Raman spectroscopy, originating from the inelastic scattering of incident photons and molecules, is an effective method to analyze the structure of molecules, and the applications have covered the fields of physics, chemistry, materials, life science, etc. Due to the small Raman scattering cross section, Raman examination sensitivity by far-field excitation is relatively low. Surface-enhanced Raman spectroscopy (SERS) based on the surface plasmonic resonance effect can improve Raman examination sensitivity to single molecule level. SERS relies on the local electric-field enhancement of noble metal nanostructure to improve the Raman scattering intensity of the target analytes. Linearly polarized beam (LPB) has been widely used in SERS. However, for the noble metal nanostructures with special spatial symmetry, the monotonic polarization distribution of LPB lacks the ability to efficiently excite the surface plasmonic resonance effects. Compared with LPB, the azimuthal vector beam (AVB) has more abundant polarization components perpendicular to the surface of the metal nanostructures. Therefore, AVB used as an excitation light source can significantly improve SERS performance.
In the Chinese Optics Letters, Vol. 3, No. 21 (L. Zhang, et al., Azimuthal vector beam illuminating plasmonic tips circular cluster for surface-enhanced Raman spectroscopy). AVB was adopted to illuminate the plasmonic tips circular (PTCC) to significantly enhances the electric near-field intensity of PTCC array by exciting the hot spot between the adjacent nanotips in the cluster, and thus improves the SERS sensitivity. Self-assembly and inductively coupled plasma (ICP) etching were combined to prepare PTCC. Simulation results show that, compared with LPB, the AVB excitation can obtain the optimized electric near-field enhancement, no matter whether it is the electric field enhancement factor or the hot -spots number. Experimental results proved that, the SERS sensitivity of the PTCC illuminated by AVB has been increased by two orders of magnitude to 10−13 mol/L, and Raman enhancement factor reached ~2.4×108, as shown in Figure 1. This vector light field enhanced Raman spectroscopy is expected to be applied to trace detection and other sensing technologies.
Figure 1 Sketch map of AVB illuminating PTCC and SERS examination results
