Abstract
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, TM0 and TM1 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 (<lambda). 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.