Abdelwaheb Ourir

All-metamaterial-based subwavelength cavities (λ∕60) for ultrathin directive antennas

In this letter, we present the characterization and modeling of a metamaterial-based resonant cavity for ultrathin directive printed antennas. A planar artificial magnetic conductor is used for the two reflectors of the Fabry–Perot-type resonant cavity. One reflector behaves as a high impedance surface, and serves as a substrate for the printed antenna. The other reflector is a partially reflective surface used as a transmitting window. The cavity is operated on subwavelength modes, the smallest cavity thickness being of the order of λ∕60. A drastic enhancement of the antenna directivity and gain is obtained over a relatively wide band from 7.5to10.1GHz, corresponding to a range of cavity thicknesses from ∼λ∕3 to ∼λ∕60. The cavity resonance is seen to be correctly predicted from the standard ray theory approach.

TUNABLE TRAPPED MODE IN SYMMETRIC RESONATOR DESIGNED FOR METAMATERIALS

The excitation of an antisymmetric trapped mode on a symmetric metamaterial resonator is experimentally demonstrated. We use an active electronic device to break the electrical symmetry and therefore to generate this trapped mode on a symmetric spilt ring resonator. Even more, with such a tunable mode coupling resonator, we can precisely tune the resonant mode frequency. In this way, a shift of up to 15 percent is observed.

Tunneling of electromagnetic energy in multiple connected leads using ϵ-near-zero materials

A realization of a reflectionless power splitter is proposed by use of a metamaterial junction. To design the junction, the electromagnetic wave transmission in multiple connected leads is investigated theoretically and numerically. A closed analytical form is derived for the scattering matrix of any geometry of the interconnected leads. We show that the use of a junction made of ϵ-near-zero (ENZ) material allows production of perfect transmission. This can be achieved by reducing the area of the ENZ junction (squeezing effect) and by tuning the widths of the output leads with respect to the input lead. It is also shown that the same effect is obtained without squeezed junction by using a match impedance zero index material (MIZIM junction).

Negative refractive index in symmetric cut-wire pair metamaterial

Plasmonic metamaterial with a negative refractive index is achieved in a periodical symmetric cut-wire pair without any structural asymmetry. The main control parameter is the lateral lattice size. The designed structure presents two discrete resonance modes due to hybridization. The electric resonance depends strongly on the structural parameter, while the magnetic resonance remains unchanged with this parameter. The hybridization scheme can be inverted by increasing the lateral lattice width. Theoretical and experimental results demonstrate a negative refraction index for the designed structure when the electric resonance frequency occurs below the magnetic one. This spatial arrangement presents many advantages since it is easier, repetitive and well-adapted for realizations in microwave and optical regimes.

Discontinuous wavelength super-refraction in photonic crystal superprism

Experimental results on wavelength-dependent angular dispersion in InGaAsP triangular lattice planar photonic crystals are presented. An abrupt variation of the angular dispersion is observed for TM-polarized waves whose frequencies are comprised between those of the fourth and sixth allowed bands. According to the crystal period, the measured angle of refraction is found to either decrease or increase by 30 degrees within a wavelength range smaller than 30 nm. Experimental results are reproduced well from 2D finite difference time domain calculations. The observed phenomena are interpreted from the coupling of the incident light to different modes of the photonic crystal that travel with different group velocities and propagate in different directions within the crystal. Mode dispersion curves and mode patterns are calculated along with isofrequency curves to support this explanation. The observed discontinuous wavelength super-refraction opens a new approach to the application of superprisms.

Far field subwavelength imaging of magnetic patterns

Far field imaging of subwavelength magnetic objects in real time is a very challenging issue. We propose an original solution based on a planar array of closely spaced split ring resonators. Hybridization between the resonators of such metalens induces subwavelength modes with different frequencies. Thanks to these high Q resonating modes, Purcell like effect allows an evanescent source, close to the metalens, to emit waves that can be collected efficiently in the far field. We present the first microwave experimental demonstration of such metalens to image of a subwavelength magnetic pattern. Numerical simulation shows that this approach is still valid at THz frequencies.

Time Reversal in Subwavelength-Scaled Resonant Media: Beating the Diffraction Limit

Time reversal is a physical concept that can focus waves both spatially and temporally regardless of the complexity of the propagation medium. Time reversal mirrors have been demonstrated first in acoustics, then with electromagnetic waves, and are being intensively studied in many fields ranging from underwater communications to sensing. In this paper, we will review the principles of time reversal and in particular its ability to focus waves in complex media. We will show that this focusing effect depends on the complexity of the propagation medium rather than on the time reversal mirror itself. A modal approach will be utilized to explain the physical mechanism underlying the concept. A particular focus will be given on the possibility to break the diffraction barrier from the far field using time reversal. We will show that finite size media made out of coupled subwavelength resonators support modes which can radiate efficiently in the far field spatial information of the near field of a source. We will show through various examples that such a process, due to reversibility, permits to beat the diffraction limit using far field time reversal, and especially that this result occurs owing to the broadband inherent nature of time reversal.

High Impedance Surfaces Based Antennas for High Data Rate Communications at 40 GHz

Millimeter wave High Impedance Surfaces (HIS) based antennas are designed, fabricated, and characterized for high data rate communications at frequencies around 40GHz. HIS with difierent flnite surface area sizes are used as a ground plane for the microstrip patch antennas to suppress the surface waves. The antenna measurements and full wave electromagnetic simulations demonstrate a wide bandwidth of 12{15% in the frequency range of 38{44GHz with a high gain of »6dB and a very low cross polar contribution better than i20dB.

Homogenization of ultrathin metallo-dielectric structures leading to transmission conditions at an equivalent interface

We present a method of homogenization of thin metallo-dielectric structures as used in the design of artificial surfaces, or metasurfaces. The approach is based on a so-called matched asymptotic expansion technique, leading to parameters being characteristic of an equivalent interface associated to jump conditions. It is applied to an array of metal strips on top of a metal-backed dielectric slab, with the strips having a small but possibly finite thickness. Solving the equivalent problem provides explicit expressions (1) of the reflection coefficient for a wave at oblique incidence and (2) of the dispersion relation of the surface waves. The results are shown to be in agreement with results coming from the transmission line theory in the limit of vanishing thickness of the metallization and for normal incidence of the wave. The influence of the finite thickness of the metallization is exemplified and validated by comparison with full wave simulations.

Classical homogenization to analyse the dispersion relations of spoof plasmons with geometrical and compositional effects

We show that the classical homogenization is able to describe the dispersion relation of spoof plasmons in structured thick interfaces with periodic unit cell being at the subwavelength scale. This is because the interface in the real problem is replaced by a slab of an homogeneous birefringent medium, with an effective mass density tensor and an effective bulk modulus. Thus, explicit dispersion relation can be derived, corresponding to guided waves in the homogenized problem. Contrary to previous effective medium theories or retrieval methods, the homogenization gives effective parameters depending only on the properties of the material and on the geometry of the microstructure. Although resonances in the unit cell cannot be accounted for within this low-frequency homogenization, it is able to account for resonances occurring because of the thickness of the interface and thus, to capture the behaviour of the spoof plasmons. Beyond the case of simple grooves in a hard material, we inspect the influence of tilting the grooves and the influence of the material properties.