Non-Local Effects in Epsilon Near-Zero Photonic Gap Antennas Boost Field Enhancement

A material’s non-local response to an electromagnetic field becomes important when its dimensions are comparable to the Fermi wavelength of electrons within. This regime is particularly relevant for optical antennas, where fields are confined to deep subwavelength regions of the device. Particularly, optical antenna devices based on epsilon-near zero (ENZ) materials are further susceptible to non-localities, given their long electronic Fermi wavelengths. Simulating the extent to which non-localities affect the performance of these devices is challenging as it requires a proper account of this complex phenomena along with conventional electromagnetics.<br/>We observed the impact of non-localities on the far-field and near-field optical properties of epsilon near-zero photonic gap antennas (ENZ PGA). These devices consist of dielectric pillars (a-Si) within which a 10-nm thin slab of indium tin oxide (ITO) material is embedded. In these, hybrid dielectric-ENZ modes emerge from the strong coupling between the dielectric antenna modes and the thin film’s ENZ mode. These hybrid modes efficiently couple to free space and strongly confine the incident electric field within the ITO slab over nearly an octave bandwidth. The far-field response of single ENZ PGAs is probed using hyperspectral dark field scattering spectroscopy (DFSS) and their near-field response using third-harmonic generation (THG). Sharp resonances invisible in DFSS are observed atop of a broad band in the THG efficiency spectra and shift when changing the dimensions of the antennas. Local simulations using conventional electromagnetics fail to reproduce this phenomenon and instead predict broad featureless THG efficiency spectra. However, in addition to yielding a better agreement with the experimental DFSS, non-local simulations using a local analogue model qualitatively capture the resonant character of THG efficiency spectra as well as their dependence on device geometry. This work opens up the possibility of leveraging non-local effects to further enhance electromagnetic field confinement in optical antennas. It shows how a proper theoretical account of non-localities in simulations is crucial to guide the design of these non-local devices.