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Luneburg lens is a fascinating gradient refractive index (GRIN) lens that can focus parallel light rays on a perfect geometrical point without aberration in geometrical optics, compared with conventional lens made of homogeneous dielectric materials with specially designed surfaces. Constructing a three dimensional (3D) Luneburg lens at optical frequencies is a challenging task due to the difficulty of fabricating the desired GRIN materials in the lab. Reported experimental realizations of Luneburg lens at optical frequencies are mainly based on two dimensional metamaterials structures, or plasmonic structures. Here, we present the first practical implementation of the 3D version of Luneburg lens at optical frequencies based on the effective medium approximation. The 3D Luneburg lens is designed with GRIN 3D simple cubic metamaterials (SCMs) structures, and fabricated with no-resonant dielectric metamaterials by laser direct writing method in the commercial negative photoresist IP-L. We have experimentally demonstrated the feasibility of tailoring inhomogeneous metamaterials structures to realize 3D Luneburg lens with the effective GRIN profiles. The effective refractive index has been spatially and gradually modified by tailoring the volume filling fraction of SCMs structures. Simulated and experimental results simultaneously exhibit interesting 3D ideal focusing performance of the 3D Luneburg lens for the infrared light at wavelengths of 6.25m. This study would provide the protocol for developing the 3D Luneburg lens with wide field-of-view and ideal focusing theoretically and experimentally, which would further prompt the potential applications in integrated light-coupled devices and lab-on-chip integrated biological sensors based on infrared light.
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