Simulation and Testing of Metallic Aperture Arrays at Terahertz Frequencies

Abstract

Since the demonstration of Extraordinary Optical Transmission (EOT) in sub-wavelength aperture arrays, first at optical then at terahertz (THz) frequencies, the role of Surface Plasmon Polaritons (SPPs) in this phenomenon has been under investigation. By studying the interaction between THz radiation and free standing metal foils, which contain periodic arrays of apertures, this thesis explores the interaction between rectangular waveguided modes controlled by aperture dimensions and SPP modes defined by aperture periodicity.
Fabrication of a free standing metal foil perforated by micron scale 5:1 aspect ratio rectangular holes with varied array spacings is performed by photolithography and electroplating. Such free standing metal foils are shown to demonstrate EOT at THz frequencies when studied by terahertz time-domain spectroscopy (THz-TDS). Results obtained by the systematic variation of aperture size and lattice spacing are presented, accompanied by finite-difference time-domain (FDTD) simulation data. The changing aperture array dimensions allow for the isolation of the different resonant modes present in the EOT phenomena.
Further exploitation of the interaction between the two resonant modes is provided by altering the incident angle between the free standing metal aperture array and the THz field. The high frequency resolution provided by THz Vector Network Analyser measurements allows for small changes in transmission spectra to be observed as the resonant modes are brought into close proximity.
The removal of a fixed periodic spacing between the sub-wavelength apertures perforating the metal foil removes a necessary component of SPP excitation. The design of an array of aperiodic sub wavelength apertures is presented. This array demonstrates EOT in the absence of SPPs.
Furthermore, SPPs induced by the periodic x-axis aperture spacing, and controlled by THz field incidence angle, are used to extinguish the sharp transmission resonance produced by the sub-wavelength apertures. To this author’s knowledge this is the first reporting of resonant transmission extinguished by SPP excitation and manipulation.