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-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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