Abstract-All‐Dielectric Silicon Metasurface with Strong Subterahertz Toroidal Dipole Resonance

Dimitrios C. Zografopoulos  Antonio Ferraro  José Francisco Algorri  Pedro Martín‐Mateos  Braulio García‐Cámara  Aldo Moreno‐Oyervides  Viktor Krozer  Pablo Acedo  Ricardo Vergaz  José Manuel Sánchez‐Pena  Romeo Beccherelli,

https://onlinelibrary.wiley.com/doi/abs/10.1002/adom.201900777

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

Abstract-Electrically Tunable Metal-Semiconductor-Metal Terahertz Metasurface Modulators

Goran Isic,  Georgios Sinatkas,  Dimitrios C. Zografopoulos,  Borislav Vasic, Antonio Ferraro,  Romeo Becch

https://ieeexplore.ieee.org/document/8616810

We propose metal-semiconductor-metal cavity arrays as the active element 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.

Abstract-Numerical and Experimental Time-Domain Characterization of Terahertz Conducting Polymers

Dimitrios C. Zografopoulos,   Konstantinos P. Prokopidis,   Antonio Ferraro,   Luke Peters,   Marco Peccianti,   Romeo Beccherelli,

https://ieeexplore.ieee.org/document/8424162/

A comprehensive framework for the theoretical and experimental investigation of thin conducting films for terahertz applications is presented. The electromagnetic properties of conducting polymers spin-coated on low-loss dielectric substrates are characterized by means of terahertz time-domain spectroscopy and interpreted through the Drude-Smith model. The analysis is complemented by an advanced finite-difference time-domain algorithm, which rigorously deals with both the dispersive nature of the involved materials and the extremely subwavelength thickness of the conducting films. Significant agreement is observed among experimental measurements, numerical simulations, and theoretical results. The proposed approach provides a complete toolbox for the engineering of terahertz optoelectronic devices.

Abstract-Terahertz frequency-selective surface and guided-mode resonance filters

Antonio Ferraro, Roberto Caputo,   Dimitrios C. Zografopoulos,   Romeo Beccherelli

https://ieeexplore.ieee.org/document/8116222/

We report on the experimental and theoretical investigation of a new class of terahertz filters based on frequency-selective surfaces patterned on thin foils of the low-loss cyclo-olef n polymer Zeonor. We observe both broad and narrowband resonances, which stem from the FSS response and the coupling to substrate guided modes, respectively. The filtering properties are studied as a function of the FSS geometry and the thickness of the polymer layer. Very narrow linewidths with quality factors exceeding 100 are measured experimentally and theoretically confirmed via finite-element simulations.

Abstract-Systematic Design of THz Leaky-Wave Antennas Based on Homogenized Metasurfaces

 Walter Fuscaldo, Silvia Tofani, Dimitrios C. Zografopoulos, Paolo Baccarelli,  Paolo Burghignoli, Romeo Beccherelli, Alessandro Galli

https://ieeexplore.ieee.org/document/8259457/

In this paper, a systematic design of Fabry-Perot cavity antennas based on leaky waves is proposed in the THz range. The use of different topologies for the synthesis of homogenized metasurfaces shows that a specific fishnetlike unit cell is particularly suitable for the design of efficient THz radiating devices. Accurate full-wave simulations highlight the advantages and disadvantages of the proposed geometries, thoroughly considering the bounds dictated by technological constraints and the homogenization limit as well. The radiative performance of different designs for achieving theoretical directivities ranging from 15 to 30 dB is evaluated with reliable analytical and numerical methods, and completely validated with full-wave simulations. The relevant results corroborate the proposed systematic design, consolidating the validity and the usefulness of the leaky-wave approach, well established at microwave frequencies, to the more challenging and still unexplored THz range.

Abstract-Broad- and Narrow-Line Terahertz Filtering in Frequency-Selective Surfaces Patterned on Thin Low-Loss Polymer Substrates

 Antonio Ferraro,  Dimitrios C. Zografopoulos,  Roberto Caputo,   Romeo Beccherell

http://ieeexplore.ieee.org/document/7847400/

A new class of frequency-selective surface filters (FSS) for terahertz (THz) applications is proposed and investigated both numerically and experimentally. A periodic FSS array of cross-shaped apertures is patterned on aluminum, deposited on thin foils of the low-loss cyclo-olefin polymer Zeonor. Apart from the fundamental filtering response of the FSS elements, we also observe very narrow-linewidth peaks with high transmittance, associated with guided-mode resonances in the dielectric substrate. The effect of the filter's geometrical parameters on its performance is systematically studied via finite-element method simulation and confirmed by time-domain spectroscopy characterization of the fabricated samples. Finally, thanks to the flexibility of the employed substrates, THz-FSS filters are also characterized in bent configuration, revealing a robust response in terms of the fundamental FSS passband filter and a high sensitivity of the GMR peaks. These features can be exploited in the design of novel THz filters or sensors.

Abstract-Mechanically tunable Bragg filters for terahertz applications

Antonio Ferraro, Dimitrios C. Zografopoulos, Roberto Caputo

http://ieeexplore.ieee.org/document/7758662/?reload=true

Mechanically tunable Bragg filters at terahertz frequencies are designed, fabricated by a simple procedure, and characterized via THz time-domain spectroscopy. The Bragg reflectors are composed by stacking thin cyclo-olefin Zeonor polymer layers, which are separated by spacers made of a bi-adhesive tape. The defect cavity is void and its thickness is mechanically tuned by means of a translation stage. The filters show high transmittance and quality factors, owing to the selected very low-loss polymer.

Abstract-Tunable terahertz fishnet metamaterials based on thin nematic liquid crystal layers for fast switching

• Dimitrios C. Zografopoulos
•  & Romeo Beccherelli

http://www.nature.com/articles/srep13137

The electrically tunable properties of liquid-crystal fishnet metamaterials are theoretically investigated in the terahertz spectrum. A nematic liquid crystal layer is introduced between two fishnet metallic structures, forming a voltage-controlled metamaterial cavity. Tuning of the nematic molecular orientation is shown to shift the magnetic resonance frequency of the metamaterial and its overall electromagnetic response. A shift higher than 150 GHz is predicted for common dielectric and liquid crystalline materials used in terahertz technology and for low applied voltage values. Owing to the few micron-thick liquid crystal cell, the response speed of the tunable metamaterial is calculated as orders of magnitude faster than in demonstrated liquid-crystal based non-resonant terahertz components. Such tunable metamaterial elements are proposed for the advanced control of electromagnetic wave propagation in terahertz applications

Abstract-Electrically Tunable Critically Coupled Terahertz Metamaterial Absorber Based on Nematic Liquid Crystals

PH

Goran Isić, Borislav Vasić, Dimitrios C. Zografopoulos, Romeo Beccherelli, and Radoš Gajić

Phys. Rev. Applied 3, 064007 – Published 11 June 2015

http://journals.aps.org/prapplied/abstract/10.1103/PhysRevApplied.3.064007

Liquid-crystal devices are a promising cheap alternative for terahertz light modulation, albeit they suffer from problems associated with thick cells. Here we describe a few-micron-thick polarization-independent nematic liquid-crystal metamaterial device displaying terahertz reflectance modulation depths above 23 dB, millisecond response times, low operating voltages, and a spectral tuning of more than 15%. The dramatic performance improvement is based on invoking critical coupling with external fields, which rests on a suitable choice of resonator geometry. We analyze the coupling mechanism to conclude that perfect absorption can be reached with a wide range of parameters and liquid-crystal materials. The proposed device performance, microscopic details, and the nematic molecule switching dynamics are evaluated with the use of a rigorous tensorial formulation of the Landau–de Gennes theory and shown to be robust to small parameter deviations.

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