Broadband surface plasmon polariton directional coupling via asymmetric optical slot nanoantenna pair.

Surface plasmon polariton (SPP) coupling is a basic subject for plasmonic study and applications. Optical nanoantennas enable downscaling of the SPP coupling to subwavelength scales. In this study, asymmetric optical slot nanoantenna pairs composed of two optical slot nanoantennas with different lengths are proposed for SPP directional coupling. Broadband unidirectional launching of SPPs is achieved, and the extinction ratio obtained experimentally reaches up to 44. The bandwidth is larger than 157 nm. Furthermore, SPP direction-selective radiation is demonstrated using the asymmetric optical slot nanoantenna pairs. A novel plasmonic display device showing the propagation direction of SPPs is achieved by employing asymmetric optical slot nanoantenna pairs without any electric device. Asymmetric optical slot nanoantenna pairs have large potential in the directional control of SPP launching and radiation and can be very useful in compact optical circuits and other photonic integrations.