  Plasmonic dimer antennas create strong field enhancement by squeezing light
into a nanoscale gap. These optical hotspots are highly attractive for boosting
nonlinear processes, such as harmonic generation, photoelectron emission, and
ultrafast electron transport. Alongside large field enhancement, such phenomena
often require control over the field asymmetry in the hotspot, which is
challenging considering the nanometer length scales. Here, by means of strongly
enhanced second harmonic generation, we demonstrate unprecedented control over
the field distribution in a hotspot by systematically introducing geometrical
asymmetry to the antenna gap. We use focused helium ion beam milling of
mono-crystalline gold to realize asymmetric-gap dimer antennas in which an
ultra-sharp tip with 3 nm apex radius faces a flat counterpart, conserving the
bonding antenna mode and the concomitant field enhancement at the fundamental
frequency. By decreasing the tip opening angle, we are able to systematically
increase both field enhancement and asymmetry, thus enhancing second harmonic
radiation to the far-field, which is nearly completely suppressed for
equivalent symmetric dimer antennas. Combining these findings with second
harmonic radiation patterns as well as quantitative nonlinear simulations, we
further obtain remarkably detailed insights into the mechanism of second
harmonic generation at the nanoscale. Our results open new opportunities for
the realization of novel nonlinear nanoscale systems, where the control over
local field asymmetry in combination with large field enhancement is essential
to create nonreciprocal functionalities.
