Dimitrios C. Zografopoulos

Photonic crystal-liquid crystal fibers for single-polarization or high-birefringence guidance

The dispersive characteristics of a photonic crystal fiber enhanced with a liquid crystal core are studied using a planewave expansion method. Numerical results demonstrate that by appropriate design such fibers can function in a single-mode/single-polarization operation, exhibit high- or low- birefringence behavior, or switch between an on-state and an off-state (no guided modes supported). All of the above can be controlled by the application of an external electric field, the specific liquid crystal anchoring conditions and the fiber structural parameters.

Guided-wave liquid-crystal photonics

In this paper we review the state of the art in the field of liquid-crystal tunable guided-wave photonic devices, a unique type of fill-once, molecular-level actuated, optofluidic systems. These have recently attracted significant research interest as potential candidates for low-cost, highly functional photonic elements. We cover a full range of structures, which span from micromachined liquid-crystal on silicon devices to periodic structures and liquid-crystal infiltrated photonic crystal fibers, with focus on key-applications for photonics. Various approaches on the control of the LC molecular orientation are assessed, including electro-, thermo- and all-optical switching. Special attention is paid to practical issues regarding liquid-crystal infiltration, molecular alignment and actuation, low-power operation, as well as their integrability in chip-scale or fiber-based devices.

Tunable highly birefringent bandgap-guiding liquid-crystal microstructured fibers

A new type of nematic liquid-crystal infiltrated photonic bandgap-guiding fiber for single polarization or high-birefringence guidance is proposed. Numerical studies demonstrate that modal birefringence can be tuned by proper selection of the structural and material parameters as well as by the application of an external electric field in conjunction with the specific liquid-crystal anchoring conditions

Liquid crystal-based dielectric loaded surface plasmon polariton optical switches

An optical switch based on liquid crystal dielectric loaded surface plasmon polariton waveguides is proposed and theoretically analyzed. The infiltration of the plasmonic structure with a nematic liquid crystalline material serving as the dielectric loading is shown to allow for extensive electrical tuning of its waveguiding characteristics. Both the electrical switching and optical properties of the proposed waveguide are investigated in the context of designing a directional coupler optical switch, which is found to combine efficient voltage control, low power consumption, high extinction ratio, and relatively low insertion losses.

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

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.

A Unified FDTD/PML Scheme Based on Critical Points for Accurate Studies of Plasmonic Structures

A generalized auxiliary differential equation (ADE) finite-difference time-domain (FDTD) dispersive scheme is introduced for the rigorous simulation of wave propagation in metallic structures at optical frequencies, where material dispersion is described via an arbitrary number of Drude and critical point terms. The implementation of an efficient perfectly matched layer for the termination of such media is also discussed and demonstrated. The models validity is directly compared with both analytical and numerical results that employ known dispersion schemes, for the case of two benchmark examples, transmission through a thin metal film and scattering from a metallic nanocylinder. Furthermore, the accuracy of the proposed method is also demonstrated in the study of the optical properties of Ag and Au metal-insulator-metal waveguides, filters, and resonators, which also involve dielectrics whose material dispersion is described by the Sellmeier model.

In-Line Polarization Controller Based on Liquid-Crystal Photonic Crystal Fibers

Compact polarization control elements based on index-guiding soft-glass photonic crystal fibers infiltrated with nematic liquid crystals are proposed and thoroughly studied. The nematic director profiles at the fibers cross section are consistently calculated by solving the coupled electrostatic and elastic problem, in the context of an analysis on the tunability of liquid-crystal-infiltrated photonic crystal fibers. The fibers dispersive properties and light propagation in the proposed polarization controller are studied by means of a fully anisotropic finite-element-based beam propagation method. The electrically induced evolution of the state of polarization is mapped on the Poincaré sphere. Efficient polarization conversion is demonstrated, with a crosstalk of -50 dB, for a total device length of 4.65 mm and a maximum applied voltage of 150 V. Crosstalk values lower than -20 dB are achieved over a 30 nm window. The proposed devices are envisaged as compact all-in-fiber dynamic polarization controllers.

Tunable one-dimensional photonic crystal slabs based on preferential etching of silicon-on-insulator

We design and assess a one-dimensional photonic crystal slab fabricated by preferential etching of a silicon-on-insulator substrate. The etched grooves are considered to be infiltrated by a highly-birefringent nematic liquid crystalline material. A detailed analysis of the nematic director response within the grooves is presented. We investigate different configurations and demonstrate large band gap shifting when switching the liquid crystal with an applied voltage. Furthermore, we assess this type of device as an efficient alternative for compact refractometric optical sensing applications.

Liquid-crystal-tunable metal?insulator?metal plasmonic waveguides and Bragg resonators

The dispersive properties of liquid-crystal-tunable metal?insulator?metal plasmonic waveguides are theoretically investigated as a function of the applied control voltage. The LC reorientation study is rigorously coupled to the optical studies via a multiphysics finite-element analysis. A Bragg grating resonator is designed based on the proposed waveguide, which shows an extensive tuning efficiency of more than 100?nm?V?1 for low operating voltages up to 2?V.

Design of a vertically coupled liquid-crystal long-range plasmonic optical switch

An optical switch based on liquid-crystal (LC) tunable long-range metal stripe waveguides is proposed and theoretically investigated. A nematic liquid crystal layer placed between a vertical configuration consisting of two gold stripes is shown to allow for the extensive electro-optic tuning of the couplers waveguiding characteristics. Rigorous liquid-crystal switching studies are coupled with the investigation of the optical properties of the proposed plasmonic structure, taking into account different excitation conditions and the impact of LC-scattering losses. A directional coupler optical switch is demonstrated, which combines low power consumption, low cross-talk, short coupling lengths, along with sufficiently reduced insertion losses.