Enhanced Surface Raman Scattering Effect Of Quasi Three-Dimensional Metallic Half Shell Arrays Microstructures

In this paper, we study the surface-enhanced Raman scattering (SERS) effect of plasmonic crystals (PCs) which composed of quasi three-dimensional (3D) metallic periodic microstructures. Through the Raman scattering of the PCs and the finite-difference time-domain calculations, we investigate the relationship between enhanced Raman scatting and the properties of the structural, which can be divided into the following three parts:In the first part, an approach of nanosphere lithography (NSL) to fabricate quasi-3D PCs by depositing a thin gold layer onto a two-dimensional (2D) array of silica microspheres self-assembled on a quartz substrate, followed by removal of the sacrificial template. The novel quasi-3D PCs are composed of interconnected metal hollow half shells supported on a planar dielectric substrate. Based on different methods of removal of the sacrificial template, the different opening orientation of interconnected metal hollow half shells supported on a planar dielectric substrate are acquired.In the second part, we have investigated the resonant transmission properties of the novel quasi-3D PCs which are composed of interconnected metal hollow half shells supported on a planar dielectric substrate. In this paper, we further study the SERS of the novel quasi-3D PCs. It is found that the novel quasi-3D PCs show a large SERS effect and the measured Raman signal enhancement shows a noticeable difference on the half shell orientation relative to the pump beam wavevector. At the excitation wavelength outside the plasmonic mode, the SERS on an upward gold half shell array substrate can be two-fold than that on a downward one when the incident pump beam impinges on the substrate vertically from the top of the substrate. But when the excitation wavelength is tuned in the vicinity of the main plasmonic resonant mode of the PCs, the enhancement can be further increased to five-fold.In the third part, it is seen from the transmission spectra that the locations of the transmission resonance peaks are nearly the same when the incident light beam impinges vertically from the top of the substrate, whatever the opening direction of the half shell arrays might be upward or downward. We believe that excited outside the plasmonic mode, the difference of Raman signal of gold half shell array substrate with different opening is result from the nanotip effect at the rim of upward opening structure. The electric field around the rim of the upward opening structure is enhanced which can introduce the enhancement of Raman signal. Excited at the vicinity of the main plasmonic resonant mode of the PCs, the a strong electric field inside cavity of the gold half shell shown in finite-difference time-domain calculations lead to significant increase in molecule's absorption rate. Such upward half shell arrays substrate that support a strong electric field inside cavity together with a nanotip effect at the rim of bowls attribute to the stronger Raman signal of upward opening half shell arrays excited at the vicinity of the main plasmonic resonant mode.