Controlling Field Asymmetry in Nanoscale Gaps for Second Harmonic Generation

Field hotspots in nanoscale gaps of plasmonic antennas can boost nonlinear processes such as harmonic generation, photoelectron emission, and ultrafast electron transport. Alongside large field enhancement, such phenomena often require control over the field symmetry in the hotspot, which is challenging considering the nanometer length scales. Here, by means of strongly enhanced second harmonic generation, unprecedented control over the field distribution in a hotspot is demonstrated by systematically introducing geometrical asymmetry to the antenna gap. Focused helium ion beam milling of mono-crystalline gold is used to realize asymmetric-gap dimer antennas in which an ultra-sharp tip faces a flat counterpart. This allows to increase both field enhancement and asymmetry while maintaining the bonding antenna mode at the fundamental frequency, thereby enhancing second harmonic radiation to the far-field. Combining these findings with second harmonic radiation patterns as well as quantitative nonlinear simulations, remarkably detailed insight into the mechanism of second harmonic generation at the nanoscale is obtained. These results open new opportunities for the realization of nonreciprocal nanoscale systems.Field hotspots in nanoscale gaps of plasmonic antennas can boost nonlinear processes, which often require high field enhancement combined with an asymmetric field distribution. This work demonstrates unprecedented control over the symmetry of the field distribution by systematically introducing geometrical asymmetry in the antenna gap, thereby strongly enhancing second harmonic generation. These results open new opportunities for the realization of nonreciprocal nanoscale systems.image