Ultrabroadband and Wide-Angle Unidirectional Coupling of Surface Plasmons Based on Chirped-Nanoslits Grating

We experimentally demonstrate a novel ultrabroadband and wide-angle unidirectional coupling scheme by combining the coupling effect of single nanoslit and chirped grating effect. The operation principle is numerically and experimentally clarified through analysis and compare. Numerical analysis shows that the fabricated chirped-nanoslits grating could unidirectionally couple surface plasmon polaritons from free-space light over an ultrabroadband (1.3  $\mu$ m) and wide-angle ( ${\text{30}}^{\circ }$ ) ranges. Excellent unidirectional coupling performance with extinction ratio ER $>10$  dB (a peak value ER = 22 dB) over an ultrabroad operating bandwidth of 300 nm and wide angular range of ${\text{26}}^{\circ }$ is experimentally demonstrated.