Near-field control of gold nanostructure by the interaction of SPP and incident light

Abstract

Localized Surface Plasmon (LSP) in nanostructure excited by Surface Plasmon Polariton (SPP) corresponds to stronger near-field enhancement and special spectral and dynamic responses that provides a new path to explore the interaction between light and matter. Meanwhile, this scheme can also release the signal background noise and structural thermal effect, and improve the performance of plasmonic components and sensing detectors based on LSP. However, the current research on this aspect is still insufficient. In this paper, we investigated the near-field characteristics of a plasmon composite structure composed of plasmon focusing lens and gold nanorod under the excitation of dual-beam using Finite-Difference Time-Domain (FDTD) method. The result shows that the near-field intensity control on the upper surface and in the gap position of the nanorod can be achieved by adjusting the relative time delay between the first light beam (used to excite SPP) and the second light beam (used to excite LSP). Specifically, the maximum adjustment range of the near-field intensity corresponding to 770 nm resonant mode in the gap position is about 23, and the adjustment period is about 2.4 fs. In a resonant mode dominated by SPP at a wavelength of 999 nm, the near-field intensity adjustment range is as small as 6, and the adjustment period is about 4 fs. On the upper surface of the structure, the adjustment range of the near-field intensity of the two resonant modes (719 nm and 802 nm) is basically the same (about 15), and the adjustment period is 2.4 fs and 2.8 fs. The achievement of the near field control is attributed to the coherent superposition of SPP-excited LSP with light-excited LSP. In addition, the dephasing time of the coupling field was investigated using quasi- normal mode. It is found that the nanorod structure will correspond to different dephasing time under different relative time delay between two excitation light beams. Specifically, for the time delay of 0.72 fs (Δt=0.72 fs), the corresponding dephasing time for both modes is the same of 6.0 fs. For Δt=1.92 fs, the dephasing time of the longer-wavelength mode is 7.1 fs, and the one of the shorter-wavelength mode is 5.8 fs. We attribute the variation of the dephasing time to different coupling strength between the two modes at different delay times. This study may further promote the application of plasmons in the fields of surface-enhanced Raman scattering and plasmon assisted catalysis.