  We present numerical studies, nano-fabrication and optical characterization
of bowtie nanoantennas demonstrating their superior performance with respect to
the electric field enhancement as compared to other Au nanoparticle shapes. For
optimized parameters, we found mean intensity enhancement factors >2300x in the
feed-gap of the antenna, decreasing to 1300x when introducing a 5nm titanium
adhesion layer. Using electron beam lithography we fabricated gold bowties on
various substrates with feed-gaps and tip radii as small as 10nm. In
polarization resolved measurement we experimentally observed a blue shift of
the surface plasmon resonance from 1.72eV to 1.35eV combined with a strong
modification of the electric field enhancement in the feed-gap. Under
excitation with a 100fs pulsed laser source, we observed non-linear light
emission arising from two-photon photoluminescence and second harmonic
generation from the gold. The bowtie nanoantenna shows a high potential for
outstanding conversion efficiencies and the enhancement of other optical
effects which could be exploited in future nanophotonic devices.
