Experimental Research On The Key Technology And Detection Of Transmission-/Reflection-mode Tip-enhanced Raman Spectroscopy

Tip-enhanced Raman spectroscopy (TERS) is a promising method for exploringphysical property and chemical information of material and structure on the nanoscale.Combining the advantages of near-field optics and surface enhanced Ramanspectroscopy, TERS is capable of obtaining corresponding topographical and spectralinformation with the optical resolution beyond the diffraction limit and remarkablelocalized Raman signal enhancement. In this dissertation, a TERS setup in thetransmission-mode and reflection-mode is designed and built up for measurements ofvarious types of samples. In this setup, an accurate multi-dimensional adjusting devicefor the optical path is designed to realize the precise alignment of the excitation fieldand tip. A dual-scanner/dual closed-loop controller configuration is adopted to achievenanometer precision scanning with long-term stability. In order to further develop thesetup and improve its performance, the key technologies of TERS are investigated andoptimized. Then, the TERS setup is used in experimental research on nano-materials.A tip spectral response detection setup, based on unilateral total internal reflectionillumination, is an effective way to detect the optical responses of TERS tips and checkthe performances of TERS tips. It is integrated with the TERS setup on the sameplatform. The excitation condition of the tip detection setup is very close to theexperimental TERS. Thus the detected spectral response of the tip apex will moreaccurately reflect the performance of practical applications for TERS. The spectralresponses of silver-and gold-coated AFM silicon tips are detected experimentally.TERS experiments with these two tips under532nm laser excitation are carried out. Thecomparative experiment indicates that using these tips with a resonance response at anillumination wavelength more strongly enhances the near-field Raman spectrum. Thisverifies the corresponding relationship between the measured spectral response and thetip’s enhancement performance in TERS.Longitudinal electric field illumination is quite effective for enhancing the localelectric field and Raman signal at the tip apex due to the lighting-rod effect. In order tooptimize the excitation field in the transmission-mode TERS, the longitudinal fieldgenerated by tightly focused radially polarized (RP) beam is used instead of the linearlypolarized beam. The focused light fields are numerically simulated according to the vector diffraction theory and their vectorial analysis is also carried out. And thelongitudinal fields on the focal plane are experimentally detected using a scattering-typescanning near-field optical microscopy. The simulation and experimental resultsindicate that the tightly focused radially polarized beam is suited to generate a strongerand purer longitudinal optical field at the focus for TERS excitation. The enhancementfactor,1.7×104, with RP illumination is achieved in the transmission-mode TERS. It is6times higher than that with the focused linearly polarized excitation. Thus, the RPillumination could be advantageous in TERS excitation. The corresponding TERSmapping and topography are simultaneously characterized. The experimental resultsdemonstrate that the TERS setup with RP excitation is capable of providing effectiveenhancement and reasonable detection sensitivity.A TERS setup based on plasmonic lens (PL) excitation is proposed in this work. APL expected to realize a strong longitudinal electric field focus is designed according tothe phase matching conditions. The focusing performance of the PL is calculated viafinite-difference time-domain (FDTD) simulation and experimentally detected by ascattering-type scanning near-field optical microscope. The PL is introduced to a TERSsystem as a focusing device. Experimental results with carbon nanotube samplesindicate that the Raman scatting signal is significantly enhanced. The enhancementfactor is4×103. It proves experimentally that the combination of a PL focused excitationfield with a metallic tip in a TERS system is a promising method.