| Surface-enhanced Raman spectroscopy is an important tool for molecular detection and analysis due to its ability to significantly enhance the intrinsic Raman scattering of matter through localized surface plasmon resonance based on metal nanostructures.The tremendous development and progress of nanoscale in materials,biology,chemistry and other fields have put forward higher requirements for high-sensitivity detection technology.Considering that the optical fiber has the characteristics of anti-interference,low noise,easy integration,and special polarization distributions of cylindrical vector beams,in this dissertation a method is proposed for improving the detection sensitivity of SERS technology by the synergy of fiber-mode cylindrical vector beam and the well-matched metallized fiber probe.The process of internal illumination of fiber-mode cylindrical vector beams in the plasmonic fiber probe has been investigated by theoretical calculation and simulation.The excitation,transmission and enhanced localization models of surface plasmon polaritons（SPPs）during the process have been achieved,which helps to design the plasmonic fiber with high electric field enhancement factors.The designed plasmonic fiber has been applied in SERS examination.SERS experiment system based on the plasmonic fiber internally excited by cylindrical vector beam has been developed.The preparation process for the plasmonic fiber with high SERS activity has been explored and optimized.The highly-sensitive Raman examination method based on plasmonic fiber internally excited by cylindrical vector beam has been proposed.The main work and research results are listed as follows:（1）A method for highly sensitive SERS examination through tip-based nanofocusing has been proposed.We theoretically achieve the matching rule between optical modes and SPPs under the condition that linearly polarized beam（LPB）and radially polarized vector beam（RVB）exciting the silver film coated fiber taper,respectively.The calculation results show that tip nanofocusing can be achieved under RVB excitation,and an electric field intensity enhancement can reach～2×103,but the SPPs under LPB excitation is cut off.Experimentally,the acoustically-induced fiber grating（AIFG）-based method was adopted to generate RVB in fiber,the system of which was further developed to be the SERS examination system based on fiber-mode RVB excited metallized fiber tip.The Raman intensity of malachite green（MG）examined by this system under RVB excitation was observed to be 15 times as strong as that of LPB excitation.A detection sensitivity up to 10-14 M is achieved,and the energy transversion efficiency was estimated to be～17%with RVB excitation which was much higher than that by aperture-based tips（typically 0.001-0.01%）and hybrid tips（typically0.15%）.（2）A method through Ag nanoparticles（NPs）coated fiber tip excited by fiber-mode azimuthally polarized vector beam（AVB）has been proposed for highly sensitive SERS examination.Theoretical analysis shows that the Ag-NPs-coated fiber probe internally excited via the AVB can provide a larger SERS active surface than that of LPB excitation because uniform gap modes between adjacent NPs at any azimuthal position at the surface of the plasmonic fiber tip can be excited.The experimental result showed that the intensity of the Raman spectrum of MG by internally exciting the Ag-NPs-coated fiber tip via an AVB was about 8 times as strong as that by LPB excitation,and the detection sensitivity under AVB excitation can reach 10-11 M.In addition,the time stability and reliability were tested,the relative standard deviation（RSD）was calculated to be 4.4%and 5.8%,respectively.（3）A method for Raman detection with high sensitivity has been presented adopting a Ag NPs coated D-shaped fiber（DSF）internally excited via fiber-mode AVB.Simulation results demonstrate that the electric field intensity can be effectively improved by AVB excitation compared with LPB excitation,because the strong gap-mode is generated between two adjacent Ag NPs on the surface of the DSF planar side.The configuration was applied to examine Raman spectroscopy,where the target analyte molecule was methylene blue（MB）.In the case of LPB excitation,the maximum Raman intensity and the minimum Raman intensity were achieved at two orthogonal polarization direction（～100°and 10°）.When the fiber-mode AVB generated by AIFG method was adopted as the excitation source,the Raman intensity was～4.5 times stronger than that of LPB excitation with the optimal polarization direction.A detection sensitivity of 10-9 M was achieved under the APB illumination,and the RSD corresponding to time stability was also examined to be 9.7%.（4）A method for Raman examination based on a Ag-pyramid coated microfiber internally excited via RVB has been developed.Theoretically,the Ag-pyramid coated microfiber is designed to be matched with the RVB,and simulation results showed that the electric-field intensity enhancement of the cylindrical metalized microfiber excited by RVB can reach 2.5×103,being much higher than that for LPB excitation.In experiment,the Raman intensity for MG detected by the Ag-pyramid coated microfiber under RVB excitation was 5 times more intense than that under LPB excitation,where the RVB was generated by AIFG method.The Raman sensitivity for MG under RVB excitation was as low as 10-10 M.The excellent homogeneity of the Ag-pyramid coated cylindrical micro-fibers and time stability of the method were confirmed in the examination of Raman imaging. |
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