Directional coupling of surface plasmon polaritons using single nano-slit with asymmetric sidewall structure and material composition

We report on the performance of the asymmetric nano-slit that we design and fabricated with electron beam lithography (EBL) and glancing angle deposition techniques (GLAD) for directional coupling of surface plasmon polariton (SPP) on Ag surfaces. The slit structure includes asymmetric sidewalls in terms of material composition as well as structural morphology. The overall width of the slit was varied for optimization. We illuminated the slit with a focused 532nm laser beam and characterized the SPP signal on the Ag surface near the slit with nearfield scanning optical microscopy (NSOM). We demonstrate that optimal directional coupling of SPP toward either side of the slit can be achieved by selecting proper slit widths, with the best extinction ratio of 79000 ± 18000. We also carried out numerical calculations to simulate the interaction between the incident light and the slit structure. The results reproduced the experiment qualitatively. Detailed analysis of the distribution of the E-field and the time-averaged Poynting vector indicates that SPP excited on the Ag pad substructure in the slit plays an important role in the directional coupling of SPP.