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

Abstract

Abstract We numerically study the excitation of a Symmetry-Protected Vertical Mode (SPVM) 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 Electro Magnetic (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 particularly sensitive to mechanical perturbation (acoustic wave for instance) of the geometry, enabling the design of highly efficient THz modulators operating at λ ≈ 300 μm with a sensitivity as great as 12.4 μm/◦ of the tilt angle of its metal core.