Optical antennas integrated with concentric ring gratings: electric field enhancement and directional radiation.

We describe a means for improving the coupling of illumination to, and the collection of scattered radiation from, an optical antenna. This is achieved by integrating optical antennas with concentric ring gratings. Electromagnetic simulations demonstrate that the ring grating improves the coupling to the antenna, even if the incident illumination is focused by an aplanatic lens such as a microscope objective. Dipole radiation from the center of the structure is well collimated. Various aspects of field enhancement and dipole radiation behavior are analyzed. We propose this device for Raman scattering enhancement.