Dependence of the coupling properties between a plasmonic antenna array and a sub-wavelength epsilon-near-zero film on structural and material parameters

Abstract

The resonance properties of a plasmonic dipole antenna array depend on its geometry and the properties of its surrounding medium. The linear optical properties of an array of plasmonic dipole antennas can be modified with the inclusion of an epsilon-near-zero (ENZ) thin film. In this work, we numerically investigate the roles of the antenna dimensions, the ENZ film thickness and loss, and the separation between the antenna and the ENZ film in determining the linear optical response of the antenna–ENZ metasurface. The results show that for a sufficiently small separation, the linear optical properties of the antenna array are determined by the strong or ultrastrong coupling with the ENZ film and are only weakly dependent on the antenna geometry. We show that for metasurfaces with thick, lossy ENZ films, the lower polariton branch is not observable due to the high loss of ENZ films. Since the dependence of the upper polariton on antenna length is weak, this results in a single antenna-length-invariant resonance. However, in the presence of low-loss ENZ films, the lower polariton branch is also visible for antenna–ENZ metasurfaces with thicker ENZ films, indicating a strong coupling between the antenna array and the ENZ film. For a given antenna geometry, the coupling strength increases with increasing thickness of the ENZ film and can reach up to ∼50% of the zero-permittivity frequency of the ENZ film, indicating an ultrastrong coupling between the plasmonic antenna array and the ENZ film.