Novel approach to cloaking, which allows to realize directly the idea of wave guiding and to eliminate the reflection
from the cloaked structure, is proposed. Cloaking structure is composed of metal wires guiding TEM modes
around the object. In high conductive metal wires at microwave frequencies, the TEM modes are dispersionless
and the energy propagates along the wires. A plane wave incident onto a wire medium (WM) under any angle,
excites both TEM and TM modes, which have similar polarizations. The TEM modes provide full transmission
through the cloaking structure if the total length of wires equals a number of half-wavelengths. The TM mode
attenuates in WM at frequencies below the plasma frequency of WM and does not contribute to reflection if the
WM is dense enough. The angle between WM and the propagation direction of the incoming wave is chosen
so that the difference in paths of waves in WM and free space outside the cloak is a multiple of wavelengths in
order to eliminate distortion of the phase front. In the infrared range quasi-TEM modes, supporting propagation
of plasmons, play role of TEM modes. Parameters of WM are chosen so that the quasi-TEM modes have
low dispersion and their phase velocity is slightly less than the speed of light. Results of HFSS simulations
demonstrate considerable cloaking effect.
In recent years a lot of attention has been paid to metal nanoscale structures because of new phenomena and
potential applications in waveguide and antenna techniques. Especially in the optical region new effects arise
based on plasmon resonances. It is known that in the optical region some noble metals behave like free-electron
plasma with low losses. In this study field propagation in nanoporous metal structures is considered. We consider
propagation in regular arrays of pores in metal in the presence of an interface. Although the field is decaying
outside the pores, these inclusions are so close to each other that there is interaction with the neighboring pores.
In addition the metal-insulator interface causes coupling. Near the plasmonic resonance these interactions are
strong enough, and there exist guided wave modes along the array. Properties of these modes are investigated.
The allowed frequency range where the guided modes exist depends on the geometry, i.e., on the size of the pores
and on the distance between them. In such structures there exist three propagating modes, two transversely and
one longitudinally polarized. The transversely polarized fields propagate as forward waves and the longitudinally
polarized fields form a backward wave. When the chain of pores is far from the interface, the two transversely
polarized modes become decoupled and have the same dispersion due to degeneracy.
Interest of using metal wire structures as transmission-lines in frequencies much higher than microwave region is recently arised. In this theoretical study guided waves in circular metal wire is investigated in infrared frequency range or even in optical region for transmission-line applications. In this frequency region the
permittivity of the metal is not any more described by the conductivity of the metal but obeys the Drude model. The operational frequency region is between the electron scattering frequency and the plasma frequency. Just below the plasma frequency the permittivity is negative and almost a real number. The axial field components are written in terms of modified Bessel functions. The eigenvalue equation is evaluated for the guided modes by equating the continuity condition of the tangential field components at the interface of the free space and the wire region. The dispersion curves are calculated numerically and the propagation factors inside and outside of the wire region is illustrated as well as the propagation factor in axial direction. Also the field distribution in the cross-section of the structure is analysed. Analysing the properties of the propagating
fields in metal wire, it is found that some modes are more and more tightly bound into the surface of the metal wire when the frequency approaces to the plasma resonant frequency. Using typical values for metals, the proper radius of the wire is in a range of about a hundred nanometers which is much below the wavelength of the guided mode even in optical region. The effect is analogous to that for guided fields in microwave frequency range for the circularly symmetric mode propagating in the metal wire surrounded by a thin dielectric layer. Now the similar effect is occured in subwavelenth region due to plasma phenomena. In this study it is demonstrated that subwavelength metal wire structures may have applications as transmission-line structures in infrared or optical region.
Recently discovered extraordinary transmission of light through a thick metal plate perforated by regular hole array is in this paper studied for switching applications. Although this extraordinary transmission effect was first found in optical regime, it also occurs at microwave frequencies. In this study transmission of microwave radiation at normal incidence is theoretically studied through a semiconductor layer covered on both sides by a metal plate with regular hole array. At first, an analytical model for a metal plate perforated by dense regular hole array is presented. The metal hole array plate is modeled as a frequency dependent lumped inductance applicable for transmission-line modeling. Secondly, the properties of the semiconductor layer are considered. Silicon is a photoconducting material whose conductivity increases when exposed to optical illumination. With enough high optical illumination the perforated metal plate backed by semiconductor slab is like a metal conductor, and transmission through the structure is negligible. The whole structure, two perforated plates and the slab between them, is modeled using transmission-line techniques. The reflection and transmission coefficients are presented. Because the properties of the semiconductor slab can be controlled with optical illumination, we have optically controlled switch device for microwave transmission.