Unidirectional Coupling of Surface Plasmon Polaritons by a Single Slit on a Metal Substrate

In this letter, a theoretical model is proposed to calculate the coupling efficiency of surface plasmon polaritons (SPPs) by a single slit on the metal film. In contrast to previous work developed for normal incident light only, TM
₀
and TM
₁
modes of the slit are incorporated in the model, which makes it applicable for the case of oblique incidence. Numerical simulations confirm that this two-mode model can accurately predict the coupling efficiency of SPPs by slits with subwavelength widths (<;λ). Moreover, we demonstrate that unidirectional SPPs can be coupled by an oblique incident light beam, which can be explained by the generation and interference of these two modes in the slit.