  High-index Mie-resonant dielectric nanostructures provide a new framework to
manipulate light at the nanoscale. In particular their local field confinement
together with their inherently low losses at frequencies below their band-gap
energy allows to efficiently boost and control linear and nonlinear optical
processes. Here, we investigate nanoantennas composed of a thin indium-tin
oxide layer in the center of a dielectric Gallium Phosphide nanodisk. While the
linear response is similar to that of a pure GaP nanodisk, we show that the
second and third-harmonic signals of the nanogap antenna are boosted at
resonance. Linear and nonlinear finite-difference time-domain simulations show
that the high refractive index contrast leads to strong field confinement
inside the antenna's ITO layer. Measurement of ITO and GaP nonlinear
susceptibilities deliver insight on how to engineer nonlinear nanogap antennas
for higher efficiencies for future nanoscale devices.
