Second harmonic generation from plasmonic hotspots by controlled local symmetry breaking

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