Effects of the tip shape on the localized field enhancement and far field radiation pattern of the plasmonic inverted pyramidal nanostructures with the tips for surface-enhanced Raman scattering.

The plasmonic 2D W-shape and 3D inverted pyramidal nanostructures with and without the tips are studied. The effects of the tip height and tip tilt angle on the near field enhancement and far field radiation pattern are discussed in this paper. The localized hot spots are found around the pits and the radiation pattern can be affected by the tip structures. The inverted pyramidal nanostructures with and without the tips were fabricated and their reflection spectra and surface-enhanced Raman scattering (SERS) signals for the chemical molecules thiophenol were measured. The simulation according to the geometry parameters of the fabricated structures is demonstrated. We found that the SERS of our proposed structures with the tips can have stronger light field enhancements than the inverted pyramidal nanostructures without the tips, and the far field radiation pattern can be varied by changing the tip height and tip tilt angle. The study of surface plasmon modes and charge distributions can help the understanding of how to arrange the plasmonic structures to achieve high field enhancement and preferred far field radiation pattern. Our study can be useful for the design of the strong field enhancement SERS substrate with specific far field radiation properties. It can be also applied to the portable Raman detectors for in situ and remote measurements in specific applications.