The field of plasmonics has historically been a playground exclusively for the optics community. Primarily this is because
the response of metals becomes dominated by their large conductivities at much lower frequencies, making it difficult
to exploit the unique properties of surface plasmon (SP) modes. Indeed SPs on flat, perfectly conducting substrates
are better described as simple surface currents or grazing photons. However the realization that one can form metal-dielectric
composites to support surface waves with plasmon-like properties has opened the field of plasmonics to the
terahertz and microwave domains. Pendry et al. [Science, 305, 847 (2004)] were among the first to speculate about an
extension of plasmonics into long wavelength regimes. They demonstrated that the perforated surface of a perfect conductor
can support a SP-like mode whose behavior is determined purely by the geometry of the substrate. Beginning
with our initial experimental verification of these SP-like modes excited via grating-coupling, we present an overview of
some of our recent microwave studies. We progress to study the classical method of prism coupling and also consider
the enhanced transmission phenomenon (mediated by plasmon-like surface modes) through hole arrays. Finally the first
experimental evidence of coupled SP-like modes between two such perforated metal substrates placed in close proximity
will be presented.
Fabry-Perot cavities are perhaps the best known of the optical transmission resonators, with cavity field enhancement
accomplished by two parallel and partially reflecting planes. Recently it has been shown that arrays of narrow slits cut into a
metal substrate are similarly able to exhibit resonant transmission modes. Here, the transmission of normally incident plane
wave microwaves through a single stepped sub-wavelength slit in a thick metal plate is explored. The presence of the step
substantially increases the radiation wavelength, which may be resonantly transmitted to well beyond twice the plate
thickness. Insight into the resonant behaviour of the stepped slit is provided through the analysis of the field solutions
produced by a finite element model. This model also predicts resonant transmission which is in excellent agreement with the
Using liquid crystals to control the propagation of microwaves is a potentially interesting technology. By incorporating small amounts of liquid crystal in thin slat metal structures through which the microwaves may resonantly pass a whole new range of voltage tuned microwave devices are becoming available. Metallic sub-wavelength slit structures at microwave frequencies have been constructed which show Fabry-Perot type resonances in very thin slits. If the dielectric in such thin slits is an aligned liquid crystal it is found possible to voltage-control the resonant frequencies. Novel selective filters and structures for microwave beam steering have been fabricated leading to a new generation of liquid crystal controlled devices.
It is shown that microwave radiation can be transmitted through a wall of aluminum-alloy bricks even though the width of the gaps between the metallic elements is less than 5% of the radiation wavelength. Up to 90% of the radiation made incident upon the wall is transmitted, with both linear polarizations being passed. Experimental results are compared to theoretical predictions. Proving that the transmission mechanism relies upon self-coupled surface plasmon resonances in what are effectively Fabry-Perot cavities.