  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.
