Compact aperiodic metallic groove arrays for unidirectional launching of surface plasmons.

The ever-increasing power of computers and the development of new optimization methodologies have enabled the ability to design complex aperiodic devices, which can outperform periodic ones and offer new functionalities. Here, we describe the realization of an ultracompact aperiodic grating coupler capable of selectively launching surface plasmon polaritons (SPPs) in a desired direction. We use a transfer matrix model to facilitate the rapid optimization of such structures. We demonstrate both numerically and experimentally that a structure consisting of five subwavelength grooves patterned into silver can unidirectionally launch SPPs in the visible spectral range with a record right-to-left contrast ratio of 55. The general design principles behind this study can readily be extended to a great diversity of sophisticated aperiodic nanophotonic structures.