Bonding resonant modes of plasmonic nanoantennas with narrow gaps exhibit
very large local field enhancement. These hotspots are highly attractive for
boosting optical nonlinearities, such as second harmonic generation. However,
for resonant symmetric gap antennas, the strong second harmonic sources created
at the gap interfaces oscillate out-of-phase and therefore interfere
destructively in the far-field. Here, we use an advanced nanofabrication
approach to systematically break the local symmetry of nanoscopic antenna gaps
while retaining the bonding resonant antenna mode at the fundamental frequency
and the concomitant intensity hotspot. We find that antennas with the strongest
local symmetry breaking emit correspondingly intense second harmonic radiation
as compared to symmetric reference structures. By combining these findings with
second harmonic radiation patterns as well as quantitative nonlinear
simulations, we obtain remarkably detailed insights into the mechanism of
second harmonic generation at the nanoscale. Our findings open new perspectives
for the realization of non-reciprocal nanoscale systems, where local symmetry
breaking is crucial to create unique functionalities