Broadband field-enhancement in epsilon-near-zero photonic gap antennas

In recent years, the large electric field enhancement and tight spatial confinement supported by the so-called epsilon-near-zero (ENZ) mode have attracted significant attention for the realization of efficient nonlinear optical devices. Here, we experimentally demonstrate a new type of antenna, termed an ENZ photonic gap antenna (PGA), which consists of a dielectric pillar within which a thin slab of indium tin oxide (ITO) material is embedded. In ENZ PGAs, hybrid dielectric-ENZ modes emerge from strong coupling between the dielectric antenna modes and the ENZ bulk plasmon resonance. These hybrid modes efficiently couple to free space and allow for large enhancements of the incident electric field over nearly an octave bandwidth, without the stringent lateral nanofabrication requirements required by conventional plasmonic or dielectric nanoantennas. The linear response of single ENZ PGAs is probed using dark field scattering and interpreted using a simple coupled oscillator framework. Third harmonic generation is used to probe the field enhancement and large enhancements are observed in the THG efficiency over a broad spectral range. This proof of concept demonstrates the potential of ENZ PGAs, which we have previously shown can support electric field enhancements of up to 100--200X, which is comparable with those of the best plasmonic antennas.