Manipulating microwaves with ‘spoof’ surface plasmons

Abstract

The newly discovered ability to confine, guide, and manipulate microwaves in open structures on scales that are much smaller than their wavelengths appears to be heralding a new era in microwave technology. Since the days of Marconi, metal surfaces have been known to support surface electromagnetic waves. In many ways, such waves are identical to simple grazing radiation because the electromagnetic fields in the air above the planar metal surface extend over distances of order many tens of wavelengths and are almost completely excluded from the metal. Since these waves are only weakly localized at the surface, they are readily perturbed by objects placed some distance away. They are consequently not particularly useful for forming surface-guided wave structures. However, if the distance over which the electromagnetic field extends above the metal is considerably reduced, so that the mode is much more strongly bound to the surface, ideally over distances of less than the wavelength of the incident radiation, then surface-wave manipulation of microwaves is more readily achieved. By patterning a metal surface with grooves or holes with a characteristic dimension less than the wavelength of the incident radiation, one can alter the electromagnetic boundary conditions to strongly localize microwave radiation to that surface (see Figure 1). This highly localized wave is—using language borrowed from optical technology—a ‘spoof’ or ‘designer’ surface plasmon. In the visible regime, a surface plasmon is the trapped surface wave associated with density oscillations of the conduction electrons. (Observations of surface-plasmon excitation in the visible domain were first recorded, unknowingly, by Wood in 1902, who found that—for light impinging on a metal grating—a strong absorption of power occurred for a particular angle of incidence.) For the visible region, the degree of trapping of the mode at the metal surface and the usefulness of the surface plasmon depend strongly on the metal used. Figure 1. Schematic representation of electric fields associated with a mode propagating along the surface of a metal. (a) At microwave frequencies, the metal is almost perfectly conducting and the field (Ez) extends far beyond the metal. (b) By perforating the substrate with an array of subwavelength holes, the field is localized near the interface.