Research Article

Active coated nano rod antennas for enhanced and directive scattering phenomena

¹ Department of Electrical Engineering, Technical University of Denmark, Kgs. Lyngby 2800, Denmark
² Department of Photonics Engineering, Technical University of Denmark, Kgs. Lyngby, 2800, Denmark

^* e-mail: sar@elektro.dtu.dk

Received: 8 February 2019
Accepted: 9 September 2019
Published online: 18 October 2019

Abstract

The scattering properties of a range of symmetric and asymmetric active coated nano rod antennas are investigated numerically. The active nano rods are composed of a silica dioxide nano-core coated with a silver nano-shell, and with a canonical gain model implemented into their nano-core regions. The asymmetric nano rods are obtained through suitable perforations of their nano-shell and/or nano-core regions. In all cases, active nano rods are found to exhibit super-resonant phenomena with significantly enhanced scattered fields for an incident plane wave having the magnetic field parallel to the rod axis. While the dipole-mode response in the symmetric cases is only weakly directive, the asymmetric cases stimulate an abundant emission of higher order modes furnishing rather enhanced and directive near-fields. As the length of the symmetric nano rods decreases, more gain is needed to achieve a super-resonant response, which also was found to be blue-shifted. For asymmetric cases, the gain was lowered, and the response got blue-shifted as the asymmetry increased. The proposed active nano rod antennas provide a new class of antennas with desirable wavelength tunability and polarization-dependent scattering properties; this makes them interesting candidates for many nano-photonic applications. Moreover, the proposed geometries bridge the important gap between the two often considered canonical geometries, namely, spherical and infinitely long cylindrical particles. The detailed knowledge of gain values and resonant wavelengths provided in here is crucial for a successful combination of such particles with realistic gain materials.