Far-Field Tunable Nano-Focusing Based on Metallic Slits Surrounded with Nonlinear-Variant Widths and Linear-Variant Depths of Circular Dielectric Grating

In this work, we present a new design of a tunable nanofocusing lens using a circular grating of linear-variant depths and nonlinear-variant widths. Constructive interference of cylindrical surface plasmon launched by the sub-wavelength metallic structure forms a sub-diffraction-limited focus, the focal length can be adjusted by varying the geometry of each groove in the circular grating. According to the numerical calculation, the range of focusing points shift is much more than other plasmonic lens, and the relative phase of emitting light scattered by surface plasmon coupling circular grating can be modulated by the nonlinear-variant width and linear-variant depth. The simulation result indicates that the different relative phase of emitting light lead to variant focal length. We firstly show a unique phenomenon for the linear-variant depths and nonlinear-variant widths of the circular grating that the positive change and negative change of the depths and widths of grooves can result in different of variation trend between relative phases and focal lengths. These results paved the road for utilizing the plasmonic lens in high-density optical storage, nanolithography, super-resolution optical microscopic imaging, optical trapping, and sensing.