  In this work, we design a new tunable nanofocusing lens by the linear-variant
depths and nonlinear-variant widths of circular grating for far field practical
applications. The constructively interference of cylindrical surface plasmon
launched by the subwavelength metallic structure can form a
subdiffraction-limited focus, and the focal length of the this structures can
be adjusted if the each groove depth and width of circular grating are arranged
in traced profile. 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 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, superresolution optical
microscopic imaging, optical trapping, and sensing.
