Plasmonic Particle-on-Film Nanocavity in Tightly Focused Vector Beam: a Full-Wave Theoretical Analysis from Near-Field Enhancement to Far-Field Radiation

Based on the local electric field enhancement of gap-mode surface plasmon resonance, the plasmonic particle-on-film nanocavity (PPoFN) can realize Raman signal enhancement of multiple orders of magnitude, even single molecule detection. The tightly focused radially polarized (Bessel-Gaussian) beam (TFRPB), containing a cylindrical symmetry longitudinal component, is a superior excitation source for PPoFN to generate a more intriguing electromagnetic hot spot in the gap. For the optical response of PPoFN in TFRPB, some theoretical and experimental studies at a certain wavelength have been reported, but a full-wave analysis about the near- and far-field optical properties is still lacking because it is still difficult to generate a full-wave radially polarized beam and achieve a dispersion-free confocal light field in the experiment. In this paper, we analyze the full-wave near- and far-field optical properties of PPoFN in TFRPB by using numerical simulation with the finite element method (FEM). For comparison, a full-wave analysis of PPoFN in tightly focused linearly polarized (Gaussian) beam (TFLPB) is also implemented. Based on the comprehensive results of local field enhancement, nanofocusing property, and far-field radiation, it is found that the PPoFN in TFRPB with the strongest toroidal dipole (TD) mode at a certain wavelength can achieve good performance in Raman signal excitation and detection. Importantly, this result will provide a theoretical reference for the application of PPoFN in tightly focused vector beam.