Efficient and Directional Excitation of Surface Plasmon Polaritons by Oblique Incidence on Metallic Ridges

For many years, the search for efficient surface plasmon polariton (SPP) excitation mechanisms has been a recurring matter in the development of compact plasmonic devices. In this work, we excited SPPs illuminating a subwavelength metallic ridge with a focused spot to characterize the coupling efficiency by varying the incidence angle of the excitation beam from -50 to 50 degrees. The intensity distribution of the excited SPPs was measured using leakage radiation microscopy to determine the relative coupling efficiency in the wavelength interval from 740 to 840 nm. We modeled the excitation efficiency as a function of the incidence angle using a simple analytical diffraction model. Two ridges of different width (200 and 500 nm) were used to compare results and validate the model. The experimental results show a higher coupling efficiency at oblique incidence, where the coupling was enhanced by factors of 2x for the 500-nm-wide ridge, and 3x for the 200-nm-wide ridge, as well as unidirectional SPP excitation. The experimental results are in good agreement with the proposed model.