Design of wide-angle broadband Luneburg lens based optical couplers for plasmonic slot nano-waveguides

Gradient index (GRIN) structures have attracted great interests since their invention. Especially, the recent advance in the fields of transformation optics, plasmonics, and nanofabrication techniques has opened new directions for the applications of GRIN structures in nano-photonic devices. In this paper, we apply Luneburg lens and its transformed counterpart to realize efficient coupling to plasmonic nano-waveguides. We first briefly present the general structures of Luneburg lens and generalized Luneburg lens, as well as the design process of flattened Luneburg lens applying quasi-conformal mapping techniques. After that, we study the performance of these lenses for coupling electromagnetic signals to nano-waveguides (the metal-insulator-metal nano-waveguide). Different coupling schemes are investigated. It is found that the proposed Luneburg lens based optical couplers can be used to provide broadband light couplings to plasmonic nano-waveguides under wide incident angles.