Emmanouil E. Kriezis

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.

Light wave propagation in liquid crystal displays by the 2-D finite-difference time-domain method

Abstract Light wave propagation within liquid crystal (LC) devices is studied using a two-dimensional (2-D) finite-difference time-domain (FDTD) method. Computational space termination is provided by a combination of the perfectly matched layer absorbing boundary condition and periodic boundary conditions, to overcome the limitations imposed by previously proposed FDTD methods for LC optics. Both normal and oblique incidence cases are successfully implemented and a consistent method for magnitude and phase extraction is made available. This provides a rigorous numerical solution for the light wave propagating within LC devices, and a sample application for a small twisted nematic pixel is given.

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.

Finite-difference time domain method for light wave propagation within liquid crystal devices

Light wave propagation within liquid crystal devices is determined by the application of an appropriate finite-difference time-domain (FDTD) method, accounting rigorously for electromagnetic wave propagation. The introduction of FDTD calculations is aimed to substitute the matrix-type solvers in cases where the stratified-medium approximation fails. Such cases are commonly encountered when a liquid crystal device exhibits variations of the director orientation along the transverse direction on the scale of the propagating optical wavelength, for instance, at pixel edges. The formulation and sample numerical application are focused on planar liquid crystal devices exhibiting bend/splay deformation.

Theoretical analysis of thermally tunable microring resonator filters made of dielectric-loaded plasmonic waveguides

Microring resonator filters, which are made of dielectric-loaded surface plasmon polariton waveguides and operate in the telecom spectral range, are thoroughly analyzed by means of vectorial three dimensional (3D) finite element method (FEM) simulations. The filters’ functional characteristics, such as the resonant frequencies where the transmission minima occur, the free spectral range, the extinction ratio, and the minima linewidth associated with the quality factor of the resonances, are investigated for different values of the key structural parameters, namely, the ring radius and the gap separating the bus waveguide from the ring. The rigorous 3D-FEM simulations are qualitatively complemented by a simplified model. Apart from the harmonic propagation simulations, the uncoupled microring is treated as an eigenvalue problem, and the frequencies of the resonances are compared with those of the transmission minima. Furthermore, the possibility of exploiting the thermally tuned microring resonator filter ...

Thermo-optic plasmo-photonic mode interference switches based on dielectric loaded waveguides

We demonstrate an efficient thermo-optic dielectric loaded surface plasmon polariton waveguide (DLSPPW) 2 × 2 switch using a high thermo-optic coefficient polymer and a dual mode interference configuration. Unlike previous configurations relying on single-mode waveguide circuitry, the switch we consider is based on the interference between a plasmonic and a low-damping photonic mode of the DLSPPW, thus leading to the minimization of insertion losses of the device. Switching extinction ratios of 7 dB are measured for a compact 119 μm-long device. The overall device performances are in good agreement with numerical simulations performed using the beam propagation method.

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

Analysis of tunable photonic crystal devices comprising liquid crystal materials as defects

The tuning properties of two-dimensional dielectric and metallic photonic crystals, which contain nematic liquid crystal materials as defect elements or layers, are thoroughly analyzed using appropriate formulations of the finite difference time domain (FDTD) method. Our methodology correctly incorporates the anisotropy introduced by the liquid crystal materials together with the dispersive properties of the metallic elements; it is used for calculating both the dispersion diagrams of the defect-free photonic crystal as well as the device response in the presence of the defect elements. Numerical simulations reveal that defect states originating from the liquid crystal impurities can be effectively tuned by the application of a local static electric field. Indeed, tuning ranges up to almost 100 nm can be achieved requiring operating voltages lower than 4 V. It is also concluded that the placement of a defect mode relative to the bandgap edges greatly influences both its linewidth as well as its tuning range.

Interfacing Dielectric-Loaded Plasmonic and Silicon Photonic Waveguides: Theoretical Analysis and Experimental Demonstration

A comprehensive theoretical analysis of end-fire coupling between dielectric-loaded surface plasmon polariton and rib/wire silicon-on-insulator (SOI) waveguides is presented. Simulations are based on the 3-D vector finite element method. The geometrical parameters of the interface are varied in order to identify the ones leading to optimum performance, i.e., maximum coupling efficiency. Fabrication tolerances about the optimum parameter values are also assessed. In addition, the effect of a longitudinal metallic stripe gap on coupling efficiency is quantified, since such gaps have been observed in fabricated structures. Finally, theoretical results are compared against insertion loss measurements, carried out for two distinct sets of samples comprising rib and wire SOI waveguides, respectively.

Diversity Combining in Hybrid RF/FSO Systems with PSK Modulation

We present a novel architecture for hybrid radio frequency (RF)/ free space optical (FSO) wireless systems without feedback or channel state information (CSI) at the transmitter. Under the assumption that 60 GHz RF and FSO systems support the same data rates, the proposed implementation transmits the same data over both links, using phase shift keying (PSK) as a common modulation scheme, and combines the signals from each individual link at the receiver on a symbol-by-symbol basis. Two popular diversity combining schemes are considered, namely, selection combining (SC) and maximal ratio combining (MRC), while tractable analytical approximations for the bit error rate (BER) are obtained. Investigations over various weather conditions and link distances revealed that the proposed implementation fully exploits the complementary nature of RF and FSO channels, even when one of the two available links fails. Furthermore, the comparison of the combining schemes demonstrates MRC as the optimum combining scheme, offering link distance gains compared to SC.