Efficient unidirectional SPP launcher: coupling the SPP to a smooth surface for propagation.

We propose a unidirectional surface plasmon polariton (SPP) launcher with high coupling efficiency and long propagation length. The structure consists of a metallic slanted grating and a metal film that are separated by a SiO2 layer on a quartz substrate. By inserting the SiO2 layer, strong interaction between the metal grating and metal film can significantly increase the conversion of incident light into SPPs. Meanwhile, due to the characteristics of the smooth interface between metal film and substrate, the dissipation of SPP originating from the propagation process will be remarkably reduced. Numerical simulations show that this structure with 11 grating periods will obtain high contrast and efficiency of launching. Compared with the rough metal-quartz interface, the smooth one can improve the efficiency of light conversion to SPPs by more than 22.6% and extend the propagating distance by approximately 158%.