Asymmetrical excitation of surface plasmon polaritons on blazed gratings at normal incidence

We present results of numerical simulations and preliminary experiments to investigate and characterize the effect of asymmetrical coupling of normally incident light to surface plasmon polaritons (SPPs) on metallic blazed gratings. Two types of blazed gratings are investigated, a two-dimensional (2D) area-coded binary grating and a one-dimensional (1D) slanted sinusoidal grating. The 2D blazed grating, which can be fabricated with standard e-beam lithography, is shown to have the same ability as the classical 1D blazed grating to enhance the strength of the -1st(+1st) evanescent order over the +1st(-1st) counterpart, which leads to the asymmetrical excitation of two counterpropagating SPP modes on the grating surface. The 1D blazed grating, as a reference, is also studied experimentally to verify the previous theoretical predictions. In our first experiments, the observed asymmetrical coupling effect is relatively weak compared with the optimal designs due to many practical limitations. However, good agreement between theory and experiment has been obtained, and physical insight concerning the observed SPP coupling phenomena has been gained. Further measures to realize stronger asymmetrical excitation of SPPs on blazed gratings at normal incidence are discussed.