Light-induced symmetry breaking for enhancing second-harmonic generation from an ultrathin plasmonic nanocavity

Abstract Efficient frequency up-conversion of coherent light at the nanoscale is highly demanded for a variety modern photonic applications, but it remains challenging in nanophotonics. Surface second-order nonlinearity noble metals can be significantly boosted up by plasmon-induced field enhancement, however related far-field second-harmonic generation (SHG) may also quenched symmetric plasmonic nanostructures despite huge near-field amplification. Here, we demonstrate that SHG from single gold nanosphere enhanced when tightly coupled to metal film, even absence plasmon resonance SH frequency. The light-induced electromagnetic asymmetry nanogap junction efficiently suppresses cancelling locally generated fields and emission further amplified through preferential coupling bright, bonding dipolar mode nanocavity. conversion efficiency $$3.56\times 10^{-7}$$ 3.56 × 1 0 ? 7 W ?1 demonstrated 100 nm diameter, two orders magnitude higher than complex double-resonant nanostructures. Such efficient nanocavity constitutes an ultrasensitive nonlinear nanoprobe map distribution longitudinal vectorial nanophotonic systems.