Plasmonic nano-comb structures for efficient large-area second harmonic generation

We propose and demonstrate plasmonic nano-comb (PNC) structures for efficient large-area second-harmonic generation (SHG). The PNCs are made of 250 nm-thick gold film and have equally-spaced 30 nm-slits filled with ployvinylidene fluoride-co-trifluoroethylene (P(VDF-TrFE)). The PNC with 1.0 μm-spacing couples resonantly with 1.56 μm 100-fs laser beams. For the 1.0 μm-spacing PNCs under the fixed-pump-power condition, the nonlinear SHG power remains almost independent of the pump diameter ranging from 2 μm to 6 μm. The SHG power from the resonant PNC is measured to be 8 times larger than that of the single-nano-gap metallic structure, when the pump beam is tightly-focused to 2 μm in diameter in both cases. This relative enhancement of the total nonlinear SHG signal power reaches up to >200 when the pump beam diameter is increased to 6 μm. We attribute this unusual phenomenon to the resonant coupling of the finite-size pump wave with the finite-size one-dimensional plasmonic mode.