Mode matching in multiresonant plasmonic nanoantennas for enhanced second harmonic generation

An asymmetric plasmonic nanoantenna featuring a double resonant mode that overlaps with both the excitation fundamental wavelength and the second harmonic emission displays a remarkably large nonlinear coefficient for second harmonic generation. Boosting nonlinear frequency conversion in extremely confined volumes remains a challenge in nano-optics research1, but can enable applications in nanomedicine2, photocatalysis3 and background-free biosensing4. To obtain brighter nonlinear nanoscale sources, approaches that enhance the electromagnetic field intensity and counter the lack of phase matching in nanoplasmonic systems are often employed5,6,7,8. However, the high degree of symmetry in the crystalline structure of plasmonic materials (metals in particular) and in nanoantenna designs strongly quenches second harmonic generation5. Here, we describe doubly-resonant single-crystalline gold nanostructures with no axial symmetry9 displaying spatial mode overlap at both the excitation and second harmonic wavelengths. The combination of these features allows the attainment of a nonlinear coefficient for second harmonic generation of ∼5 × 10–10 W–1, enabling a second harmonic photon yield higher than 3 × 106 photons per second. Theoretical estimations point toward the use of our nonlinear plasmonic nanoantennas as efficient platforms for label-free molecular sensing.