Sierpiński Fractals as Plasmonic Metastructures for Second-Harmonic Generation

Metallic Sierpiński fractal metastructures made of gold nanoprisms that exhibit compatibility for both surface-enhanced Raman spectroscopy and second-harmonic generation have been prepared by electron-beam lithography. A sequential series of Sierpiński patterns were investigated up to the eighth-order generation. The presence of localized surface plasmon modes in the visible spectral range is due to the arrangement of the individual triangles forming the Sierpiński geometry and was first exploited for surface-enhanced Raman measurements after functionalization of the plasmonic platform with p-nitrothiophenol. Because of the noncentrosymmetric arrangement of the individual nanoprisms within the Sierpiński pattern, second-harmonic generation (SHG) microscopy measurements were conducted to evaluate the second-order nonlinear optical activity and its relationship to the spatial distributions of electromagnetic enhancement over the surface of the metastructure. Functionalization of the surface with molecules with an electron-withdrawing group appears to further enhance the surface SHG signal.