Off-angle illumination induced surface plasmon coupling in subwavelength metallic slits.

A directional coupling effect of surface plasmon waves (SPW) in subwavelength metallic slits is studied. The p-polarized wave generates two SPW modes in the subwavelength slit. As the angle of incidence is changed, coupling arises between both SPW modes. The coupling length increases exponentially with the width of the slit but is independent of the angle of incidence. At a coupling length and an incident angle of ~30 degrees , light is emitted from one side of the slit. The single side emission light has a width smaller than 50nm and double peak intensity than at normal incidence. The SPW coupling effect reveals a simple way for producing a nanometer light source of high transmission intensity.