Surface Waves on Periodic Structures at Microwave Frequencies

Abstract

Experimental investigations of structurally-dictated surface waves supported by periodically textured metallic substrates with different symmetries, are the primary focus of the work presented in this thesis. The electromagnetic response of three near perfectly conducting substrates perforated with arrays of holes with different geometries,together with a low-profile high-impedance structure are characterised. Experimental measurement techniques are employed to record the transmission, and reflection from the structures under investigation, together with phase-resolved measurements to directly obtain the dispersion of the surface waves supported by these structures. From these measurements information about the nature of the surface modes supported by the structures under investigation can be observed. A study of diffractively coupled surface waves supported by a close-packed array of square cross-section, close-ended holes in the limit where the wavelength of incident radiation and periodicity of the hole array are comparable, is presented. An additional grating, which has a periodicity comparable to the hole array is used to control the strength of diffractive coupling to the mode. Using a free-space measurement technique,information about the dispersion of the modes supported by the structure is obtained by recording the azimuthal-dependent reflection from the structure. It is found that the relative positions of the hole array and `coupling-in' grating is significant, a key issue not addressed in the literature when investigating grating-coupling to surface modes. Good agreement with numerical predictions is demonstrated. Structurally-dictated surface waves on a metallic substrate pierced by a close-packed array of deep, rectangular holes is characterised. In this arrangement, the fundamental resonance in the holes in the orthogonal directions is different and the frequency therefore to which the dispersion of the surface waves supported by the structure is limited, varies with sample orientation. The anisotropic dispersion, resulting from an ellipsoid of limiting frequencies, is directly mapped using a phase-resolved measurement technique. Furthermore by exploiting the anisotropy of the unit cell, a family of higher order surface waves associated with the quantisation of the electromagnetic fields within the holes is explored in this chapter. Once again good agreement with numerical predictions is shown.The `enhanced transmission' recorded through a `zigzag' hole array, attributed to the excitation of diffractively coupled surface waves, is explored. Due to the specific symmetry of the unit cell of the zigzag hole array it is shown that coupling to these surface waves can be achieved with both transverse magnetic and transverse electric polarised incident radiation. Further, incident radiation can directly couple to the surface modes supported by the zigzag hole array, via scattering from its inherent in-plane periodicity. The observed polarisation-selective excitation of individual surface wave bands, agrees well with numerical predictions and is shown to be a direct consequence of the reduced symmetry of the system. Finally, the dispersion of the modes supported by an ultra-thin, high-impedance surface in the form of a Sievenpiper `mushroom' structure, with rectangular geometry is directly recorded. The behaviour of the Sievenpiper structure is rather complex and to aid understanding of the electromagnetic response of the structure, the results are compared with the modes supported by a simpler patch array structure. The anisotropy arising from the rectangular geometry is characterised and an in depth discussion of the origin of the modes presented.