Optimization Design Of Transmission Tip-enhanced Raman Spectroscopy Detection System

Tip-enhanced Raman spectroscopy(TERS)technology is an important research method to detect molecular vibration spectra at the nanoscale structure.Based on scanning tunneling microscope(STM)and atomic force microscope(AFM),TERS technology greatly confined and enhanced electric field near the tip apex,utilizing the localized surface plasmon gap-mode hot-spot formed at the gap between tip apex and sample.Meanwhile,the hot-spot greatly enhanced the scattering cross section of the sample,which can break through the optical diffraction limit and obtain nanoscale spatial resolution and high detection sensitivity at the same time.While the spatial resolution and signal sensitivity of the transmitted AFM-TERS system are widely limited by the excitation light field mode and the enhancement effect of only the metal tip.Therefore,how to improve electromagnetic field enhancement factor and spatial resolution of the transmitted AFM-TERS system has always been the research forefront on the TERS.In our work,the metal thin layer was introduced in the transmittance AFM-TERS system to build the gap-mode TERS model,whose electromagnetic field was obtained by the finite element method(FEM).Through analyzing the performance parameters,such as electromagnetic enhancement factor,mode distribution and spatial resolution,to design and construct the optimized AFM-TERS system in terms of excitation method and metal thin layer parameters.The main framework of the dissertation is divided into two parts:First,the optimization of excitation field in the transmission TERS system.According to the vector diffraction theory,the radial polarized light excited from the bottom is focused by the objective lens with high numerical aperture to form a longitudinal tight focused light field paralleling to the axial direction of the probe.It was found that,compared with common linear polarized beam,the radial polarized beam can enhance the localized electromagnetic field by 103 times,the TERS enhancement factor by 1.4 × 108,the spatial resolution by 2 times,and the horizontal resolution by 7.4 nm.Therefore,the radial polarized optical field is an optimized excitation light field to improve the enhancement performance and transverse spatial resolution of the TERS in the nano cavity,which is suitable for the transmission TERS detection system.Second,the optimization of metal thin layer in the transmission TERS system.The thickness of metal thin layer is the source of the competition between the light transmittance and hot-spot coupling enhancement.By varying the thickness of the metal thin layer and theoretically analyzing the light transmittance and the coupling strength of hot spot,the maximum enhancement factor of the TERS system was obtained.And the optimal thickness of the metal thin layer in the transmission TERS system excited by radial polarized beam is about 30 nm.The research in this paper provides reference for the realization of high efficiency gap hot spot mode in transmission TERS system,and also contributes to the spatial resolution and signal sensitivity of TERS.