Deep subwavelength optical nanoantenna with enhanced characteristics for near and far field applications

Extraordinary optical transmission through a subwavelength aperture was discovered more than a decade ago. A single subwavelength aperture surrounded by a finite array of grooves on a thin metallic film is a design used by many authors to show subwavelength focusing. In this paper, a modified version of this design is introduced that, to the best of our knowledge, gives best results of this design in terms of the peak power and the full width at half maximum FWHM in the near field as well as the far field of the lens. Numerical simulations using Finite-Difference Time-Domain (FDTD) method coupled with perfectly matched layer (PML) boundary conditions verify that the proposed metallic lens can give a near (far) field focal point 125 nm (1.39 μm) away from lens with FWHM of 245 (299) nm at incident wavelength of 760 (610) nm with power enhancement of at least 2 times over the unmodified design. The dependence of this resonant focusing ability with a certain geometrical parameter defining the modified structure is extensively analyzed in the visible range of spectrum. Such a focusing plasmonic device has potential practical applications like NSOM and FSOM due to the simplicity in its design and fabrication and due to superior results in near and far fields of the lens.