Liquid–crystal tunable waveguides for integrated plasmonic components

A broad range of liquid–crystal tunable plasmonic waveguides, based on long-range, dielectric-loaded, and channel surface plasmon polaritons, are theoretically designed and investigated. Liquid–crystal switching is rigorously modeled by solving for the coupled elastic/electrostatic problem, whereas the optical studies are conducted via the finite-element method. Extensive tunability of key optical properties, such as modal index, propagation losses, and modal confinement is demonstrated for waveguides of different optical confinement scale. These highly functional waveguiding structures are proposed as building blocks for the design of functional components, e.g. optical attenuators, directional couplers and switches, in integrated plasmonic chips.     

Optimizing silicon-plasmonic waveguides for \chi ^{(3)} nonlinear applications

Hybrid silicon-plasmonic waveguides constitute an appealing platform for integrated photonic circuitry. They merge the technical maturity and prevalence of the SOI platform with the subwavelength confinement of plasmonic waveguides, essential for accessing enhanced nonlinear response at micron length-scales. Employing full-wave numerical simulations complemented with Schrödinger equation techniques, we propose nonlinear waveguide designs for Kerr-effect applications exhibiting minimized impairments due to free-carrier effects, thus raising the power-ceiling imposed on standard silicon waveguides.     

Light wave propagation in periodic tilted liquid crystal structures: a periodic beam propagation method

Periodically deformed liquid crystal profiles are commonly encountered either spontaneously under the influence of an external excitation, or intentionally, with the introduction of relief type gratings on the supporting surfaces. These types of periodicity are often characterized by a pitch around one micron or less. As the pitch is directly comparable to the optical wavelength, it is necessary to develop rigorous numerical methods to predict light wave propagation within such structures. A wide angle periodic beam propagation method (BPM) is proposed for the simulation of light propagation in purely bend/splay periodic director deformations. A common occurrence of this type of deformation is encountered in smectic A materials under the Helfrich effect. More importantly, bistable nematic cells with zenithal deformation have been developed for interesting display applications. Both aforementioned deformations are examined to verify the applicability of the proposed periodic BPM.     

Computational techniques for the analysis and design of dielectric-loaded plasmonic circuitry

The finite element and the beam propagation method, two widely used methods in photonics, are utilized for the analysis of plasmonic components based on the dielectric-loaded plasmonic waveguide. Two components are chosen as examples and are subsequently numerically investigated by employing the aforementioned methods, in order to demonstrate their applicability in plasmonics. Specifically, a microring resonator add-drop filter and a Mach–Zehnder interferometric switch are analyzed by means of the finite element and the beam propagation method, respectively. The formulation adopted is clearly presented in both cases and the case-dependent implementation details are thoroughly discussed.     

Quasi-soliton propagation in dispersion-engineered silicon nanowires

Soliton-like propagation of ultra-short pulses in dispersion-engineered silicon photonic wires is theoretically investigated via the nonlinear Schrödinger equation. It is shown that by proper patterning of silicon waveguides, the engineering of group velocity dispersion can effectively compensate for both linear and two-photon absorption-induced nonlinear losses. Quasi-soliton propagation is demonstrated for 100-fs pulses over large propagation lengths for a realistic silicon wire of optimally patterned waveguide width.     

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.     

FDTD analysis of photonic crystal defect layers filled with liquid crystals

Dielectric and metallic photonic crystals comprising nematic liquid crystal materials as defect layers or elements are investigated by the Finite Difference Time Domain (FDTD) method. Appropriate formulations of the FDTD algorithm, for the simulation of anisotropic and dispersive media as well as periodic geometries, are utilised and combined with the proper absorbing and periodic boundary conditions. The spectral properties of the presented structures are tuned by means of applying static electric fields across the defect layers, thus affecting the molecular orientation of the liquid crystal material. Numerical results show that sufficient tuning ranges are achieved, requiring low operating voltages. Moreover, high and sharp resonance peaks are observed.     

Diffraction grating with suppressed zero order fabricated using dielectric forces

An electric-field-assisted method to produce diffractive optical devices is demonstrated. A uniform film of liquid UV curable resin was produced as a drying ring from an organic solvent. Dielectrophoresis forces maintained the stability of the thin film and also imprinted a periodic corrugation deformation of pitch 20 μm on the film surface. Continuous in situ voltage-controlled adjustment of the optical diffraction pattern was carried out simultaneously with UV curing. A fully cured solid phase grating was produced with the particular voltage-selected tailored optical property that the zero transmitted order was suppressed for laser light at 633 nm.     

Microdisk resonator filters made of dielectric-loaded plasmonic waveguides

Microdisk resonator filters, an alternative to microring resonator filters, are studied by means of vectorial three dimensional finite element method simulations. Their performance characteristics are highlighted for different microdisk radii, and compared with those of the respective, footprint-wise, microring filters. We show that microdisk filters are advantageous, as the resonator involved exhibits smaller radiation losses. Extinction ratios as high as 30 dB are possible by properly tuning the gap separating the waveguide from the microdisk in each case. Transmission dips due to higher-radial-order modes that drastically change the transmission picture appear only for very large microdisk radii.     

Modeling light propagation in liquid crystal devices with a 3-D full-vector finite-element beam propagation method

A full-vector finite-element beam propagation method in 3-D is introduced for the simulation of light propagation in liquid crystal (LC) devices. The three electric field components are expressed in terms of mixed finite elements, providing the correct enforcement of boundary conditions. Moreover, the optical dielectric tensor of the medium can have all its nine elements nonzero, thus allowing the LC director to have an arbitrary orientation. A photonic crystal fiber with a LC infiltrated core and a homeotropic to multi-domain cell are analyzed. Comparison with other existing simulation techniques is provided, in order to validate the accuracy of the proposed method.