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Recently, nanostructures composed of tapered apertures have been researched for electromagnetic field enhancement. Tapered plasmonic aperture antenna can concentrate transverse electric dipoles and longitudinal magnetic dipoles in tiny volume of plasmonic waveguide or metasurface. It is an important element for efficient nonlinear optics, modification of spontaneous decay rates, and sensitive nanophotonic sensors to simultaneously intensify both of electric and magnetic fields. However, even though their advantages include apparent theory based on constructive interference of surface plasmons and relatively simple fabrication, the enhancement performances are not strong as much as those of conventional bridged bowtie aperture antenna. Here, we propose a novel design with reflection type metasurface patterned by a funnel-shaped waveguide cavity array. This supports longitudinal cavity mode along the direction of incident light in addition to perimeter cavity mode or transverse cavity mode of conventional tapered apertures. Longitudinal cavity mode contributes to electric and magnetic fields on transverse plane and transverse cavity mode with funnel shape induces strong circulating currents around tiny volume that generate magnetic dipoles on longitudinal plane. To demonstrate our proposed design, we carry out three-dimensional finite element method for numerical calculation. It shows resonant average (maximum) enhancements of electric and magnetic intensities reach about 177 (1484) and 91 (274), respectively, at the wavelength of 1120 nm. Furthermore, the enhancement of spontaneous decay rates and Purcell effect are verified numerically. Our design can offer a new approach of various nano-optical applications such as biochemical sensor, nonlinear optics, and photoluminescence application.
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