Maximizing the forward scattering of dielectric nanoantennas through surface impedance coatings

In this letter, we discuss a novel approach for designing passive nanoantennas with maximum forward and almost-zero backward scattering. The proposed approach is based on the use of high-index dielectric spheres, supporting dipolar magnetic resonances, coated by ultra-thin surface impedance coatings. It is shown that, by properly engineering the radius of the coat and its surface reactance, it is possible to introduce an additional electric dipolar resonance and makes it overlapped with the magnetic one sustained by the high-index dielectric sphere. A realistic design based on graphene and working in the low-THz range is also proposed and verified with full-wave simulations. Compared to earlier techniques, based on the combination of multipoles or on the use of ellipsoidal particles, the proposed one is quite robust toward realistic Ohmic losses and preserves the isotropic behavior of the nanoantenna.