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-Electrically tunable terahertz polarization converter based on overcoupled metal-isolator-metal metamaterials infiltrated with liquid crystals

Borislav Vasić1, Dimitrios C Zografopoulos2, Goran Isić1, Romeo Beccherelli2  Radoš Gajić1

Published 21 February 2017 • © 2017 IOP Publishing Ltd 

http://iopscience.iop.org/article/10.1088/1361-6528/aa5bbd

Large birefringence and its electrical modulation by means of Fréedericksz transition makes nematic liquid crystals (LCs) a promising platform for tunable terahertz (THz) devices. The thickness of standard LC cells is in the order of the wavelength, requiring high driving voltages and allowing only a very slow modulation at THz frequencies. Here, we first present the concept of overcoupled metal-isolator-metal (MIM) cavities that allow for achieving simultaneously both very high phase difference between orthogonal electric field components and large reflectance. We then apply this concept to LC-infiltrated MIM-based metamaterials aiming at the design of electrically tunable THz polarization converters. The optimal operation in the overcoupled regime is provided by properly selecting the thickness of the LC cell. Instead of the LC natural birefringence, the polarization-dependent functionality stems from the optical anisotropy of ultrathin and deeply subwavelength MIM structures. The dynamic electro-optic control of the LC refractive index enables the spectral shift of the resonant mode and, consequently, the tuning of the phase difference between the two orthogonal field components. This tunability is further enhanced by the large confinement of the resonant electromagnetic fields within the MIM cavity. We show that for an appropriately chosen linearly polarized incident field, the polarization state of the reflected field at the target operation frequency can be continuously swept between the north and south pole of the Poincaré sphere. Using a rigorous Q-tensor model to simulate the LC electro-optic switching, we demonstrate that the enhanced light–matter interaction in the MIM resonant cavity allows the polarization converter to operate at driving voltages below 10 Volt and with millisecond switching times.

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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