Anna C. Tasolamprou

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.

Toroidal eigenmodes in all-dielectric metamolecules

We present a thorough investigation of the electromagnetic resonant modes supported by systems of polaritonic rods placed at the vertices of canonical polygons. The study is conducted with rigorous finite-element eigenvalue simulations. To provide physical insight, the simulations are complemented with coupled mode theory (the analog of LCAO in molecular and solid state physics) and a lumped wire model capturing the coupling-caused reorganizations of the currents in each rod. The systems of rods, which form all-dielectric cyclic metamolecules, are found to support the unconventional toroidal dipole mode, consisting of the magnetic dipole mode in each rod. Besides the toroidal modes, the spectrally adjacent collective modes are identified. The evolution of all resonant frequencies with rod separation is examined. They are found to oscillate about the single-rod magnetic dipole resonance, a feature attributed to the leaky nature of the constituent modes. Importantly, we observe that ensembles of an odd number of rods produce larger frequency separation between the toroidal mode and its neighbor than the ones with even number of rods. This increased spectral isolation, along with the low quality factor exhibited by the toroidal mode, favors the coupling of the commonly silent toroidal dipole to the outside world, rendering the proposed structure a prime candidate for controlling the observation of toroidal excitations and their interaction with the usually present electric dipole

Frequency splitter based on the directional emission from surface modes in dielectric photonic crystal structures

We demonstrate the numerical design and the experimental validation of frequency dependent directional emission from a dielectric photonic crystal structure. The wave propagates through a photonic crystal line-defect waveguide, while a surface layer at the termination of the photonic crystal enables the excitation of surface modes and a subsequent grating layer transforms the surface energy into outgoing propagating waves of the form of a directional beam. The angle of the beam is controlled by the frequency and the structure operates as a frequency splitter in the intermediate and far field region.

Experimentally excellent beaming in a two-layer dielectric structure

We demonstrate both experimentally and theoretically that a two-layer dielectric structure can provide collimation and enhanced transmission of a Gaussian beam passing through it. This is due to formation of surface localized states along the layered structure and the coupling of these states to outgoing propagating waves. A system of multiple cascading two-layers can sustain the beaming for large propagation distances.

Near infrared and optical beam steering and frequency splitting in air holes-in-silicon inverse photonic crystals

We present the design of a dielectric inverse photonic crystal structure that couples line-defect waveguide propagating modes into highly directional beams of controllable directionality. The structure utilizes a triangular lattice made of air holes drilled in an infinitely thick Si slab, and it is designed for operation in the near-infrared and optical regime. The structure operation is based on the excitation and manipulation of dark dielectric surface states, in particular on the tailoring of the dark states’ coupling to outgoing radiation. This coupling is achieved with the use of properly designed external corrugations. The structure adapts and matches modes that travel through the photonic crystal and the free space. Moreover it facilitates the steering of the outgoing waves, is found to generate well-defined, spatially and spectrally isolated beams, and may serve as a frequency splitting component designed for operation in the near-infrared regime and in particular the telecom optical wavelength band. The design complies with the state-of-the-art Si nanofabrication technology and can be directly scaled for operation in the optical regime.

Silicon photonic crystal beam steering and frequency splitting at telecom wavelengths based on the manipulation of surface states (Conference Presentation)

Dielectric, ohmic-loss-free, finite photonic crystals (PCs) may sustain the propagation of highly confined surface waves that propagate bound to the interface of the bulk structure and the free space. For many years the dielectric photonic crystals surface states have been treated as a subsidiary effect related to the inevitable finite size of the PCs in realistic implementations. However, in the recent years it has been realized that the features of the dielectric surface states and their impact to the wave exit from the photonic crystal structure render the relevant structures suitable components for a variety of applications, including optical spectroscopy, sensing, intercomponent coupling, etc. In this work we present the design of a silicon-based PC component that couples the modes that propagate through the bulk structure into outgoing, free space propagating beams with high directionality. In addition to previous works involving silicon-based PCs for beam collimation in the near infrared and optical regime, we demonstrate here, that the (frequency depended) emission angle of the generated beams can be controlled by properly engineering the PC termination. Thus the component may serve as a beam steering structure or a de-multiplexer in the optical telecommunications wavelength band (~1.5 μm). Our design takes into account the state-of-the-art nano-fabrication technology and all the constraints arising from the treatment of silicon-based periodic media in this frequency regime. It consists of air holes drilled through an infinite silicon slab, arranged in a standard hexagonal lattice with periodicity α = 320 nm. Within the bulk photonic crystal we assume a line-defect waveguide that leads to the PC-air interface, where a properly designed termination layer of air holes is imprinted. The termination is designed to induce surface states at the PC-air interface with desirable dispersion and spatial characteristics. The line-defect waveguide is an area of unperforated silicon slab and it is chosen since it is a widely used scheme for guiding energy through the reflective photonic crystals; therefore, our design is compatible with the majority of the silicon-based PCs circuit components. By properly designing the interfacial termination layer we can tailor the properties of the non-radiating, dark, surface states in order to adapt and match the waveguide propagating modes and the free-space modes. As a result, we demonstrate a silicon-based photonic crystal structure that provides (a) the generation of well-defined and highly directional beams at the exit of the photonic crystal structure; (b) efficient beam multiplexing, i.e., the formation of two well defined beams that exhibit sufficiently high spectral isolation, as well as sufficiently high spatial separation. In particular we present two design approaches; the first is able to generate two beams at the operation wavelengths λ1 = 1.37 μm and λ2 = 1.5 μm, with high spatial separation defined by the emission angles φ1 = +20 deg and φ2 = -23 deg and the second generates two beams at λa = 1.42 μm and λb = 1.52 μm with emission angle φa = +1 deg and φb = 23 deg. The wavelengths of operation, as well as the emission angle, can be engineered at will.

Pairing Toroidal and Magnetic Dipole Resonances in Elliptic Dielectric Rod Metasurfaces for Reconfigurable Wavefront Manipulation in Reflection

Abstract A novel approach for reconfigurable wavefront manipulation with gradient metasurfaces based on permittivity‐modulated elliptic dielectric rods is proposed. It is shown that the required 2π phase span in the local electromagnetic response of the metasurface can be achieved by pairing the lowest magnetic dipole Mie resonance with a toroidal dipole Mie resonance, instead of using the lowest two Mie resonances corresponding to fundamental electric and magnetic dipole resonances as customarily exercised. This approach allows for the precise matching of both the resonance frequencies and quality factors. Moreover, the accurate matching is preserved if the rod permittivity is varied, allowing for constructing reconfigurable gradient metasurfaces by locally modulating the permittivity in each rod. Highly efficient tunable beam steering and beam focusing with ultrashort focal lengths are numerically demonstrated, highlighting the advantage of the low‐profile metasurfaces over bulky conventional lenses. Notably, despite using a matched pair of Mie resonances, the presence of an electric polarizability background allows to perform the wavefront shaping operations in reflection, rather than transmission. This has the advantage that any control circuitry necessary in an experimental realization can be accommodated behind the metasurface without affecting the electromagnetic response.

THz polarization control with chiral and bianisotropic metamaterials and metasurfaces

Chiral metamaterials, where magnetoelectric coupling is orders of magnitude larger than that of natural chiral media, offer unique possibilities for the control and manipulation of the electromagnetic wave polarization. Such a control is particularly useful in the THz range, where natural materials do not show strong response, and therefore the optical components available are limited, while the possibilities in terms of applications are huge.

Beam shaping and manipulation in photonic crystal structures (presentation video)

Photonic crystals may support the propagation of surface waves, provided that they are properly terminated. An additional grating like-layer may facilitate the coupling of the surface waves to outgoing propagating waves leading to enhanced transmission and directionality of the beam. This work investigates and demonstrates how the proper design of the grating layer provides control over the beam shape and emission angle. We also demonstrate both experimentally and theoretically, that a single bilayer dielectric structure allows for the collimation and enhanced transmission of a Gaussian incident beam, while a system of multiple cascading bilayers can sustain the beam for large propagation distances.