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-Graphene-Covered Photonic Structures for Optical Chemical Sensing

Borislav Vasić and Radoš Gajić

http://journals.aps.org/prapplied/abstract/10.1103/PhysRevApplied.4.024007

Graphene applications in chemical sensing are based on the chemical doping of graphene. In this process, molecules adsorbed on graphene serve as charge-carrier donors or acceptors, thus changing the graphene conductivity. While the previous studies have been focused on chemical sensors with electrical detection, we theoretically investigate chemical sensing based on photonic structures covered with graphene. By considering chemical doping of graphene as a small perturbation, we show that optimal photonic structures operate at low-terahertz frequencies, with the reflectance intensity as the output signal. In order to achieve an efficient chemical sensing, photonic structures should provide the electric-field enhancement within the graphene plane. As a result, the proposed structure consists of the metallic mirror and quarter-wavelength-thick dielectric spacer with graphene on the top of it. The sensitivity is maximized when the Fermi energy in the graphene not exposed to the environment is around 30 meV. By taking the resolution for the reflectance measurement of 1%, we show that the proposed sensing structure can detect graphene doping by 150 electrons or holes per square micrometer in the dynamic range of around 3000 charge carriers.

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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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