Selective-mode optical nanofilters based on plasmonic complementary split-ring resonators.

A nanoplasmonic optical filtering technique based on a complementary split-ring resonator structure is proposed. The basic and modal properties of the square-nanoring are studied using the group theory. Degeneracy and non-degeneracy of the possible TM odd- and even-modes are characterized based on the symmetry elements of the ring structure. Distinctively, the proposed technique allows selecting and exciting the proper plasmonic modes of the nanoring in the side-coupled arrangement. It is found that the non-integer modes can be excited due to the presence of a metallic nano-wall. These modes are highly sensitive to the nano-wall dimensions, in contrast to the regular integer modes. Moreover, the transmission-line theory is used to derive the resonance condition of the modes. The results show the optical transmission spectrum of the investigated filter can be efficiently modified and tuned either by manipulation of the position or by variation of the width of the employed nano-wall inside the ring. The numerical results support the theoretical analysis.