Unidirectional Coupling of Surface Plasmons Over a Broad-Angle Range Based on Joint Effects of Asymmetric Mode Excitation and Grating Resonance

A new coupling scheme for unidirectional coupling of surface plasmons from free-space light over a broad-angle range is proposed and theoretically investigated. Firstly, the property of strong asymmetric excitation is found in the single wide defect, especially the single dielectric ridge, which is due to the excitation of asymmetric mode in the wide defect under oblique incidence. Then we show that the strong asymmetric excitation by single ridge still exists in the periodic dielectric ridge grating. By cascading the asymmetric mode excitation peak and the grating resonance angle peak, the angular-full-width-half-maximum (AFWHM) is broadened to $39^\circ$ . Finally, the AFWHM is further broadened to $52^\circ$ by utilizing the chirped ridge grating and a remarkable property of wide flat-top angular response is achieved.