Impedance Matching of THz Plasmonic Antennas

Plasmonic antennas with subwavelength gaps work as sensing devices for molecules for the optical and terahertz (THz) frequency range. In such a configuration, the sensing gap creates a high impedance, in contrast to the antenna itself, which is designed for low ohmic losses. Besides metals, highly doped semiconductors can be used as plasmonic materials for the THz range, which increase the impedance of the antenna while keeping the strong electromagnetic intensity enhancement in the sensing gap. Still, a substantial impedance mismatch remains. In this paper, an approach for matching the gap to the antenna impedance is proposed. First, a germanium semiconductor slab antenna is designed for THz operation, with a dipole resonance at 4 THz. Based on an impedance analysis, an impedance matching element consisting of a graphene sheet is inserted in parallel to the gap. Employing impedance matching, a strong intensity enhancement occurs and the normal dipole mode of the plasmonic antenna is split into a symmetric one at 0.67 THz and an antisymmetric one at 4 THz. The symmetric mode provides a very high-quality factor and a substantial enhancement; furthermore, its resonance can be tuned by the Fermi level adjustment in the graphene. First designs of this structure are computed analytically with a circuit model and are verified by 3D full-wave simulations.