Millimeter-wave extraordinary transmission: connection to metamaterials and technological applications

Abstract

The phenomenon of Enhanced Transmission also happens in the millimeter wave range, as it happened in optical frequencies. This experimental fact shows that this is a phenomenon mostly linked to the periodic structure regardless of the model used to describe the metal. The number of illuminated holes is a key parameter to observe the Enhanced Transmission band. Measurements in Fresnel zone show a weak band, and with the farfield illumination a good level is obtained. It has been shown that ET is mainly governed by one of the transversal periodicities, the one in the direction of the electric field (under normal incidence). The presence of dielectrics can produce a great field confinement and therefore a more efficient illumination of holes. Enhancement of transmission through a narrow slot on a metallic plane achieved by corrugating the metallic plane has been experimentally proven in the range of microwaves and millimeter waves. This result links the enhancement to the geometry of the metallic substrate rather than to the metallic model. It has been checked the ability to produce a strong beaming at broadside in the configuration of corrugations drilled on the output face. Several low-profile and all-metallic antenna prototypes have been designed and measured in the microwave range. The feeding is made by means of a waveguide. By changing the central (ideally) infinite slit to a finite slot several improvements can be done, fundamentally the thickness of the antenna can be reduced and a dual-band operation can be achieved. A great variety of farfield characteristics can be obtained by changing the number of corrugations. High gain antennas can be attained by using annular corrugations, the socalled Bull’s-Eye antenna. Further improvements deal with the insertion of dielectric inside the corrugations and with changes in the shape of the corrugations, for example sinusoidal profile. An artificial waveguide defined by introducing a set of parallel electric and magnetic conductors can be employed to analyze the diffraction problem of an incident plane wave normally to a sub-wavelength hole array. Evanescent higher order modes play a key role in the ET effect. Even more unexpected results appear if a periodic structure is made by stacking several of such plates: a left-handed metamaterial can be achieved by the periodic stacking of sub-wavelength hole array plates to form a photonic band-gap structure. The stack period with LH behavior can be made much smaller than the operating wavelength, and therefore it can be safely stated that the structure works in that dimension as an effective metamaterial. Simulation and experimental results presented show that left-handed propagation effects appearing in the band where EOT happens can be allowed or inhibited by a proper engineering of the band gap position of the photonic crystal made of stacked subwavelength hole-arrays. In the transition from LH to RH behavior a nearly zero slope band is observed, which can evidence a frozen mode propagation regime inside the structure. A simplified model based on inverse line equivalent circuit has been exposed to explain the LH and RH behavior. The stacked hole array structure can be engineered to construct prisms with anomalous behavior. Other geometries such as parabolic lenses can also be achieved. It has been seen that surfaces made by the periodical arrangement of Split- Ring Resonators (SRRs) and its complementary particle (CSRRs) illuminated by a plane wave show a high variety of cross-polarization effects. The analytical discussion based on the homogenization principle has been compared with the experimental results. It is able to catch the qualitative features in the response of the screens. The complementarity in the response of SRRs and CSRRs has been checked. Applications of the studied devices to frequency selective surfaces, polarizers and polarization converters can be envisaged. The existence of electroinductive waves (EIWs) in chains of electrically coupled CSRRs has been demonstrated both theoretically and experimentally. The duality between EIWs and previously reported MIWs has been discussed, and the ability of long CSRR chains to transport electromagnetic energy along many periods has been shown. A practical transducer between electromagnetic and EIWs in planar technology has been proposed and analyzed. Regarding practical applications, EIWs can be an alternative to MIWs for the guidance of electromagnetic energy, as well as for the design of couplers, delay lines and other planar devices, when electric couplings rather than magnetic couplings are imposed, or simply desired for the design. Taking as a basis the resonators used to implement an EI waveguide, a further step has been given in the design of a UWB filter in microstrip technology. This filter can be improved by inserting CSRRs to reject the higher frequencies. A final word fundamentally related to the ET results presented here: The reported results have been achieved for the millimetre range, but similar results are expected to happen at optical frequencies since extraordinary transmission has been shown at optical frequencies and the kind of structure presented here will present low losses in higher frequency regime. The control of the EOT-LHM could lead to a new class of practical devices both in the microwave and in the optical range.