Azimuthal vector beam illuminating plasmonic tips circular cluster for surface-enhanced Raman spectroscopy

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.

表面增强拉曼光谱，痕量检测再突破

表面增强拉曼光谱

拉曼光谱源于入射光子和分子的非弹性散射，是解析分子结构的一种有效方法，相关应用已涵盖物理学、化学、材料和生命科学等领域。但拉曼散射截面小，采用远场激发的拉曼检测灵敏度低。基于表面等离激元共振效应发展的表面增强拉曼光谱术（SERS），可将拉曼检测灵敏度提高到单分子水平。

SERS主要依赖贵金属纳米结构表面的局域电场增强，以提高目标分析物的拉曼散射强度。具有高斯模态分布的线性偏振光束（LPB）已被广泛用于SERS。然而，对于具有特殊空间对称性的贵金属纳米结构，LPB单调的偏振分布特性缺乏高效激发表面等离激元共振效应的能力。与LPB相比，角向矢量光场（AVB）具有更丰富的垂直于金属纳米结构表面的偏振分量。因此，将AVB用作激发光源，可以显著提高SERS性能。

痕量检测再突破

西北工业大学张文定教授课题组将贵金属纳米结构的特殊空间分布特性与角向矢量光场特殊的电矢量分布特性相结合，实现贵金属纳米结构表面局域模式电近场强度的显著提高，进而实现高灵敏度拉曼检测。相关研究成果发表在Chinese Optics Letters2023年第21卷第3期上（L. Zhang, et al., Azimuthal vector beam illuminating plasmonic tips circular cluster for surface-enhanced Raman spectroscopy），并被选为当期封面。

该课题组利用自由空间AVB照射等离子体探针圆簇（PTCC），通过激发PTCC中相邻纳米尖端之间的热点，显著增强了电近场强度，进而将其用于提高SERS检测灵敏度。将自组装和电感耦合等离子体（ICP）蚀刻相结合，制备了圆周型排布的PTCC。仿真结果表明，与LPB相比，AVB照明PTCC，无论是电场增强因子还是热点数量，都能够获得显著提高。实验结果表明，与LPB相比，AVB照明PTCC，将SERS灵敏度提高了两个数量级，达到10⁻¹³ mol/L，且拉曼增强因子达到~2.4×10⁸，如图1 所示。该矢量光场增强拉曼光谱技术有望应用于痕量检测等传感技术领域。