The Numerical Simulation Studies On Surface Electromagnatic Enhanced Raman Scattering Of Spherical Metallic Nanometer Particles-metallic Film

With the discovery of the laser, the application of the detection of the weak signal and the application of the computer, the applications of the Raman spectrum in many research domains are developed rapidly. And some techniques, such as Fourier Transfer Raman, Resonance Raman, co-force micro-Raman, high temperature Raman, PARS, and SERS, made the sensitivity and the ratio of signal to noise of Raman spectrum improvement. Among of them, SERS which can efficiently and highly enhance Raman signal attracts more and more attentions. The high sensitivity of SERS makes its applications extension from the material, chemical, biology to the medicine.This thesis concentrates on the numerical simulations of the near-field enhancement distribution of different kinds of metallic nano-structures under the near-field excitation and aims to explain the electric enhancement in the SERS theoretically and provides scientific basis for the SERS experiment.The thesis mainly includes the following four parts:1. The frequency-dependence finite-difference time-domain formulation is extended to deal with the metallic material. With a complex frequency-dependent permittivity, the metallic material can be described by Drude model. The equivalent incident wave method which is widely used in the multi-layer medium system is used to resolve the setting problem of evanescent wave excitation arisen from the total internal reflection above the interface. Thus a complete numerical simulation method is got to deal with the interaction between the metallic nano-structures with the random shape and the electric field. And its validity is approved in this thesis.2. The near-field enhancement distribution of the metallic nano-structures with the random shape under the near-field excitation is simulated. Through the numerical simulation of the near-field distribution of the different metallic nano-structures under the near-field excitation, the dependence of the field enhancement factor on the shape of configurations is illuminated with the concept of electric free path, dielectric function, plasma frequency and so on.The key elements which are related to the Raman signal in SERS are the metallic optical properties, configuration, size, congregation, and the orientation of the metallic colloids. The near-field enhancement distributions of bi-spherical nanoparticles and the multi- spherical nanoparticles system under the excitation of the plane wave are given to explain the problems brought forward by Nie in his paper in 1997 and provide the theory basis for the experiments.3. Design of the super sensitivity near-field SERS sample cell is the key technique for extending the application areas of the SERS technique and especially for the single molecule SERS technique. Though the intense field enhancement can be got for the metallic nano-structures with more complex shape, immature producing technique limits their applications in an extensive way. Spherical metallic nanometer particles colloids are still the best choice for the field enhancement. The field distribution for different arrays of bi-spherical Ag particles under the excitation inspires us to design the sample cell composed of two layers of Ag film and Ag particles between them. The optimal design parameters are also simulated in the thesis and the validation of the sample cell has been done experimentally. Several problems of the sample cell on the experiment are pointed out and the means are put forward in this thesis.4. The other important application of SERS technique is the probe coated with metallic film, around the end of which the strong local field enhancement effect can be produced. The effect made the probe coated with metallic film extensively used from data storage, surface modification, multi-photo molecular fluorescence spectroscopy, to optical tweezing at nanometer scale resolution. A kind of pyramidal dielectric probe fully coated with thin metallic film is simulated and the results show that the structure can provide not only the nanometer spot but the greatly enhanced field at the tip of the probe.A kind of more effective numerical simulation method is introduced to deal with the interaction between the metallic nano-structures with more complex shapes and the electric field.