Three dimensional Luneburg lens at optical frequencies
A Luneburg lens is a fascinating gradient refractive index (GRIN) lens that can focus parallel light on a perfect point without aberration in geometrical optics. Constructing a three dimensional (3D) Luneburg lens at optical frequencies is a challenging task due to the difficulty of fabricating the desired GRIN materials. Here, we present the practical implementation of a 3D Luneburg lens at optical frequencies. Such a 3D Luneburg lens is designed with GRIN 3D simple cubic metamaterial structures, and fabricated with dielectric metamaterials by femtosecond laser direct writing in the commercial negative photoresist IP L. Simulated and experimental results exhibit an interesting 3D ideal focus for the infrared light. The protocol for developing the 3D Luneburg lens with ideal focus would prompt the potential applications in integrated light coupled devices and lab on chip integrated biological sensors based on infrared light.