Excitation of symmetry-protected vertical modes through arrays of coaxial sub-λ apertures for THz applications

Abstract We numerically study the excitation of a specific Symmetry-Protected (SP) mode within a specific periodic structure of coaxial apertures made in gold. The geometrical parameters are chosen to operate in the THz domain. Contrarily to classical SP modes that correspond to a mode propagating in the direction of periodicity of the structure, here the SP mode consists on the Transverse ElectroMagnetic (TEM) guided mode that propagates vertically inside the coaxial aperture along the metal thickness. This feature makes the spectral properties of the excited mode less-sensitive to the spatial extension of the grating allowing thus the design of less-cumbersome devices. We demonstrate that the spectrally localized resonance corresponding to this mode is specially sensitive to mechanical perturbation (acoustic wave for instance) of the geometry allowing to design very efficient THz modulators operating at λ ≈ 300 µm with sensitivity as large as 12.4 µm/° of the tilt angle of its metallic central part.