Focusing behavior of 2-dimensional plasmonic conical zone plate

A conical configuration plasmonic zone plate based on Fresnel zones made up of Au thin film slits is proposed for focusing in the free space with visible illumination. The surface plasmons enable propagation of radiating modes to distances equal to several wavelengths of the illumination field. Through numerical simulations, the conical structure found to yield focal spot beating the diffraction barrier encountered by conventional focusing elements. The focal spot size measured as full-width at half-maximum (FWHM) is observed to be as small as 0.31 times the illumination wavelength at the focal distance of 8 wavelength. Moreover, the simple design rules make it possible to predict and control the focal distances accurately.