Tunable directional beaming assisted by asymmetrical SPP excitation in a subwavelength metallic double slit

Abstract

We present a design for tunable directional beaming through a subwavelength metallic double slit surrounded by dielectric surface-relief gratings. On-axis and off-axis beaming can be switched by controlling the incident angle to asymmetrically excite surface plasmon polaritons (SPPs) that are subsequently coupled out to propagating beams by the two gratings on the left and the right sides of the double slit. Furthermore, the division of optical power into two off-axis beaming directions can be tuned smoothly by varying the incident angle while keeping the total power almost unchanged. The mechanism of this effect is analyzed theoretically and verified using rigorous numerical simulations.OCIS Light emerging from a subwavelength slit or aperture is normally diffracted into all directions. Recently, the directional beaming effect from a subwavelength metallic slit due to the resonant interactions between surface plasmon polaritons (SPPs) and the surrounding grating structures has stimulated much interest due to its possibility to steer and confine emitted light beams in a small spatial range of angle [1,2] , which has potential applications in laser beam shaping [3−4] , optical interconnection [5] , secure communication, optical data storage, and optical imaging, among other areas. Considerable prior research related to optical on-axis and off-axis directional beaming has been conducted [6−13] , with the mechanism applied to mid-infrared [3,10] , THz wavebands [4] , acoustic wave [9] , and photonic crystals [12] . However, little work has been done on tunable directional beaming (i.e., modulating the beaming angle or power distribution by controlling signals), although tunable beaming with the use of permittivity-modulating material has been proposed before [8] . Hence, it is of great significance to realize tunably control of the directional beaming, which is highly desirable in various applications.Previously, various plasmonic devices have been used to exploit the angular-modulation of the incident light [14−16] . For example, Kim et al. designed an unidirectional SPP launcher consisting of a single slit with its width comparable to or larger than the incident wavelength [15] . However, such a wide slit has a low SPP excitation efficiency because most of the light illuminating the slit would be directly transmitted through the slit rather than coupling into SPPs [17] . Recently, we demonstrated efficient directional excitation of SPPs on a metal film through a subwavelength double nanoslit [16] . The SPPs can be directed into two opposite propagating directions with a predetermined splitting ratio by varying the incident angle. Such dynamic tunability of SPPs can be used to realize some useful applications. In this letter, a simple and effective design for tunable directional beaming through a metallic double slit surrounded by dielectric surface-relief gratings is proposed. On-axis and off-axis beaming can be switched by controlling the incident angle to asymmetrically excite SPPs that are subsequently coup...