X-shaped quasi-3D plasmonic nanostructure arrays for enhancing electric field and Raman scattering.

We propose and demonstrate strongly enhancing electric field and Raman scattering with a large tolerance to the light incident angle and polarization by using x-shaped quasi-3D plasmonic nanostructure arrays (X-Q3D-PNAs). The finite-difference time-domain simulations were used to study the reflectance spectra and electric field profiles of X-Q3D-PNAs. Results show that both surface plasmon polaritons and localized surface plasmon polaritons (LSPPs) can be generated at the metal/dielectric interfaces of the top gold thin film with square grating x-shaped nanoholes. The resonance of the LSPPs generated at the gold islands formed between x-shaped nanoholes at the top gold thin film greatly enhance the electric fields at the tips of the cross-sectors of the x-shaped nanoholes. Both plasmon resonances and electric field enhancements are affected by the structural dimensions. The strong electric field enhancement and the large tolerance to the laser polarization were demonstrated by surface-enhanced Raman scattering experiments. This unique plasmonic property of X-Q3D-PNAs could be attractive for photovoltaics and biosensing applications.