Study On Property Of Slective Surface Enhanced Raman Scattering Based On Surface Plasmon Rasonance

Optical antenna has been widely investigated due to their novel optical phenomena and capability of manipulating light field at the nanometer scale.It has been used in many fields such as nano-scale light manipulation,surface enhanced Raman spectroscopy（SERS）,fluorescence enhancement,near-field imaging and near-field optical nanolithography.With the development of SERS in the past several years,structures based on surface plasmon resonance have been widely used for the high sensitivity.However,most researchs mainly focused on how to realize higher enhancement of Raman signals.To solve another important problem,the recognition of target molecules,plasmonic structures with multi-resonances are proposed to realize selective SERS enhancement,which can enhance specific Raman resonant modes to improve the identification of target molecules.In this paper,based on the surface plasmon localized and coupling enhancement characteristics of optical antenna,structure with high Q value and multiple wavelengths:periodic metal nano-unit-dielectric layer-metal film,is designed and prepared.By exciting the localized surface plasmon resonance（LSPR）mode of the metal nano structure and the propagation surface plasmon resonance（SPR）mode introduced by the periodic structure,and making the hybrid coupling between different resonance modes to form Fano resonance,a surface-enhanced Raman Scattering（SERS）substrate with multiple resonance wavelength and high enhancement factor was realized.Meanwhile,due to the introduction of the intermediate dielectric layer,these mode fields are coupled with the light field excited by the mirror image to further enhance the electromagnetic field intensity around the structure.Moreover,due to the mutual coupling of multi-level resonance modes,the high enhancement factor is realized and the resonant peak spectrum width is effectively compressed at the same time,which greatly improves the quality factor Q of the structure.In addition,this structure also has the fine characteristics of small model volume and high spatial overlap.This kind of Raman sensor with high sensitivity and selectivity can be widely used in non-chemical,non-fluorescence probe,high sensitivity and universal sensing of various molecules and other fields.The details of this thesis are shown as following:（1）In order to realize the selective enhancement of substrate with high enhancement factor,periodic gold disk dimer-SiO₂-gold membrane structure with dual resonance wavelength is designed according to the hybrid coupling theory of SPP and LSPR.The structure parameters are optimized by Comsol Multiphysics software,and the structure is prepared by lift-off technology of electron beam exposure.It is found that the structure not only exhibits a large field enhancement factor,but also has two resonance modes,which can enhance the excitation field as well as the scattering field.Experimental results show that this structure could achieve the selective Raman enhancement of probe R6G molecules.However,since the resonance peak of LSPR mode was still too wide,the selective Raman enhancement effect needed to be improved.（2）Because the LSPR mode of the periodic gold disk dimer is too wide,periodic gold disk dimer is optimized to periodic silver hole according to the emission characteristics of optical antenna.It was found that compared with the gold disk dimer,this structure not only effectively compress the line width of the plasmon resonance peak,but also greatly improves the quality factor Q of the structure,and provides three resonance modes for the realization of the selective Raman enhancement effect.Experimental results show that the signal intensity of R6G molecule was enhanced about an order of magnitude compared with the traditional single resonance structure（periodic hole-silicon）.In addition,compared with the characteristic spectrum measured on the silicon substrate,the Raman signals matching the resonance peaks are increased by about 3-5 times,while the Raman signals not matching are only slightly enhanced.When considering the two selected Raman signals,the detection sensitivity of the probe molecule can be improved at least one order of magnitude.（3）Although the quality factor of periodic holes can be effectively improved,its characteristic of localized electric field still needs to be enhanced.Based on this,the circular hole unit is optimized to bowtie hole.It was found that the structure can also provide three resonance modes with high quality factor,small mode volume and high spatial overlap in the near infrared spectrum.In addition,due to the existence of gap,the three resonance modes all have great field enhancement factors.Compared with the circular hole structure,this structure not only realizes the high quality factor but also shows a larger field enhancement factor,which increases the average field enhancement factor from 150 to 5000.Compared with the gold disk dimer structure,the central field enhancement factor was increased from 4000 to 35,000,and the quality factor was greatly improved.These features provide a better solution for implementing selective Raman enhancement.