Surface-Enhanced Raman Scattering Research Based On Surface Plasmon Resonance

Many efforts have been made to increase the enhancement factor (EF) of surface-enhanced Raman scattering (SERS) by the design of various substrates, such as nanoparticles, dimers, and clusters. In most cases, the electromagnetic enhancement mechanism dominates the SERS process, and hot spots come from the excitation of localized surface plasmon resonance (LSPR). More confinement of electrical field and novel physical mechanisms are needed for further enhancement. One-dimensional (1D) subwavelength metallic grating is a simple structure, but possesses rich physical phenomena. Apart from the LSPR, the surface plasmon polaritons (SPPs) can be generated in a 1D subwavelength metallic grating owing to the momentum matching between far-field and near-field electromagnetic waves. Gap plasmon polaritons and magnetic plasmon polaritons can also be generated in 1D subwavelength deep metallic gratings. Moreover, the coupling between them can occur. Here, we investigate the coupling effect between the LSPR and SPP in 1D subwavelength Ag gratings. The electrical field enhancement behavior of 1D subwavelength Ag gratings is revealed by both finite-difference time-domain (FDTD) simulation and SERS measurements. The optimized duty ratio of 1D subwavelength Ag grating is 0.4 for the laser incidence of 532 nm wavelength. The calculated SERS EF is on the order of 106 at coupling. In experiments, the 1D subwavelength Ag grating structures are fabricated by focused ion beam (FIB) milling. A single molecule layer of 4-aminothiophenol (4-ABT) is then adsorbed on the 1D subwavelength Ag gratings, and the SERS measurements are performed with a confocal Raman system.The experimental SERS EF is obtained on the order of 104.The main achievements are listed below:1. Based on the coupling effects of surface plasmon plaritons ans localized surface plasmon, we put forward the structure of the subwavelength silver nanometer gratings. Due to the "hot spot" effect, the sharp edges and corners make the local electric field higer than other place. The local electric field has been greatly enhanced to get a higher enhancement factor.2. We ascribe the discrepancy between the simulated and the experimental results mainly to the Ag film evaporation and the followed FIB milling techniques. The fluctuation of Ag film surface engenders a distribution of width and height values of gratings, which is confirmed by scanning electron microscopy (SEM) and atomic force microscopy (AFM).3. In light of this consideration, the experimental SERS EFs agree well with the calculated ones. Our results indicate that the coupling effect can be invoked to get significant SERS enhancement, even using a simple 1D metallic grating, and the SERS performance can be improved by optimization of fabrication technologies of metallic materials.