N. Katsarakis

Electric coupling to the magnetic resonance of split ring resonators

We study both theoretically and experimentally the transmission properties of a lattice of split ring resonators (SRRs) for different electromagnetic (EM) field polarizations and propagation directions. We find unexpectedly that the incident electric field E couples to the magnetic resonance of the SRR when the EM waves propagate perpendicular to the SRR plane and the incident E is parallel to the gap-bearing sides of the SRR. This is manifested by a dip in the transmission spectrum. A simple analytic model is introduced to explain this interesting behavior.

Magnetic response of split-ring resonators in the far-infrared frequency regime.

We report on the fabrication, through photolithography techniques, and the detailed characterization, through direct transmission measurements, of a periodic system composed of five layers of photolithographically aligned micrometer-sized Ag split-ring resonators (SRRs). The measured transmission spectra for propagation perpendicular to the SRRs plane show a gap around 6 THz for one of the two possible polarizations of the incident electric field; this indicates the existence of a magnetic resonance, which is verified by detailed theoretical analysis. To our knowledge this is the first time that a system of more than one layer of micrometer-sized SRRs has been fabricated. The measured optical spectra of the Ag microstructure are in very good agreement with the corresponding theoretical calculations.

Flexible chiral metamaterials in the terahertz regime: a comparative study of various designs

Five different chiral metamaterials in the terahertz (THz) regime, fabricated on fully flexible polyimide substrates, are comparatively studied via numerical calculations and experimental measurements. The chiral properties of these metamaterials, which are discussed based on their optical activity, circular dichroism, and the retrieved effective parameters, show pronounced pure optical activity (larger than 300°/wavelength), as well as important circular polarization generation and filtering capabilities. Negative refractive index is also obtained for all the considered designs.

Experimental demonstration of negative magnetic permeability in the far-infrared frequency regime

Using transmission and reflection measurements in a five layer micrometer-scale split-ring resonator (SRR) system, fabricated by a photolithography procedure, the authors demonstrate the occurrence of a negative magnetic permeability regime in that system at ∼6THz. The transmission and reflection were measured using oblique incidence, resulting to a magnetic field component perpendicular to the SRR plane, which excites the resonant circular currents constituting the magnetic resonance.

Optically controllable THz chiral metamaterials

Switchable and tunable chiral metamaterial response is numerically demonstrated here in different uniaxial chiral metamaterial structures operating in the THz regime. The structures are based on the bi-layer conductor design and the tunable/switchable response is achieved by replacing parts of the metallic components of the structures by photoconducting Si, which can be transformed from an insulating to an almost conducting state through photoexcitation, achievable under external optical pumping. All the structures proposed and discussed here exhibit frequency regions with giant tunable circular dichroism, as well as regions with giant tunable optical activity, showing unique potential in the achievement of active THz polarization components, like tunable polarizers and polarization filters.

Self-organization approach for THz polaritonic metamaterials

In this paper we discuss the fabrication and the electromagnetic (EM) characterization of anisotropic eutectic metamaterials, consisting of cylindrical polaritonic LiF rods embedded in either KCl or NaCl polaritonic host. The fabrication was performed using the eutectics directional solidification self-organization approach. For the EM characterization the specular reflectance at far infrared, between 3 THz and 11 THz, was measured and also calculated by numerically solving Maxwell equations, obtaining good agreement between experimental and calculated spectra. Applying an effective medium approach to describe the response of our samples, we predicted a range of frequencies in which most of our systems behave as homogeneous anisotropic media with a hyperbolic dispersion relation, opening thus possibilities for using them in negative refractive index and imaging applications at THz range.

Negative index short-slab pair and continuous wires metamaterials in the far infrared regime

Using transmission and reflection measurements under normal incidence in one and three layers of a mum-scale metamaterial consisting of pairs of short-slabs and continuous wires, fabricated by a photolithography procedure, we demonstrate the occurrence of a negative refractive index regime in the far infrared range, ~2.4-3 THz. The negative index behavior in that system at ~2.4-3 THz is further confirmed by associated simulations, which are in qualitative agreement with the experimental results.

Parametric investigation and analysis of fishnet metamaterials in the microwave regime

We study through experiments and associated simulations the electromagnetic response of microwave fishnet metamaterials and its dependence on the systems geometrical parameters. Our study verifies the validity of an earlier proposed inductor-capacitor (LC) circuit description of the fishnet design and reveals a left-handed response with high transmittance for a wide variety of geometrical structure parameters. This study paves the way to achieve optimized left-handed fishnet metamaterial designs.

Laser-machined layer-by-layer metallic photonic band-gap structures

Metallic photonic band-gap crystals operating in the microwave frequency range were fabricated by laser precision machining. They consist of stainless steel plates with a tetragonal lattice of holes and a lattice constant of 15 mm. Transmission measurements show that periodic crystals exhibit a cutoff frequency in the 8–18 GHz range, below which no propagation is allowed. The cutoff frequency can be easily tuned by varying the interlayer distance or the filling fraction of the metal. Combinations of plates with different hole diameters create defect modes with relatively sharp peaks, which are tunable. The experimental measurements are in good agreement with theoretical calculations.

Electromagnetic response of anisotropic eutectic metamaterials in THz range

We study the electromagnetic (EM) response of anisotropic eutectic metamaterials, consisting in cylindrical polaritonic LiF rods embedded in a KCl host. The specular reflectance of the samples was measured at far infrared (3–12 THz). The sample reflection was simulated by modeling the eutectic structure and solving numerically Maxwell equations for the EM fields. The reflectance was also calculated from simple effective response functions models. A good agreement was obtained between experimental and calculated spectra. From the effective response functions calculations, we obtained a range of frequencies in which the system behaves as a homogeneous effective anisotropic media, with a hyperbolic dispersion relation, opening possibilities for negative refraction and focusing applications.