Unidirectional Long-range Surface Plasmon Polaritons Coupler Based On Nanoslits

Surface plasmon polaritons (SPPs) provide the basis for plasmonics, an emerging branch in nanophotonics, aiming at the miniaturization and integration of optical devices.In this regard, one of the key issues in plasmonics is to couple the free-space light to SPPs. Conventional optical schemes utilize prisms or gratings to realize the momentum matching between photons and plasmons. However, these optical configurations are too "bulky" for the plasmonic devices in compact integrated photonic circuits. A variety of nanostructures including nanoslits, grooves, ridges and compact detuned nanoantennas can efficiently couple the SPPs from the free-space light to the specified direction. In contrast to the single-interface SPPs, the long-range surface plasmon polaritons (LRSPPs) possess much lower propagation loss, resulting in a much longer propagation distance. Hence, in view of potential applications, unidirectional LRSPPs launching is also essential and highly desired. This paper first introduces the application of SPPs in chemical and biological sensoring, optical data storage and waveguide. The states of unidirectional SPPs coupler based on different nanostructures are also reviewed. The second chapter introduces the fundamental principles of SPPs and Finite Difference Time-Domain, including Yee gridding, the numerical stability condition and absorbing boundary condition. The history and the principles of LRSPPs are presented in chapter three, as well as the common coupling strictures which are to excite SPPs. Finally, we propose a novel LRSPPs launcher based on nanoslits in the visible range. We are able to tune the direction of the generated LRSPPs by varying the incident angle of Gaussian light beams. Numerical simulations are used to systematically study the impacts of LRSPP launcher geometry on the extinction ratio and the effective angular width. The maximal extinction ratio reaches up to 28 dB with an effective angular width of 30°. The bandwidth is up to 170nm. Simply via varying the incident angle, the generated LRSPPs will propagate to the other direction. We expect that such a tunable LRSPP launcher may be of interest for various applications in the plasmonic regime.