Tip-enhanced Raman spectroscopy mapping with strong longitudinal field excitation

Longitudinal electric field illumination is greatly effective for enhancing the local electric field and Raman signal at the tip apex in tip-enhanced Raman spectroscopy (TERS). In this work, the optical field distribution of tightly focused radially polarized (RP) beam on the focal plane is calculated according to vector diffraction theory and the effectiveness of longitudinal field excitation on Raman signal enhancement is experimentally verified. An RP beam generated by a liquid crystal spatial light modulator is focused via a high numerical-aperture objective lens to form a strong longitudinal electric field at the focus. The focused field is utilized to excite single-walled nanotubes sample in TERS experiments. The experimental results demonstrate that the enhancement factor of RP excitation is obviously higher compared with linearly polarized illumination. And then, the corresponding TERS mapping and topography are simultaneously characterized.