Plasmonic Particle-on-Film Nanocavity in Tightly Focused Vector Beam: a Full-Wave Theoretical Analysis from Near-Field E
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Plasmonic Particle-on-Film Nanocavity in Tightly Focused Vector Beam: a Full-Wave Theoretical Analysis from Near-Field Enhancement to Far-Field Radiation Ping Tang1 · Xinyue Xing1 · Shengde Liu1 · Wendai Cheng1 · Xiaoxu Lu1 · Liyun Zhong1 Received: 3 June 2020 / Accepted: 24 August 2020 © Springer Science+Business Media, LLC, part of Springer Nature 2020
Abstract 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. Keywords Plasmonic particle-on-film nanocavity · Tightly focused radially polarized beam · Full-wave near- and far-field optical properties · Finite element method
Introduction When a noble metal nanoparticle is located on the surface of a thin plasmonic metal film, typically a gold (Au) nanoparticle on the Au film, the local surface plasmon resonance (LSPR) of Au nanoparticle and surface plasmonpolaritons (SPPs) of Au film will couple with each other [1]. When the distance between Au nanoparticle and Au film become small enough (typically 1–2 nm), the gapmode will generate an ultra-strong local electric field in the hot spot. This configuration is commonly referred to as the plasmonic particle-on-film nanocavity (PPoFN) or the nanoparticle-on-mirror (NPoM), which has been Liyun Zhong
[email protected] 1
Guangdong Provincial Key Laboratory of Nanophotonic Functional Materials and Devices, South China Normal University, Guangzhou 510006, China
widely used in fluorescent or Raman signal detection due to its outstanding ability for weak signal enhancement by several orders of magnitude [2–7]. As a kind of plasmonic nanocavity, PPoFN
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