Linearly polarized microwave radiation is shown to have its plane of polarization converted to the orthogonal state upon reflection from an ultrathin (λ/25) cavity array. The structure benefits from an uncomplicated design consisting of a metallic grating closely separated from a ground plane by a dielectric spacer. A single set of periodically spaced slits (monograting) exhibits polarization conversion when the normally incident electric field is aligned at 45° to the slits. Two orthogonal sets of slits (bigrating) allows this narrow-band effect to be broadened when the two orthogonal resonances are separated in frequency. We optimise the design and experimentally demonstrate near loss-less polarization conversion (95% of the incident intensity) across a 3.1 GHz frequency band. Finally, we study the dependence of the structure's performance on incident angle and slit width.

We present a thin acoustic metamaterial absorber, comprised of only rigid metal and air, that gives rise to near unity absorption of airborne sound on resonance. This simple, easily fabricated, robust structure comprising a perforated metal plate separated from a rigid wall by a deeply subwavelength channel of air is an ideal candidate for a sound absorbing panel. The strong absorption in the system is attributed to the thermo-viscous losses arising from a sound wave guided between the plate and the wall, defining the subwavelength channel.

Phase resonances are observed in the microwave response of metal transmission gratings comprised of identical but alternately orientated tapered slits. Despite each slit having identical dimensions, and being equally spaced from its neighbors, these high-Q mode are nonetheless supported even at normal incidence. The excitation of these modes is attributed to the interference between FabryPerot-like modes and diffractively coupled surface waves.

The concept of pseudo-plasmonic surfaces at microwave frequencies is extended to include structures with a high degree of surface anisotropy. The experimental sample is fabricated by patterning a metal with a rectangular array of rectangular holes and is found to support structurally dictated surface plasmon-like modes; the anisotropic dispersion of which results from an ellipsoid of limiting frequencies. By exploiting the anisotropy of the unit cell, the family of higher order surface waves associated with the quantization of the electromagnetic fields within the holes is also explored.

Surface waves at microwave frequencies are excited on a zigzag metasurface formed from unit cells comprised of a pair of identical, alternately oriented, rhombic-shaped metal tubes. This particular symmetry leads to diffractive excitation, with polarization selectivity of different orders, of a family of surface waves. Furthermore, the expected band gap at the first Brillouin zone boundary is absent.