Enhanced confined microwave transmission by single subwavelength apertures
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Grating-coupling phenomena between surface plasmons and electromagnetic waves are studied in the microwave spectrum using metallic circular apertures surrounded by an array of grooves. The measurements are performed in the microwave spectrum of 10-18 GHz, corresponding to a wavelength region of 16.7-30 mm. The metallic samples have a subwavelength hole with a diameter of 8 mm and have concentric grooves with a periodicity of 16 mm. We first present the experimental and theoretical results of enhanced microwave transmission though a subwavelength circular aperture with concentric periodic grooves around the surface plasmon resonance frequency. This is followed by transmission studies through circular annular apertures with and without concentric periodic grooves around the aperture. We demonstrate a 145-fold enhancement factor could be obtained with a subwavelength circular annular aperture surrounded by concentric periodic grooves. Moreover, we study the diffraction of electromagnetic waves from subwavelength metallic circular annular apertures in the microwave spectrum. The theoretical and experimental demonstration of the near- and far-field EM distributions for subwavelength circular apertures and circular annular apertures surrounded by concentric periodic grooves is reported. We present the angular transmission distributions from circular apertures and circular annular apertures surrounded by concentric periodic grooves. At the surface mode resonance frequency the transmitted electromagnetic waves from the subwavelength circular annular aperture surrounded by concentric periodic grooves have a strong angular confinement with an angular divergence of ±3°. This represents a fourfold reduction when compared to the angular divergence of the beam transmitted from a subwavelength aperture. These results show, that not only high transmission but also a confined beam is achieved at the surface plasmon resonance frequency using a circular annular aperture with grooves .