Optical beam manipulation through two metal subwavelength slits surrounded by dielectric surface gratings

A novel plasmonic nanolens formed by two subwavelength metal slits surrounded by surface dielectric gratings is proposed and demonstrated numerically. In the proposed method, with just the regulation of the aperture, the radiation fields of surface plasmon polaritons (SPPs) can be controlled effectively. A method to regulate the focal length is also demonstrated. Numerical simulations using the finite-difference time-domain (FDTD) method coupled with perfect-matching-layer (PML) absorbing boundary conditions verify that the method is effective for the design of a plasmonic nanolens.