Abstract
Efficient frequency up-conversion of coherent light at the nanoscale is highly demanded for a variety of modern photonic applications, but it remains challenging in nanophotonics. Surface second-order nonlinearity of noble metals can be significantly boosted up by plasmon-induced field enhancement, however the related far-field second-harmonic generation (SHG) may also be quenched in highly symmetric plasmonic nanostructures despite huge near-field amplification. Here, we demonstrate that the SHG from a single gold nanosphere is significantly enhanced when tightly coupled to a metal film, even in the absence of a plasmon resonance at the SH frequency. The light-induced electromagnetic asymmetry in the nanogap junction efficiently suppresses the cancelling of locally generated SHG fields and the SH emission is further amplified through preferential coupling to the bright, bonding dipolar resonance mode of the nanocavity. The far-field SHG conversion efficiency of up to 3.56times 10^{-7} W−1 is demonstrated from a single gold nanosphere of 100 nm diameter, two orders of magnitude higher than for complex double-resonant plasmonic nanostructures. Such highly efficient SHG from a metal nanocavity also constitutes an ultrasensitive nonlinear nanoprobe to map the distribution of longitudinal vectorial light fields in nanophotonic systems.
Highlights
Efficient frequency up-conversion of coherent light at the nanoscale is highly demanded for a variety of modern photonic applications, but it remains challenging in nanophotonics
The nonlocal, bulk electric quadrupole, and bulk magnetic dipole interactions can contribute to second-harmonic generation (SHG) by breaking the lattice centrosymmetry, but they are weaker than the contributions of the surface SHG19
The plasmonic modes of such a nanostructure originate from the near-field interaction between the localized surface plasmon (LSP) of the nanoparticle and its induced image in the gold film, which results in a prominent vertical bonding dipolar mode at a wavelength of 850 nm and a weaker, horizontal bonding dipolar mode at a shorter wavelength (Fig. 1e)[37]
Summary
Efficient frequency up-conversion of coherent light at the nanoscale is highly demanded for a variety of modern photonic applications, but it remains challenging in nanophotonics. Nonlinear nanophotonics has gained rapid appreciation as a route to control light with light at the nanoscale and realize many applications ranging from optical frequency conversion to imaging and biosensing as well as quantum technologies[1,2,3,4] Both dielectric[4,5] and metallic nanoparticles[6,7] enable strong field enhancement inside and at the surface of a particle, respectively, and offer promising ways to enhance the nonlinear optical processes, including second-harmonic generation (SHG). Plasmonic excitations, which provide resonantly enhanced surface electric fields, can significantly amplify the SH conversion near a nanoparticle surface
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