Subwavelength beam focusing by multiple-metal slits surrounded by chirped dielectric surface gratings

A novel method is proposed to actualize the beam focusing through multiple subwavelength metal slits surrounded by chirped dielectric surface gratings. The slits have equal widths, depths, and interspaces. The electro-magnetic energy is transported by the slits in the form of surface plasmon polaritons (SPPs). The dielectric gratings converge surface plasmon wave on a single point. Numerical simulation of the design example is performed through finite-difference time-domain (FDTD) method. The results show that the focal length is just several times of the incident wavelength and the cross section of focus spot is far narrower than the incident wavelength.