Elissavet P. Kosmidou

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.

AN FDTD ALGORITHM FOR WAVE PROPAGATION IN DISPERSIVE MEDIA USING HIGHER-ORDER SCHEMES

A fourth-order accurate in space and second-order accurate in time, Finite-Difference Time-Domain (FDTD) scheme for wave propagation in lossy dispersive media is presented. The formulation of Maxwells equations is fully described and an elaborate study of the stability and dispersion properties of the resulting algorithm is conducted. The efficiency of the proposed FDTD(2,4) technique compared to its conventional FDTD(2,2) counterpart is demonstrated through numerical results.

A comparative study of the biological effects of various mobile phone and wireless LAN antennas

This paper presents a comprehensive electromagnetic and thermal analysis of radiation and its impact on human beings, due to the use of various types of commonly used mobile phones and communication antennas. This is one of the first studies that deals with a wide-range comparative investigation of modern cell phones, unlike the majority of existing work, which do not extend beyond the obsolete generic phone case. The rather severe, although overlooked, case of wireless local area network antennas is also considered, due to their increasing use and the large times of exposure associated with them.

An FDTD analysis of photonic crystal waveguides comprising third-order nonlinear materials

A finite difference time domain (FDTD) study of two-dimensional photonic crystals containing nonlinear materials is presented in this paper. An appropriate Z-transform oriented formulation of the FDTD method for the simulation of third-order nonlinear Kerr- and Raman-type media is analyzed and applied to model nonlinear photonic crystal waveguide structures. For their reflectionless termination a novel perfectly matched layer (PML) is proposed and evaluated comparatively to other periodic and inhomogeneous absorbers. Furthermore, the absorbing efficiency of the proposed PML is investigated varying its parameters.

Numerical modeling of an indoor wireless environment for the performance evaluation of WLAN systems

A site-specific numerical model, based on the finite-difference time-domain method, is developed in this paper for the indoor radio channel. The scenario of interest is concerned with wave propagation in a typical office environment, for which several simulations are performed considering different placements of the transmitting antenna. Both the 2- and 5-GHz bands are examined, where contemporary wireless local area networks operate. Important channel characteristics are evaluated via the estimation of received power levels, as well as the examination of small-scale fading and time dispersion

A comparative FDTD study of various PML configurations for the termination of nonlinear photonic bandgap waveguide structures

A comparative investigation of high-performance nonlinear, homogeneous and periodic, perfectly matched layers (PMLs) for the termination of two-dimensional (2-D) nonlinear photonic bandgap waveguides is conducted. The absorbing configurations are combined with an appropriate Z-transform-based finite difference time domain (FDTD) technique for the numerical treatment of the nonlinear materials. As in the linear case, a remarkable reduction of the reflected waves is achieved when the periodic dielectric motif of the waveguide is continued inside the absorbing layers.

Analysis of tunable photonic crystal directional couplers

Tunable directional coupler structures based on triangular photonic crystal lattices are investigated using the finite difference time domain method. The infiltration of nematic liquid crystal materials into the coupler waveguides allows for the control of its properties through the application of external static electric fields, which reorient the nematic director. Strong dynamical shifting of the dispersion curves, along with the decoupling frequencies and the coupling coefficient, is demonstrated. Such features render this class of couplers suitable for a range of applications. The coupling lengths accomplished are quite short even when the interaction region between the two coupler branches is widened. Furthermore, operation as a channel interleaver in wavelength division multiplexing systems is explored, and it is revealed that by proper selection of the geometrical parameters a constant channel separation of 0.8nm is achieved, with an overall length of a few hundreds of microns in wide frequency ranges.

Studies of photonic crystal structures infiltrated with liquid crystals

This paper investigates the tunable characteristics of various photonic crystal structures infiltrated with nematic liquid crystals. A triangular lattice of air cylinders drilled into silicon provides the photonic crystal structure and the nematic material is inserted either in all or in properly selected air voids in order to create one-dimensional cavities or directional couplers. We have shown in previous studies that the spectral properties of such geometries can be tuned by means of applying appropriate static electric fields, which eventually determine the orientation of the liquid crystal molecules inside the cylindrical cavities. The essential aspect of the present study is to consider various profiles for the nematic director, which are associated with different molecular anchoring conditions at the confining surfaces, as well as electric fields of various strengths and orientation. In particular, we examine the cases of homeotropic or tangential surface anchoring orientation in the strong or weak anchoring limit, and more specifically focus on determining the impact of the transition from strong to weak anchoring, both on the operation of each structure and the associated range of tuning.

Analysis of tunable photonic crystal based directional couplers

In this paper, we introduce a liquid crystal layer in a photonic crystal directional coupler, in order to tune its characteristics. The simulations are conducted utilizing an anisotropic formulation of the finite difference time domain (FDTD) method