Metamaterials are subwavelength man-made plasmonic or dielectric structures designed to realize specific effects on the electromagnetic radiation interacting with the material. Here we aim to rotate the polarization of an incident terahertz beam using chiral metamaterials, while suppressing the circular dichroism which induces ellipticity of the output beam. An incident linearly polarized wave can be decomposed into left-circularly and right-circularly polarized waves, and the difference in propagation phase will result in rotation of the plane of polarization. Since chiral structures couple electric and magnetic fields, they are often implemented in complex geometries such as spirals or bi-layered plasmonic structures, which can achieve carefully balanced responses to the two fields. The important feature of the bi-layered plasmonic structure is the cross-coupling between the resonances of the two layers. It is precisely this coupling between the layers that induces currents in the structures that are mutually dependent producing chirality within the structure. By coupling a metallic structure to its complement, we are able to achieve strong transmission in the region of maximum polarization rotation, and relatively low ellipticity of the output state. Three different structures were fabricated for this work that will be referred to as: the plain crosses, crossed arrowheads, and crossed arcs, pictured in the figure below. The terahertz responses of the structures were compared using terahertz time-domain spectroscopy and numerical simulations using CST Microwave Studio software.
Proc. SPIE. 10103, Terahertz, RF, Millimeter, and Submillimeter-Wave Technology and Applications X
KEYWORDS: Metamaterials, Resonators, Wavefronts, Control systems, Polarization control, Microwave radiation, Technologies and applications, Electromagnetism, Flexible displays, Current controlled current source
Metasurfaces represent the most promising class of metamaterials for real applications, whereby arbitrary wavefront and polarisation control can be achieved using just a single sub-wavelength layer. Therefore, allowing tunability over their capabilities is the next step to consolidate them as technology devices for light control. In our work we propose a new platform for creating tunable microwave devices based on gradient metasurfaces. Our study shows that the integration of a patterned elastic substrate in the design of functional metasurfaces is an effective approach to enable control over their electromagnetic properties.
To demonstrate the new platform, we propose, design and experimentally realize a novel tuning mechanism that controls the focal length of an electromagnetic metasurface lens by exploiting the degree of freedom provided by the flexible substrate, which enables continuous elongation of the system. When such a metasurface is uniaxially stretched, the distance between embedded electromagnetic resonators increases, producing a change in the phase profile created by these resonators, and this leads to a change of the focal distance of the lens. Thus, the flexible metasurface displays a functionality that can be continuously controlled by unidirectional mechanical loading. We fully characterize the spherical-like aberration phenomenon which accompanies the tuning process. Finally, our study reveals that an equidistant separation between the resonators leads to reduced device performance of the operational metasurface and, therefore, the utilization of other degrees of freedom is mandatory if the efficiency needs to be preserved.
We study the radiation patterns produced by a dipole placed at the surface of a nanofiber and oriented perpendicular to it, either along the radial (r-oriented) or azimuthal (Φ-oriented) directions. We find that the dipole induces an effective circular cavity-like leaky mode in the nanofiber. The first radiation peak of the Φ-oriented dipole contributes only to TE radiation modes, while the radiation of the r-oriented dipole is composed of both TE and TM radiation modes, with relative contribution depending on the refractive index of the nanofiber. We reveal that the field pattern of the first resonance of a Φ-oriented dipole is associated with a magnetic dipole mode and strong magnetic response of an optical nanofiber.
We demonstrate the use of liquid crystal infiltration of fishnet structures for the realization of highly tunable and
nonlinear optical metamaterials. We show that fishnet metamaterials infiltrated with nematic liquid crystals can exhibit
strong nonlinear response at moderate laser powers. We also show that this nonlinear response arises due to the
molecular orientation of the liquid crystal molecules and can be therefore be fine-tuned with an electric field, opening
new opportunities for electrically tunable nonlinear metamaterials.
We revisited the problem of the existence of plasmonic modes guided by metal-dielectric-metal slot waveguides. For the case of lossless slot waveguides, we classify the guided modes in the structure with the metal dispersion and found that, in a certain parameter range, three different guided modes coexist at a fixed frequency, two (symmetric and antisymmetric) forward propagating modes and the third, backward propagating antisymmetric mode. We study the properties of the forward and backward plasmonic guided modes in the presence of realistic losses, and discuss the importance of evanescent modes in lossy structures.
We discuss a novel tuning method based on continuous adjustment of metamaterial lattice parameters. This
method provides for remarkable tuning of transmission characteristics through a subtle displacement of metamaterial
layers. While the effective medium theory predicts correctly the general tuning characteristics, it turns
out that the particular tuning pattern is determined by the peculiarities of near-field interaction between the
metamaterial elements. We describe the modes of this interaction and provide qualitative explanations to the
performance observed numerically and experimentally.
Artificial structures with periodically modulated index of refraction such as photonic crystals offer novel ways of controlling light propagation due to the existence of a range of forbidden frequencies corresponding to a photonic bandgap. We discuss physical phenomena which do not allow for the existence of complete electromagnetic band gaps in dielectric structures with variation of the refractive index in one spatial dimension. We also demonstrate that the limitations imposed on one-dimensional dielectric structures can be removed by using the so-called left-handed materials, and that such a structure can posses a full photonic band gap.
We analyze nonlinear properties of microstructured materials with
negative refraction, the so-called left-handed metamaterials. We
demonstrate that the hysteresis-type dependence of the magnetic
permeability on the field intensity allows changing the material
properties from left- to right-handed and back. We predict also
that nonlinear left-handed metamaterials can support both TE- and
TM-polarized self-trapped localized beams, spatial electromagnetic
solitons. Such solitons appear as single- and multi-hump beams,
being either symmetric or antisymmetric, and they can exist due to
the hysteresis-type magnetic nonlinearity and the effective
domains of negative magnetic permeability.