for describing the carrier behavior in the semiconductor, we find a theoretically infinite extinction ratio, insertion loss of around 10%, and picosecond intrinsic switching times at 1 THz, for a structure designed to enter the critical coupling regime once the depletion layer reaches the bottom metal contact. We also show that the proposed modulation concept can be used for devices operating at the higher end of the terahertz spectrum, discussing the limitations on their performance. ? 1995-2012 IEEE.

A single-layer, all-dielectric metasurface exhibiting a strong toroidal resonance in the low-atmospheric loss radio window of the subterahertz W-band is theoretically proposed and experimentally demonstrated. The metasurface is fabricated on a high-resistivity floating-zone silicon wafer by means of a single-process, wet anisotropic etching technique. The properties of the toroidal mode of both the constituent dielectric elements and the metasurface are rigorously investigated by means of the multipole decomposition technique and full-wave simulations. The experimental demonstration of such a compact, all-silicon metasurface opens new venues of research in the investigation of toroidal modes and the engineering of functional millimeter-wave components, which can be scaled to terahertz and higher frequencies of the electromagnetic spectrum. ? 2019 The Authors. Published by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

We theoretically investigate the possibility to load microwave waveguides with dielectric particle arrays that emulate the properties of infinite, two-dimensional, all-dielectric metasurfaces. First, we study the scattering properties and the electric and magnetic multipole modes of dielectric cuboids and identify the conditions for the excitation of the so-called anapole state. Based on the obtained results, we design metasurfaces composed of a square lattice of dielectric cuboids, which exhibit strong toroidal resonances. Then, three standard microwave waveguide types, namely parallel-plate waveguides, rectangular waveguides, and microstrip lines, loaded with dielectric cuboids are designed, in such a way that they exhibit the same resonant features as the equivalent dielectric metasurface. The analysis shows that parallel-plate and rectangular waveguides can almost perfectly reproduce the metasurface properties at the resonant frequency. The main attributes of such resonances are also observed in the case of a standard impedance-matched microstrip line, which is loaded with only a small number of dielectric particles. The results demonstrate the potential for a novel paradigm in the design of "metasurface-loaded" microwave waveguides, either as functional elements in microwave circuitry, or as a platform for the experimental study of the properties of dielectric metasurfaces. ? 2019, The Author(s).

We experimentally demonstrate that electromagnetically thin polyimide substrates can mitigate substrate-induced detrimental effects to the performance of metallic metasurfaces. A planar quarter-wave plate for the microwave K-band is fabricated on a polyimide substrate of deep subwavelength thickness by means of standard photolithography. By properly selecting the combination of the polyimide thickness and the aluminum layer thickness of the metasurface, conversion from linear to circular polarization is achieved at the design frequency. The proposed approach is generic, and it can be applied to the fabrication of mechanically robust, flexible metallic metasurfaces, which are primarily designed to work in a free-standing configuration. ? 2019 Author(s).

A new terahertz filter based on the coupling of guided-mode resonances and Fabry-Perot resonance is presented resulting in a flat top response. The spectral response lies in the sub-THz communication windows with central frequency of 300 GHz. The filter performance shows high transmittance, with less than 3dB losses, and high out-of-band rejection. The filter is fabricated via standard photolithography on thin films of the Zeonor polymer. This typology of THz components provides a cost-effective functional solution for narrowband filtering in emerging THz devices and systems for telecommunication.

In this work, we design and experimentally demonstrate a novel terahertz (THz) filter exhibiting a flattened spectral response in the atmospheric transmission window around the central frequency of 300 GHz. The innovative concept behind this filter is the coupling of Fabry-Perot and guided mode resonances. The latter arise from a two-dimensional patch array patterned on an aluminum layer deposited on a low loss cyclo-olefin polymer. The filter experimental performance shows high transmittance in the flat-top band, with less than 3 dB losses, and high out-of-band rejection, as theoretically expected. This kind of component provides a cost-effective, functional solution for narrowband filtering in emerging THz devices and systems with possible applications in wireless telecommunications.

The focusing properties of a series of polymeric zone-plate lenses are investigated around a target frequency of 1 THz. Their characterization is performed by means of terahertz (THz) time-domain spectroscopy, employing a modified knife-edge technique that compensates for asymmetries of the impinging THz beam shape of typical photoconductive antenna-based THz sources. The samples are fabricated by a three-axis milling technique on slabs of an ultralow-loss cyclo-olefin polymer. Three different zone plates are studied, a conventional binary zone plate, a conventional four-level zone plate, and a recently introduced double-sided zone plate consisting of the stack of two phase-reversal binary zone plates, which is simpler to fabricate and less sensitive to mechanical damage than multilevel zone plates. Experimental results, coupled with finite element simulations, demonstrate that the double-sided zone plate features a resolution increased by about 3 lambda with respect to the binary zone plate and comparable with that of the four-level zone plate. The double-sided zone plate has 40% lower focusing efficiency and approximately 7 lambda shorter depth of field compared to its four-level counterpart. Nevertheless, it outperforms conventional binary zone plates by 25% in power focusing efficiency and features a 10 lambda longer depth of field. (C) 2019 Optical Society of America