The Study Of Surface Plasmons Field Enhanced Raman Scattering Based On Evanescent Field Excitation

The surface plasmons (SPs) around the metal is a key factor for surface enhanced Raman scattering (SERS) in all kinds of SERS electromagnetic mechanism. The SPs enhanced SERS is a hot topic in SERS study. The SPs exhibits in two types: the PSPs and propagating and localized SPs (LSPs). Both are able to excite Raman scattering. The SERS signal and SERS detection sensitivity can be further improved by combining the PSPs with LSPs in SERS detection. Compared to the conventional SPs, the long range surface plasmons (LRSPs) provide stronger electric field and longer penetration depth that possess a key factor for getting stronger SERS signal and improving SERS detection sensitivity.Based on above view, my study is outlined as follows:1. An SPR-SERS microspectrometer was developed to simultaneous measure the SPR curve and the incident angle dependence of SERS signals excited by the evanescent field. By simultaneous measurement of the SERS and SPR spectra of analytes, the highest SERS signal intensities were observed to appear at the vicinity of the resonance angle. The SERS and SPR spectra captured simultaneously not only directly confirm the correlation between the SERS and SPR, but also present a potential technique for obtaining the structure information about the analytes in molecule level with recording their SPR curves. One of the advantagement is that the SERS signals of the analytes are excited and collected in an evanescent field, which restrains the base line of the obtained spectra and gets high signal-to-noise SERS signals. This nature of the SPR-SERS microspectrometer provides potential applications for determination and the microchip detection of the small volume sample.2. By simultaneous measurement of the SERS and SPR spectra of 4-Mpy, the highest SERS signal intensities were found to appear at the vicinity of the resonance angle. We get the SERS signal with different Ag film thickness. The experiment results show that the SERS signal is strongest when the SPR reflectance is minimal. The simulated angle of the maximal SERS intensity and the electric field with different Ag film thickness based on Fresnel equation was found to be in good agreement with the experimental results. This experiment and simulated results proved that the SPR is an important mold for SERS enhancement. The enhancement factor (EF) we get is 106 based on the SPR enhanced SERS method. The 102 EF come from the SPR effect and 104 come from the contribution of rough silver film.3. We designed a silver nanoparticles assisted LSP-PSP co-enhanced spectroscopic method to detect 4-Mpy based on the evanescent field excitation. The maximal SERS signal (1573 cm-1) excited by the silver nanoparticles assisted LSP-PSP is about 55 folds stronger than that obtained on the vacuum-deposited silver film. The maximal coupling efficiency appeared at around the resonance angles. A large SERS enhancement factor of 2.0×10 was obtained by the present method. The effective coupling of the PSPs and LSPs provides hundreds of "hot spots" between nanoparticles and metal film, which is hypothesized to be responsible for the additional enhancement of SERS signals. We detected the adenine and melamine solution by using the LSP-PSP co-enhanced spectroscopic method, the detection limit of adenine and melamine solution is 10-7M and 10-9M, respectively. The LSP-PSP co-enhanced spectroscopic method possess of potential in biomolecule detection.4. A multilayer structure was designed where, by controlling the relative thicknesses of the LiF and silver film layers, narrower angular resonance curves, longer penetrate depth and stronger SERS intensity associated with the excitation of LRSPs were achieved. The LRSPs offer a number of important advantages for the Raman detection. The first one is that the LRSPR resonance angle smaller than the conventional SPR angle, which provided some convenience in experiment. The second concerns the fact that the electric field at the metal/dielectric interface can be further enhanced, which translates directly into an enhancement in Raman detection sensitivity. Finally, the penetration depth of LRSPs is larger than that of normal SPs modes, which allows for the LRSPs excited more probe molecule. The LRSPs method presents a potential technique for ultrasensitive detection.