  Manipulation of spoof surface plasmons (SSPs) has recently intrigued enormous
interest due to the capability of guiding waves with subwavelength footsteps.
However, most of the previous studies, manifested for a single functionality,
are not suitable for multifunctional integrated devices. Herein, a bifunctional
Luneburg-fisheye lens is proposed based on a two-dimension metal pillar array.
Firstly, by tuning the geometric dimension of the metal pillars in the array,
its ability to precisely manipulate the excited SSPs along one direction is
confirmed, achieving subwavelength focusing and imaging with the resolution up
to 0.14 times the wavelength. Then, separately controlling the propagation of
the SSPs along the orthotropic directions is further implemented, and the
bifunctional Luneburg-fisheye lens is realized. The bifunctional lens is
characterized as a Luneburg one along the x-axis, whereas in the y-axis, it
presents the properties of a Maxwell fisheye lens. The experimental results
almost immaculately match with the simulation ones. This bifunctional lens can
validly reduce the system complexity and exert flexibility in multifunctional
applications, while the proposed metal pillar-based design method broadens the
application range of gradient refractive-index lens in the microwaves,
terahertz, and even optical ranges.
