Circular nanocavity substrate-assisted plasmonic tip for its enhancement in nanofocusing and optical trapping

Plasmonic tip nanofocusing has widely been applied in tip-enhanced Raman spectroscopy, optical trapping, nonlinear optics, and super-resolution imaging due to its capability of high local field enhancement. In this work, a substrate with a circular nanocavity is proposed to enhance the nanofocusing and optical trapping characteristics of the plasmonic tip. Under axial illumination of a tightly focused radial polarized beam, the circular nanohole etched on a metallic substrate can form a nanocavity to induce an interference effect and further enhance the electric field intensity. When a plasmonic tip is placed closely above such a substrate, the electric field intensity of the gap-plasmon mode can further be improved, which is 10 folds stronger than that of the conventional gap-plasmon mode. Further analysis reveals that the enhanced gap-plasmon mode can significantly strengthen the optical force exerted on a nanoparticle and stably trap a 4-nm-diameter dielectric nanoparticle. Our proposed method can improve the performance of tip-enhanced spectroscopy, plasmonic tweezers and extend their applications. We anticipate that our methods allow simultaneously manipulating and characterizing single nanoparticles in-situ.