Suppression of Near-Field Coupling in Plasmonic Antennas on Epsilon-Near-Zero Substrates

Epsilon-near-zero (ENZ) media are an emerging class of nanophotonic materials which engender electromagnetic fields with small phase variation due to their approximately zero permittivity. These quasi-static fields facilitate several unique optical properties such as subwavelength confinement, arbitrary wavefront control, and enhanced light-matter interactions, which make ENZ materials promising platforms for nanophotonic and plasmonic systems. Here, we report our analysis of single and dimer nanoantennas deposited on an aluminum-doped zinc oxide layer with an ENZ wavelength around 1.5 mm. Using near-field microscopy, far-field spectroscopy, finite-element numerical simulations, and a semi-analytic Fabry-Pérot model, we show that single nanoantennas support highly dispersive plasmonic modes with near-zero effective mode index, which consequentially fixes the resonance near the ENZ wavelength of the substrate. Furthermore, we observe a strong reduction in the near-field coupling between dimer nanoantennas via measurements of the resonance shift as a function of gap size. This reduction of near-field coupling allows one to design arrays of independently operating antennas with higher densities and thereby significantly improve the array characteristics, especially when targeting gradient metasurface implementations. Our results demonstrate the use of ENZ materials for increasing the versatility and functionality of plasmonic structures and provide foundational insight into this exotic material phenomenon.