Fluorescence excitation by enhanced plasmon upconversion under continuous wave illumination

• No SHG is observed from the cluster of 12 nm EYFP decorated centrosymmetric gold MNPs.
• No SHG is observed from the cluster of gold nanoparticles of varying sizes 70–120 nm.
• SHG is observed with a CW laser when the two clusters touch each other.
• EYFP fluorescent molecule enhances SHG via interference of two conversion paths.
• EYFP fluoresces and reports us the generation of SH photons.
• EYFP has no SH response in the pump region.

We demonstrate effective background-free continuous wave nonlinear optical excitation of molecules that are sandwiched between asymmetrically constructed plasmonic gold nanoparticle clusters. We observe that near infrared photons are converted to visible photons through efficient plasmonic second harmonic generation. Our theoretical model and simulations demonstrate that Fano resonances may be responsible for being able to observe nonlinear conversion using a continuous wave light source. We show that nonlinearity enhancement of plasmonic nanostructures via coupled quantum mechanical oscillators such as molecules can be several orders larger as compared to their classical counterparts.