Beam Adjustment with Double Subwavelength Metal Slits Surrounded by Tapered Dielectric Gratings

A beam optical focusing structure with double subwavelength metal slits surrounded by tapered surface dielectric gratings is proposed and demonstrated numerically. In the proposed structure, just with the regulation of the surface gratings, the radiation fields of surface plasmon polaritons (SPPs) can be controlled effectively to make a beam spot at several times the wavelength distance from the slit. Two methods for the control of focal length and width are proposed, and the simulation results verify that both the methods are effective for the design of nano-optical focusing devices.